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A CRITICAL EVALUATION OF FOUR METHODS
0F SODIUM ANALYSIS IN FOODS WITH SPECIAL EMPHASIS
ON THE USE OF THE SODIUM ION ELECTRODE

Thesis for the Degree of M. (S.
MICHIGAN STATE UNIVERSITY
Yolanda Nifi'o-Orozco
1964

THESIS

LIBRARY

Michigan State
University

 

nBSTRACT
A CRITICAL EVALUATION OF FOUR.METHODS OF
SODIUM ANALYSIS IN FOODS WITH SPECIAL
EMPHASIS ON THE USE OE THE SODIUM ION ELECTRODE
by

Yolanda NiHo-Orozco

Sodium concentration in foods varies from traces in
certain fresh vegetables to more than ten thousand parts per
million in some processed foods. The sodium analysis in
foods is important since sodium restricted diets are used in
the treatment of certain diseases.

Four methods were evaluated in the determination of
sodium in foods of widely different composition:

The standard A.O.A.C. magnesium-uranyl-acetate and
flame photometry methods were used as reference. Alpha-
methoxy-phenyl acetic acid (MOPA) as well as a sodium ion
electrode were compared to the above.

Very good precision was obtained when using the
reference methods.

When working with milk and buttermilk, the magnesium-
uranyl acetate method gave much higher results than those
found with other methods.

The determination of sodium in foods by the MOPA
method was not successful except where the concentrations

were above 20x102 ppm.

2

The sodium ion electrode method proved to be fast
and convenient. In the proportions usually found in foods,
fats, carbohydrates and proteins did not affect the reSponse
of the electrode, but some changes in the recovery of added
sodium occurred during canning and freezing. Some modifica-
tions will be necessary for the analysis of meats, since in
this product the results obtained differed widely from the
standard methods.

Overall, the sodium ion electrode presents a good

possibility as a standard method of analysis in foods.

A mis padres.

A Critical Evaluation of Four Methods
of Sodium Analysis in Foods with Special Emphasis

on the Use of the Sodium Ion Electrode

bv .
r‘ , '
fi“l"

I .‘f..-
Yolanda Nino-Orozco

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
IVIhSTER OF SCIENCE
Department of Food Science

1964

27‘
3 {/[g/ég/

AWIJOL‘ILEDGEZ‘IELJT

The writer wishes to eXpress her sincere gratitude
to Dr. Sigmund H. Schanderl for his tireless efforts in
guiding the work of this study.

Appreciation is extended to Dr. Pericles Markakis
for his valuable suggestions during the course of this
research, and to Dr. Dorothy Arata for reviewing the manu-
script.

The author wishes also to thank Dr. Clifford L.

Bedford for his worthy assistance.

TABLE OF CONTENTS

Page

__i_
INTRODUCTION . o o . . o . o o o . . . . . . o . . . . l
LITEm—sTURERE's/IEW..-................4
a) Sodium in foods . . . . . . . . . . . . . . 4
b) Effects of heating in sodium content . . . 5
c) Methods for sodium analysis . . . . . . . . 6
l. Gravimetric methods. . . . . . . . . 6
2. Physical-chemical methods . . . . . . 9
3. Volumetric methods . . . . . . . . . 14

METHODS AND MATERIALS . . . . o o . . o . o o o o . . 15
a) Magnesium-uranyl-acetate . . . . . . . . . 16
b) Flame photometry . . . . . . . . . . . . . .16
c) Sodium ion electrode . . . . . . . . . . . .17
d) Alpha-methoxy phenyl acetic acid . . . . . .24
RESULTS ANDIDISCUSSION . . . . . o . . o o o o . . . . 27
SUMMARY AND CONCLUSIONS . . . . . . 5 . o . . . . . . 40
APPENDIX . o . o . . o o . . o . o o . . o o . . . . . 42

BBLIOGWHY O O O O O O O O O O“ O O O O O O O O O O O 49

ii

LIST OF TABLES

Variation in the Reading with Increasing the
Time allowed for Sodium Ion Electrode
to Reach Equilibrium . . . . . . . . .
Effect of the Protein on the ReSponse of the
Electrode . . . . . . . . . . . . . . .
Effect of Carbohydrate Content on the Electrode
ReSponse . . . . . . . . . . . . . . .
Influence of Mineral Mixtures on the Electrode
ReSponse . . . . . . . . . . . . . . .
Influence of the Sample Dilution on the Sodium
Electrode Response . . . . . . . . . .
Effect of Processing Operations on the ReSponse

for Sodium . . . . . . . . . .. . . . .

42
44
.45
46
47

48

Introduction

Since Sodium plays a very important role in human
metabolism and Sodium restricted diets have been necessary
in the treatment of some cardiac and renal diseases, the
quantitative analysis of this element in foods has become a
very important topic.

Several methods have been used for sodium analysis,
but because of the high cost of the equipment and reagents of
some, and to the time consumed by others, none of these methods
has been universally accepted.

Sodium is important in foods, 1. For the maintenance
of the normal water balance and distribution, 2. for the
maintenance of the normal acid-base equilibrium, 3. for the
maintenance of the osmotic equilibrium, 4. for the maintenance
of the normal muscle irritability.

Balance eXperiments showed that approximately a liter
of water is retained for every 6 or 7 g. of sodium chloride
accumulated in the body (64). When an excess of sodium
chloride is ingested by a normal person, swelling of the body
occurs. When plasma proteins are low, the excessive use of
salt also produces edema (64). In some cardiac patients it
was observed that the sodium ingested was not excreted in
the urine, nor was an increase-of sodium in the blood noticed.
It was concluded that the sodium accumulated with water formed

edema. (66)

2

Sodium exerts its buffer action usually reacting with
either bicarbonate or Chloride ions (58), thus maintaining
the acid base equilibrium of the body. Sodium ions have an
effect on irritable tissues, such as muscle, which is
counteracted by the presence of calcium ions (64). This is,
at least in part, one of the bases for the recommendations of
dietary salt restrictions in cases of hypertension and cardiac
diseases. Sodium ions are found mainly in the extracellular,
and potassium in the intracellular, fluids, both working
together to maintain the normal osmotic pressure of the
tissues and cells. When potassium is restricted in the diet,
sodium may take its place without presenting any trouble until
a critical point is reached.

Most sodium is absorbed in the small intestine but
some is absorbed in the stomach (64). Sodium is mainly ex-
creted in the urine and perSpiration.

Sodium deficiency in rats will adversely affect
appetite, growth, and synthesis of both fat and protein, and
finally produce death. (64). In humans, undesirable effects
may be caused as a result of deficiency of sodium. However,
no case has been reported in Which a diet caused death be-
Cause of its deficiency in sodium.

The analysis of sodium in foods was made by using in-
direct methods, until around 1930 when application was made
of the Streng's reagent for the precipitation of sodium as

the triple salt: sodium-magnesium-uranyl-acetate. In the

3
past ten years, extensive study using flame SpectrOphotometry
for sodium analysis in foods has been develOped.

The method is based upon the measure of the
characteristic light emitted when a solution of a salt of
sodium is atomized as a mist into a gas flame.

The work described here gives the results of a study
of four methods applied to the analysis of sodium in foods -
the above mentioned already studied, and two whose applic-
ability has not yet been proved, namely, the alpha-methoxy-
phenyl acetic acid, and the sodium ion electrode.

The purpose of this study is to evaluate the two new

methods as possible standards for sodium analysis in foods.

 

LITERATURE REVIEW

A. Sodium in foods

In general, one can say that food of animal origin
contains much sodium, while food of vegetable origin usually
contains little sodium before processing.

In the processing Operations to which foods are sub-
jected, some sodium is generally added. Processed meats and
fish usually contain much more sodium than the fresh products,
because of the addition of salt either in the canning or
freezing. Sometimes this addition is made in the washing of
the foods before processing.

The most common sodium compound generally added is
sodium chloride, which is used as preservative and also to
improve the taste of the food. Many manufactured foods which
are not intentionally salted contain somewhat more sodium
than the natural foods from which they are prepared, because
the preservatives and additives contain compounds of sodium.

Marked differences in the sodium content of lettuce,
onion and tomatoes between different producing areas and
between different seasons were found by Homer (35). If this
is true for all vegetables, no generalization should be made
about the sodium content of different items of the same family,
and the data found for sodium content in foods would have
only a very limited application. The following dataere

the result of a series of analyses made by Bills, NcDonald,

5

and Hiedermeier (7). They found sodium to be present only as
a trace element in unprocessed fruits, grain, nuts and legume
seeds. Excluding coconut, the average content is about 2 mg
per 100 9. Coconut is somewhat high in sodium, around 35 mg
per 100 g. In leaf, stalk and root vegetables the sodium
content varied. White potatoes contained but traces, while
celery, beets, and chard contained 100-200 mg per 100 g.

The fresh meats, fish, and fowl mostly contained from
50 to 100 mg of Na per 100 g . of edible portion. In each of
the five fowls studied by Bills — chicken, duck, goose,
quail and turkey - the breast meat always contained substan-
tially less Na than the leg meat. Na may be related to the
higher metabolism of these tissues. Four Species of widely
used fish - catfish, cod, halibut and salmon - were analyzed
in the fresh state. These Species were similar in Na con-
tent ranging from 48 to 60 mg of Na per 100 g.

Most of the Na in eggs is in the white. Cow's milk
contained around 50 mg of sodium per 100 g. Butter and

cheese contained more sodium because of the salt added.

B. _§§fects of heating in sodium content
1. No matter what method of cooking is employed,
meats always shrink and eXpress a portion of their juices as
fats: as ordinarily prepared, meats may lose a fourth to a
third of their raw weight. The readily soluble ions (includ-

ing Na) are lost in prOportion to the expressed juices.

6
2. hhen cooking is by steam, weight loss or shrinkage is
very close to the percentage loss of water and of soluble
salts. 3. In boiling water, a leaching effect causes addition—
al loss of soluble salts, but this loss is not nearly as large
as might be eXpected. Neat just cooked through by boiling
may lose only a negligible additional amount of soluble salts
from leach. 4. In roasting and frying, fat is expressed
and evaporation of water is intense: however, salt losses are

low. (44).

C. Methods for Na Analysis

 

The determination of Na may be made by gravimetric
methods, volumetric methods, and by using physical-chemical
methods.

1. Gravinetric methods. Among gravimetric methods for Na

 

analysis we find indirect and direct methods.

a. Indirect methods were used first, based on the
precipitation of Na and K as salts (usually as chlorides or
sulfates): potassium was then separated, and by difference,
sodium was calculated. I

The first sodium determination was made in the 19th
century when Fresenius (27) separated Ea and K as chlorides.
Iotassium was precipitated from the mixture as chlorOplati-
nate, and by difference sodium was estimated. This method

was later modified by Talapatra (61) for sodium analysis in

foods, by precipitating potassium as K0104.

7

At the beginning of the 20th centuryaineW'method
for sodium analysis appeared. Sodium and potassium were
precipitated as sulfates, and by difference after estimation
of potassium as cobalto cyanide, sodium was calculated (26).

It was not until 1913 that the Na analysis in foods
began to take on interest to the scientists. The first
estimations were made by Forbes (26). he moistened the
sample with concentrated sulfuric or nitric acid, and ashed
to destroy organic matter. PhOSphorus, magnesium, calcium,
and barium interferred and were eliminated. Phosphorus was
precipitated with magnesium hydroxide, calcium, and barium as
carbonates and oxalates. Ammonium sulfate was used to pre-
cipitate sodium and potassium as sulfates. The latter were
filtered, washed, and weighed.

Potassium sulfate was found from the precipitation
of potassium as chlorOplatinate, and subtracted from the
weight of the combined sulfates to obtain the amount of
sodium in the sample. The method was later modified by Rhue
(53), who suggested altering the process of ashing and the
precipitation of calcium and iron: and by Husband (37),
who introduced in 1927, the precipitation of potassium as
cobalto cyanide instead of using chlorOplatinate.

b. Qirgct Methods. Indirect methods were used until
1923 when Blanchetiere made application of the Streng's
reagent for the detection and precipitation of sodium as the

triple salt sodium-magnesium-uranyl-acetate (8). In 1942,

8
Fruit (48) presented a modification of this method, prolong-
ing the precipitation period from 40 minutes to 24 hrs. at
20°C, and preparing the magnesium uranyl acetate solution so
that it was saturated at this temperature. The new'method
became official for fruits and fruit products, and later for
foods in general.

Many different methods have been deve10ped based on
the magnesium uranyl acetate method, such as the colorimetric
method described by Foulk (12), and the volumetric method
described by Vuk and Sandor (65).

Lack of correlation of the results obtained when using
magnesium uranyl acetate was pointed out by Crepaz (l9),
Perietzeana (47), and Barber and Kolthoff (2). The latter
suggested the use of zinc uranyl acetate method instead of
the magnesium uranyl acetate, since in the first case the
analysis is faster, and provision is made for the precipita-
tion of potassium.

The zinc uranyl acetate, or Barber and Kolthoff's
method, consists in the precipitation of sodium as sodium
zinc uranyl acetate previous to the removal of potassium with
ammonium perchlorate in 72% alcohol solution. According to
Kolthoff (3), lithium, phosphate, and thiosulfate ions inter-
fere and should be removed.

The method has been widely studied by Collins (18)

and Clifford, and Winkler (15, 16).

9

As in the case of the magnesium uranyl acetate method,
variations in the method have been made such as the colorimetric
methods described by McCance and Schipp (43) for high sodium
foods, and the modified zinc uranyl acetate method, converting
the precipitate to uranyl potassium ferrocyanide, as given by
Bradbury (10).

In 1931, Basset (4) used, for the first time, the
Barber and Kolthoff method for the analysis of sodium in
diets.

In 1954, Clifford (16) found very close agreement
when he compared the results obtained with the zinc uranyl
acetate method to those obtained by SpectrOphotometry, while
the magnesium uranyl acetate seemed to fail when low sodium
quantities accompanied by much potassium (as in the case of
legumes) were encountered.

It should be noted that, in Spite of the fact that
the magnesium uranyl acetate method is still the standard
method for sodium analysis in foods given by the S.O.A.C., most
of the research work on sodium analysis in foods, done in
the last 30 years, has been carried out by following either
zinc uranyl or flame photometric methods.

Physical chemical methods. among the physical-
chemical methods of sodium analysis are:

a) The different colorimetric techniques using either

magnesium-uranyl-acetate or zinc-uranyl—acetate. Caley (12)

lO
pointed out a method wherein sodium is precipitated with
magnesium uranyl acetate, as in the gravimetric method, the
precipitate dissolved in warm water and the depth of the
color compared with a standard.

McCance (43) made the precipitation with zinc uranyl
acetate, and evaluated the uranium colorimetrically with
potassium ferrocyanide. barnell (20) used the sane reagent,
and washed the precipitate with ethyl acetate and acetic
acid, then photometrically measured the color develOped by
sulfosalicylic acid and sodium acetate.

b) Determination Cf sodium by using flame photometry
is based on the measurement of the energy emitted by a sarple
at the sodium wave length as revised by Farrow (23). Flame
spectrosc0py measures the light absorbed, at a selected wave
length, when a light ray from a cathode is passed through the
flame (52).

-J

C) By using radio-activation, Smales, using a Geiger
counter, measured the intensity of the radiation from sodium24
as compared with a standard irradiated simultaneously (EO).
Amiel and Feisch (1), employing the same principle, counted
the photoneutrons in an assembly consisting of D20 and neutron
detector. Castro and Smith (14) measured sodium in citrus
fruit juices by activating neutrons, using the General atomic
TRIGA reactor.

d) The determination of sodium by ion exchange resins:

Gabrielson (23) separated alkali metals from complex

ll

cyanides, using cation exchange resins of sulfonic type.
Sodium and potassium were eluted as chlorides and measured
either volumetrically or gravimetrically.

e) Using column chromatography: Erlenmeyer (22)
analyzed sodium and potassium in blood samples, by means of a
thin column chromatography made of a Special peeking, which
produced a sharp red zone with sodium. The length of the
zone was directly preportional to the sodium concentration
in the sample. P. Hsieh (46) used Amberlite IR-120., and
measured the amount of sodium and potassium indirectly by
titrating the excess of hydrochloric acid used in the elution.

f) By means of the sodium ion electrode: the sodium
ion activity is determined by measuring the potential
whidh is developed between the sample and the solution which
is inside the body of the electrode, as published by

Leonard (39).

a. Flame Photometry
The principle of the instrument is that the sample
is introduced into a flame, and the characteristic energy
emitted at certain wave length, is measured by means of a
photometer.
This method is by far the most accepted, and has found
wideSpread application, because it is simple and a minimum

of preparation of the sample is required.

12

Lundergarth, in 1930, was the first to apply flame
photometry to sodium analysis in plants and biological
materials.

Numerous investigations have been carried out on this
subject, eSpecially in order to overcome interferences by
altering the sample preparation (23) or by introducing modi-
fications in the instruments as reviewed by Robinson (52).

Bills (7) analyzed sodium in more than 100 foods and
waters, applying, for the first time, flame photometry to the
analysis of sodium in foods. Clifford (16) found very good
correlation between results obtained with flame photometric
and zinc uranyl acetate methods.

Shoga Ogawa (57) obtained lower results than eXpected
from the references, when he analyzed sodium in 31 foods by
means of flame photometry.

In 1956, Have (32) presented the results of the com-
parison of flame photometry and zinc uranyl acetate method
for sodium analysis in milk. His results gave good agree-
ment, and suggested the adaptation of flame photometry as a
simpler method for sodium analysis in foods. It should be
pointed out that Have added calcium, in a concentration
similar to that found in milk, to the Na81 solutions used
for the drawing of the standard curve in the flame photo-
metric method.

Santini (55) found that flame photometry works better

for food and feces after wet ashing with nitric acid -

l3
perchloric acid mixture instead of dry ashing.
sagakova (54) determined sodium in canned foods and
raw materials by using a selenium photoelement with OS 12
orange filter, and 7.5% solutbn of COpper sulfate in sulfuric
acid as additional filter. The determination took from 20-

30 mins (excluding ashing time).

B. Sodium Ion_§;ectrode
As early as 1923, Horowitz (36) studied the res-

ponse of glass electrodes to monovalent ions other than hydro-
gen - e.g., sodium and potassium. Sohiller extended his wofl:
(55). However, the first comprehensive study of this subject
was made in 1934 by Lengyel and Blum (38). Their aim was to
establish the relationship between composition and the
deveIOpment of hydrogen or sodium functions in the response
of electrodes made from soda-lime glasses. They discovered
the important fact that the trivalent oxides (especially
those of aluminum and Boron) cause glasses to develop a
sodium function when used to replace part of silica in the
glass structure. Goremykin (30) studied some Lengyel and
Blum glasses, and reported some correlation between composi-
tion, pre-conditioning and reSponse time.

In 1957, Eiseman and his co-workers (21) extended,
Lengyel and Blum's work to sodium alumina silicate glasses
containing as much as 18% (mol.) alumina. They found this
glass to be highly Specific for sodium, but too refractory

for practical manufacture. Leonard (39) found that a glass

14
with much lower alumina content (4% mol.) has about the same
sensitivity and is much more adaptable to quantity manufacture
of electrodes.
Mattock (42) designed in 1962, two new electrodes
which were highly Specific for sodium, and proved workable in

5 molar. Sulfate,

the range from one molar down to nearly 10-
calcium, and barium ions should be tolerated at high excess
over sodium concentration. PhOSphate was tolerable at 50
fold excesses, over sodium at pH 7. He pointed out the pos-
sibility that phOSphate ions would not be tolerable beyond
this limit. Bower (9) proved that the sodium ion electrode

method gives accurate results for sodium analysis in saline

solutions.

3. Volumetric Hethods

Aside from the volumetric method using magnesium
uranyl acetate used by vuk (65) for sodium analysis in milk,
before 1957 the literature does not mention any other volu-
metric method which has had considerable importance.

Alpha Lethoxy_pheny1 acetic acid. Wilkin Reeve
presented, in 1957, a new reagent for sodium analysis: Alpha
methoxy phenyl acetic acid (50).

Sodium forms a precipitate (an acid salt) with an
aqueous solution of this reagent, half-neutralized with
tetra-methyl ammonium, or potassium hydroxide. The salt is
titrated with sodium hydroxide. Oesper (45) used this

method to analyze sodium in blood serum.

15
According to Reeve (50), lithium, potassium, and
magnesium do not interfere. Calcium, strontium, and barium

interfere and should be removed.

methods and Katerials

The methods for sodium analysis used here are the
official magnesium-uranyl acetate method, the flame photo-
metric method, the electrode method, and the alpha-methoxy
phenyl acetic acid method. Some regulations, however, may be
applied to all of them in general, such as the use of de-
mineralizing water whenever water is necessary, and pyrex
glassware, or any other material which does not free sodium
which may contaminate the sample. The chemicals used were
always C.P. or reagent quality. The storage flasks used were

all made of plastic or of no—solvit glass.

1. Magnesium Uranyl Acetate Method
This method is based on using magnesium uranyl
acetate to precipitate sodium as the triple salt sodium-
magnesium-uranyl acetate (Ea Mg (U02)3 (CI-I3COO)9 6HZQQ7.

The procedure followed for the determination was the
one given for foods by the A.O.A.C. official methods of
Analysis (pp. 77).

The sample was weighed (1 to 10 g.) in a quartz
crucible, then moistened with (1+10) sulfuric acid (5 ml),
dried in an oven, and ignited at BOO-600°C in order to destroy

organic matter. The ash was dissolved in (2 to 5 ml”)

l6

concentrated hydrochloric acid, and the phosphates precipi-
tated and discarded as calcium phOSphate by adding calcium
ions (as calcium chloride). Ammonium hydroxide was used to
assure precipitation of the calcium phOSphate. The sodium
salt was then precipitated with the reagent, allowed to
stand for 24 hrs., filtered (over No. 5 Whatman filter paper)
using Buchner funnel, washed with alcohol containing reagent,
dried (at 105°C for half an hour), and weighed.

The sodium content in ppm in the sample was calcu-
lated according to the following equation:

Wt of the ppte X;;0153 X 106

ppm Of SOdlum = Wt of the sample

The value .0153 is the ratio at.Wt. of sodium/mol.wt.
of sodium-magnesium-uranyl acetate, and it converts the grams
precipitated salt to grams of sodium.

Following the above procedure, samples of different

foods were analyzed for sodium.

2. The Flame:photometric Method
A Beckman D.B. SpectrOphotometer with flame
attachment was used in the analysis. The standard method
for analysis of plant materials was followed.1
”he sample was weighed (0.5 to 5.0 g. according to

the sodium content in the sample) into a 25 m1 volumetric

 

ls.o.a.c. Standard methods of Analysis, pp. 76

(1961).

17
flask with glass stOpper, and ammonium oxalate2 stock solu-
tion was added up to the mark. The solution was allowed to
react for 15 minutes, Shaking at frequent intervals to
assure the reaction between calcium and oxalate. The pre—
cipitate was filtered off (through No. 2 Whatman filter paper)
and the filtrate used for the determination.

A standard curve was made with sodium chloride, at
concentrations ranging from 0 to 0.02 Normal. The instrument
was calibrated to read zero (0) with the extracting solution
(ammonium oxalate), and 100 with 0.02 N sodium chloride
solution, at 577 mp which proved to be the wave length of
maximum absorption in this instrument. (The wave length
suggested in the references was 589.0 mp).

The standard curve is shown in graph No. 1.

3. Sodium Ion Electrode. a Beckman sodium ion
electrode #39278, attached to a Beckman model 176 eXpanded
scale pH meter, was the instrument used in measuring sodium
in this method. This electrode is sensitive to sodium,
ammonium, potassium, silver and hydrogen ions, giving the same
theoretical relationship between electromotive forces and
cationic activities as the already known pH meter.

The main parts of the Beckman glass electrode are a

sodium-alumino-silicate glass bulb filled with 0.1 N NaCl,

 

2Ammonium oxalate.24N (made dissolving 17g. of

C 0 H 0 in water and dilute to one liter.)

(EH45 6 4 2

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Luminosity

nritlunlc, 2 x 2 Cycles

l9

ir1t:<:) which is immersed a silver-Silver chloride electrode.
£3 sstzandard saturated potassium chloride calomel half cell,
wvj;t:r1 fiber-type bridge, serves as the reference electrode.

The measurement of the sodium ion activity is accom-
plished with this electrode by determining the potential
vikugixjh is develOped between the sample in the beaker and the
ergpeecial solution inside the body of the electrode (0.1 N
sodium chloride).

The pdential observed for any given activity of
sac><iium ions will be according to the Nernst equation:

E = 3° - 3:1; 1n 12." NE] (Equation No. 1)

n?
ufrmeere

the potential observed for any given activity.
E = the potential of the standard state. It

should be noticed that E0 5 is zero when the

b
activity of the sodium ions is equal to the
standard state.

R = the universal gas constant given in joules.

T = Absolute temperature in degrees Kelvin.

n = Number of electrons transferred in the rever-
sible reaction.

F = Faraday constant.

In A; Na;7 = Natural logarithm of the activity of

the sodium ion to be determined. In dilute solutions, ion

El'z3“-‘-:i.vzi.ty is equal to ion concentration. Since, in the eXperiments

 

 

 

f

 

—— —--.——._ h— o

 

 

 

.._._ *.__.._-.—.._ -

JO

 

.aJIEZ)’.

OO

00
l-Lognrlthmlc

qum the find:

 

    
   

ISO

l25'

I00

75

50

21
described here, the measurement was made in dilute solutions,
the term concentration will be used instead of activity.
Standard Curve. A standard curve was made by record-
ing the change in electromotive force (millivolts) against
variation in sodium concentration. A series of sodium
chloride solutions varying in ten fold concentrations from

10"1

N to 10-5N were prepared, and the emf read on the eXpanded
scale of the pH meter. 0.1 3 sodium chloride solution was
adjusted to read zero (0) at room temperature, for convenience.
The curve was plotted as electromotive force (mv.)
on the arithmetic scale, x axis) against concentration of
sodium (normality or ppm, on the logarithm scale, Y axis).
It may be seen in Graph E0. 2.
The Rernst equation (Equation No. 1) predicts that,
at 25°C, the potential at the glass membrane will change 59 mv.
for each decade change in sodium ion activity. Because of
the difference between activity and concentration, at usually
measured concentration levels, the literature (9) predicts a
change of 56 mv. per decade change in actual concentration.
In the experiment described here, 52 mv. per decade change in
sodium ion concentration was achieved by the instrument, and
this was the curve used for the readings made in this work
See Graph Ko. 3).
Preparation of calcium tetraphenyl boron. An excess
of calcium—tetraphenyl boron was uSed to precipitate potas-

sium from the sample in order to avoid interferences from

 

 

~ 0 on 'of- I'IPc

V‘.
.0.“

|25

I
\

75
l-l‘I- l...-

50

25

23
this element. The reagent was prepared from .1 N sodium
tetraphenyl boron by means of an ion exchange column. Dowex
Sode-lz resin in the calcium form was used for this. The
eluent portion of calcium-tetraphenyl-boron with lowest
sodium concentration (20 ppm) was collected for the eXperi—
ment.

Procedure. The sample was either weighed or measured
volumetrically (in such an amount as to have a concentration
of sodium fan within the range at which the curve was
plotted) in a plastic container (250 ml capacity), and enough
calcium tetraphenylboron added to precipitate all potassium
ions. The pH was then adjusted to a concentration of hydrogen
ions of at least 4 powers of ten smaller than the sodium ion
concentration, in order to avoid hydrogen ion interference
using .04 N_calcium hydroxide. After 15 minutes reaction,
the mixture was quantitatively transfered into a 100 m1 volu-
metric flask and brought up to the mark with demineralized
water: the readings for the different samples were taken
following the same rules as when sodium chloride solutions were
read for the standard curve; and the equivalent of the mv. to
the sodium ion concentration was read from this curve.

When the sample solution to be read contained suSpended
particles, filtration was carried out before the immersion of
the electrode.

The electrode was immersed in 0.1 N sodium chloride

solution at least 2 hrs. previous to the reading, and was

24
kept dry when not in use.

4. Alpha-Methoxy-Phenyl-Acetic Agid. Alpha-methoxy-
phenyl-acetic acid is one of the two new tentative methods
which were tried for sodium determination in foods. The re-
agent is commonly called MOPA. It forms a scarcely soluble
sodium acid salt of the formula:

C6H5CH (OCH3) coon C6H5CH (0033) cooma

The method was first described by Reeve (50), and it
has only been applied to sodium analysis in blood, where the
concentration of sodium is in the range of 2000 ppm (45).

Liquid samples were preferred in this method. In
the case of solid or semi-solid materials, the sample was
weighed and dispersed in a determined volume of water. The
solution was allowed to reach the equilibrium (for one wedc,
in the present case) centrifuged or filtered, and a known
volume was then taken for analysis. The procedure followed
at first was the one given by OeSper and Pauerstein (45) for
sodium analysis in blood.

A two ml sample and 2.5 ml acetone were mixed in a
40 ml glass-stoppered centrifuge tube, centrifuged for 2
minutes at 2000 rpm to separate protein. a known volume of
supernatant (3 ml) was placed in a pyrex test tuba (150 x 15
mm) and 4 ml KOPs reagent, were added. The tube was placed
at -200C bath (dry ice in ethyl alcohol, water 40:60 V/V) and

stirred until precipitation began. After 20 minutes of

25

reaction at this temperature, the precipitate was separated
by centrifuging at 2000 rpm for 2 minutes. The sodium salt
was washed three times with cold acetone (~200C), and then
dissolved in small portions of cold water and titrated with
.05 N NaOH, using phenolphtalein as indicator.

The concentration of sodium in the sample was calcu-
lated from the following equation:

ppm Na present in the sample =

 

= ml of NaOH x N x .023 x 4.5 x 106
3 s

where
N is the normality of NaOH used in the titration.
.023 is the milliequivalent weight of sodium.
4:5 is the ratio total volume of original solu-
3 tion to the volume taken for the analysis.
5 milliters of sample taken.

The first experiment was performed in order to obtain
the minimum of concentration at which reliable results were
obtained. Samples of sodium chloride at concentrations be-
tween .1N and .001 N were analyzed.

The percentage of recovery was calculated from the
formula:

ml NaOH x N x .023 x 4.5 x 100 = % recovery.
Wt of sodium in sample 3

26

Application of the method to the analysis of foods.

In liquid foods such as broths, juices and canned
vegetables, the serum was separated by centrifugation or by
filtration, and 2 ml of the filtrate were collected for
analysis. When the foods to be analyzed were semisolid, or
not enough liquid phase was obtainable, the sample was
treated as described at the beginning of this section. The
procedure for analysis followed was the one given by OeSper

and Pauerstein (45).

27
RESULTS 3535 D DIS CUSS ION

A. Magnesium-uranyl-acetate. The following table
presents the results obtained when the method was applied to
different foods in order to obtain the sodium content. The

eXperiments were made in duplicate.

Sample Na Content Range
in 100 ppm in 100 ppm
1. Pea Soup 66.0 i .31 65.8-66.2
2. Beef broth 65.0 i .1 65.0-65.1
3. Chicken broth 55.5 3 .4 55.2-55.8
4. Lima beans 37.8 I .2 37.6-37.9
5. Sweet peas 36.0 I .1 35.9-36.0
6. Wax beans 33.0 1 .1 32.9-33.1
7. Tomato juice 28.0 I .5 27.7-28.4
8. Buttermilk 12.4 3 .1 12.3-12.5
9. Whole milk 8.651 .1 8.6- 8.7

With the exception of tomato juice, the method pre-
sented very good duplicates (less than 1.2% difference).
The results obtained for milk and buttermilk are higher than
the results obtained by using other methods. This may be
due to the high phOSphate content in milk.

The data found are taken as standard, and will be used
as comparisons with the results obtained by using other

methods.

 

1Standard Deviation.

28

FLE-‘L’ZE PIiOTCE-IET RY

 

Flame photometry proved to be the simplest method of
the four tried. The duplication was exceptionally good, as
can be seen from the next table. Here the results obtained
when samples of foods were analyzed for sodium, according

to the n,O.A.C. procedure for plant materials, are shown:

Sample Sodium Content Range Repetitions
in 100 ppm in 100 ppm

1. sea Soup (canned) 74.4 i .11 74.3-74.5 3
Campbell

2. Beef broth (canned) 69.9 i .3 69.5-70.0 3
Campbell

3. Chicken broth “ 55.2 3 .1 55.1-55.3 3
Campbell

+

4. Lima Beans (canned) 38.0 - 0 38.0-38.0 3
Campbell

5. Sweet peas ( " ) 38.2 t .4 37.7-38.2 3
Campbell

6. Wax beans ( " ) 30.5 i .l 30.4-30.6 2

7. Tomato juice " 31.2 i .1 31.1-31.3 3

8. buttermilk serum 9.60: .4 9.3-10.0 3
1-1. S.U.

9. Whole milk serum 6.00i .2 5.8- 6.0 3
14.5.U.

+
lO.Carrot juice 2.40- .03 2.35-2.40 3

The maximum percent error in two successive analysis

was 1.5% except in the case of milks.

 

l- - . .
standard Dev1ation.

29
leha-fethoxy—Yhenyl-Acetic icid Tetbod
sodium was recovered from sodium chloride solutions
at different concentrations by using OeSper's procedure (45)
with Alpha-Methoxy—Phenyl Acetic acid.

The percent recovery was calculated from the formula:

 

ml N303 x ho_4el;§y x .Q_§ x 100 = Eercentage of
weight (g.) sodium in ample Recovery
Concentration of NaC1 Av. Sodium found Percentage of
added,in 100 ppm in 100 ppm Recovery
22.90 22 95
16.03 15 95
11.45 10 90
9.16 8 80
4.58 4 90
2.29 2 70
0.23 No precipitation

r1he data showed that precipitation could only be ob-
tained in the foods tried here, when concentrations higher
than 0.05 h in sodium were analyzed. The salt did not pre-
cipitate when sodium was present in concentrations lower than
0.01 N or 200 ppm: although the sodium salt precipitated when
sodium was present in concentrations between .01 and .05h,
the recoveries varied from 70 to 90.5%.

It was noticed that the time of stirring is critical.

Only those solutions with two thousand ppm of sodium, or above,

precipitated within 2 minutes. When the concentration was

30
lower, the precipitation was either too slow or did not occur
if the sample was not shaken for a longer time.

Variations of the method. Using the same quantity
ratio between the sample, acetone and reagent as the original
method, the first modified method was carried out in order
to decrease the error by using larger amounts of sample. No
success was achieved with this eXperiment. The prOportion
of substances in this case was 10 ml sample, 12.5 ml acetone,
15 ml supernatant and 20 ml reagent.

A final modification was tested. In this, the amount
of acetone and SUpernatant was increased. NaCl solutions at
different concentrations wereauelyzed for sodium. To 10 ml
sample, 20 m1 acetone were added for the precipitation of
proteins: 25 m1 supernatant taken, and 20 m1 reagent added.
No precipitation was seen When using a .01 N NaC1 solution.

Since no advantages were obtained with the variations
tried, the rest of the eXperiments were carried on following
the procedure used by OeSper (45).

The results of the analysis of different foods were:

 

Sample Sodium concentra- Range Repetitions
tion in 100 ppm in 100 ppm
1. Pea Soup 60 E 41 56-65 6
2. Beef Broth 80 1 6 74787 6
3. Chicken broth 65 ; 6 60-73 6
4. Lima beans 30 ; 6 25—40 6
5. Sweet peas 32 ; 7 25-39 6
6. Wax beans 30 ; 6 25-36 6
7. Tomato juice 26 - 2 24-28 6
8. Buttermilk .50 O-.75 3
9. Whole milk .50 0-.75 3
1

Standard Deviation.

31

The data found for milk should be disregarded since
they are not comparable with the actual sodium content in
milk. At first, it was believed that the high calcium con~
tent of milk could be the explanation of this phenomenon, for
this element reacts with MOPA more rapidly than sodium, in-
hibiting the formation of the sodium salt. Higher concentra-
tions of reagent were used, and although the amount of pre-
cipitate increased, it was not in the proportion expected.
Therefore, besides calcium there must be another factor in
milk which affects the formation of the precipitate.

Vodivm Ion Electrode

 

Es a preparation for the actual testing of foods, a
study about the influence of different factors which could
be involved in the natural or treated food was deve10ped.

a. Equilibrium time. The time required for the elec—
trode to reach the equilibrium point was checked by using
different kinds of milk at different concentrations. The
sample was treated according to the procedure described, and
the reading recorded after different intervals of time, with
the current passing through the solution. Milk was chosen
since this food is available with different amounts of fat
and proteins, and is rich in mineral content.

The longest time for reaching equilibrium point was
required for the sample with suSpended caseinate present.

"half and half"1 also required considerable time,

 

l"i—Ialf and half" is a cream—milk mixture Which contains
at least 10% fat.

 

 

”—1

 

32
although not as long as the former. These two factors, curd
and fat, may surround the electrode, making it more difficult
to reach the equilibrium.
The dilution factor does not seem to have any effect

on the equilibrium time. (See Appendix Tables I to VII)

Sample Concentration of Equilibrium time
Sample (percent) (minutes)
Buttermilk 30 0
Whole milk 30 2
Whole milk serum 15 2
Skim milk 30 3
Buttermilk serum 25 3
"Half & Half" 30 6

Whole milk with precipitated
caseinate 25 10

Three minutes was considered the average time at which
the electrode reaches the equilibrium point. This was the
time given to the samples tested in these experiments.

b. Effect of the_protein concentration. The influ-
ence of different amounts of protein in the reSponse of the
electrode to the sodium concentrations of milk, milk serum,
buttermilk serum, half and half serum, and sodium chloride
solutions was recorded.

It was noticed that when the caseinate was added to
milk or any kind of milk serum, the response of the electrode

is affected. When caseinate solution was added to a .1 N NaCl

33
solution, only a slight difference in reSponse was noticed
which could well be due to procedure errors (see Appendix
Table VIII).

A study of the results led to the conclusion that
the effect of the protein in the reSponse of the electrode
is related to the concentration of milk in the dilution.

The effect may be appreciated in the following table.
Calcium-caseinate is added to be present in a concentration

of 4g. in 100 ml of solution.

 

% milk ppm Na Concentration ppm Na Concen- Percent differ-
when no caseinate tration when ence between
added caseinate the two read-

present ings

1.48 432.5 525 +17.6

4.16 379 379 0

10 - . 432 383 -ll.3

c. Fffect of carbohydrates. Since the possibility
existed that carbohydrates affect the electrode reSponse,
experiments were devised to study this problem. A solution
of potato starch (10E) was prepared, and different volumes of
it were added to 5 m1 of .1N NaC1. The mixtures were
analyzed for sodium content by following the procedure ad-
vised by Bower (9), giving to the electrode 3 minutes of
equilibrium time. The readings were recorded and parts per
million calculated from the standard curve.

The results did not show any change in the electrode

34
reSponse with change in carbohydrate content. (See Table Ix).

d. Effect of fat. To measure the effect of fat, if
any, on the electrode reSponse, 50 m1 of whole milk were
divided in two equal portions (25 ml each). One of these
portions was defatted by extraction with ethyl ether (25 ml)
three times.

The sodium content of the two different portions was
then analyzed separately, and found to be the same, from which
it was concluded that fat in the concentration found in milk
does not affect the sodium electrode reSponse.

e. Effect of mineral saltppq A salt mixture N.B.Co.
(-W- which contains KH P0 = 31%, CaCO = 21%, Ca3(PO

2 4 3
14.995: KCl = 125-3: blaCl = 10.5%., big 30

4’2 =
4 = 9%) solution, at a
concentration of 5% by Weight, was used for this purpose.
To 2.5 ml .1 N NaCl, different quantities (from 1 to 12.5 ml)
of the above salt mixture solution were added to determine
whether or not ions besides potassium and hydrogen ions affected
the electrode. Since the actual value of sodium ion concentra-
tion in the salt mixture was not known, the effect was only
relatively observed. The values were compared as the res-
ponse of the electrode to different concentrations of mineral
salt solution added. (See Table X .

From the experiments it was noticed that, by increasing
the ion concentration, the electrode response increased. This

could be eXplained, 1) as an increase in sodium reSponse

with the increase of actual sodium ion concentration or,

 

35
2) as an enhancing effect of one molecule or ion present or,
3) by the existence of a molecule or ion which would bind
sodium ion.

The first possibility was disregarded when the percent-
age of reSponse of the electrode was checked. 10 m1 aliquots
of sodium chloride at different concentrations were used for
this purpose. The procedure for the analysis of sodium was
the same followed in the preceding experiments. From the data
obtained (See Table XII), it was concluded that this method
gives a reSponse of around 85% of the sodium concentration.
Since the analytical data show a regular negative deviation, a
correction factor was prOposed (1.17) which gave the desired
results i 1.7.

f. Egggentages of recoveryiwhen mineral mixture present.

An experiment was performed Whereni the mineral
mixture was kept constant, and different amounts of .l N NaC1
solution were added to it (from 1 ml to 50 ml). The variation
in the recovery may be seen from the following table:

Percent recovery when different amounts of .1 N EaCl

solution were added to 5 ml of 5% salt mixture (N.B. & Co.)

 

 

m1 .123 Natl Reading (av. 33 recovery-t
1 76 not recovered
2 71 52.5
5 61 77.0
10 51 85.5
50 18 96.5

 

*Correction factor was applied.

36
The increase in percentage of recovery without exceed-
ing 100, with increase in sodium chloride added, would be
explained by the existence of a compound or ion in the salt
mixture, which in some ways affects the reSponse for sodium,
giving higher results when the prOportion of salt mixture to
sodium ions present in the sample becomes lower.

. Effect of proce-sing_on the recovery of sodium.

£0

The effect that the processing of foods (canning, freezing,
etc.) exerts on the sodium reSponse of the electrode is
important from the point of view of studying the difference
in the effect that the ions or compounds present in foods
have on the electrode reSponse, or on the sodium present,
before and after processing.

To elucidate this problem, the most commonly used
treatments, canning and freezing, were studied.

Beef broth was prepared by boiling 3 pounds of beef
in 6 liters of water for 2% hrs in a lO-liter steam Open
pot. The broth was cooled and defatted and freed from
particles by filtration. Sodium chloride, monosodium gluta-
mate, and sugar were added to part of the filtrate. The
samples were divided according to the additions made, and
either canned or frozen. For the canning process, the samples
were heated for 3 minutes (to near boiling point), then canned,

and retorted for 45 minutes at 115°C. The canned samples

37
were then cooled and kept for one week at room temperature.
The frozen samples were kept in the freezer for the

same period of time (one week).

Effect of beef broth processing on sodium recovery
by the electrode method

 

Process % Recovered
None (reference) 100
Freezing 94
Canning 95.5

Canning (with .05%.M.S.G.
and .4% sugar) 110*

Both canning and freezing decreased the recovery of
sodium when only NaCl was added. When M.S.G. and sugar
were added in addition to sodium chloride, the recovery* was
noticeably increased. (See Table XII)

h. Sodium Analysis in Foods. Samples representative
of four groups of foods, of animal and vegetable origin high
and low in proteins, were analyzed for sodium by using the
sodium ion electrode according to Bower (9) (described at the
beginning of this section). The electrode was allowed to
reach equilibrium (3 minutes). Before taking any reading a

.1N NaCl solution was calibrated to read zero in the mv.

 

*Over and above the sodium content due to M.S.G.

38
eXpanded scale. Between two different readings the electrode
was wiped very carefully, without rinsing. The results were
obtained by multiplying the readings from the curve by 1.17,

as suggested in the last eXperiments, and are given below:

 

Sample Sodium Range Repetitions
in 100 ppm in 100 ppm

1. Pea Soup 62.0 i 2.21 58.9-62.4 6
2. Beef broth 77.2 i 3.0 74.0-82.0 6
3. Chicken broth 60.3 i 1.2 59.1-61.6 6
4. Lima beans 34.4 1 1.4 33.8-35.2 6
5. Sweet peas 33.0 i 3.5 32.2-34.0 6
6. hax beans 34.8 I .8 34.0-35.0 3
7. Tomato juice 26.8 i .5 25.4-27.1 6
8. buttermilk 7.451t .16 7.40-7.60 6
9. hho1e milk 4.86 i .17 4.80-5.00 6
10. Carrot juice 2.80 i .1 2.76-2.90 3

1

Standard Deviation.

39
The following table gives the results obtained when
the four methods were applied to determine sodium in foods,

and their percent difference from the standard n.0.n.C. pro-

 

 

 

cedure.
Sample nagnesium Flame *MMOPA .-.
Uranyl- fid’ Na
«acetate Lia in :3 1,.“ in ,0
in 100 Loin 100 pom Diff. 100 mm Di’ff. 100 ppm Diff.
Eea soup 66.0 74.4 +11 62 -6 60 +19
Beef broth 65.0 69.9 + 7 78 +16 80 +19
Chicken
broth 55.5 55.2 -.1 60 + 8 65 +15
Lima beans 37.8 38.0 + 1 34 - 9 30 -20
Sweet peas 36.0 38.2 + 6 33 — 8 30 -ll
Wax beans 33.0 30.5 - 8 35 - 5 30 - 5
Tomato juice 28 31.2 +10 26 - 6 25 - 6

Whole milk 8.6 6.0 -31 4.8 -44 -— -93

SHEILARY 2‘.le CONCLUSIONS

Four methods were applied to determine sodium in
foods: Magnesium uranyl-acetate, used as standard method;
flame photometry used as known comparison: alpha-methoxy-
phenyl-acetic acid; and sodium ion electrode. They were
studied to evaluate their application as standard methods of
analysis.

The simplicity and reproducibility obtained by flame
photometry make this method the most desirable of the four
studied. The relatively high cost of the instrument makes
it unavailable to some laboratories.

The magnesium-uranyl-acetate method gave good reproduc-
ibility, but the cost of the reagents and the time consumed
in the procedure are high.

The alpha-methoxy-phenyl-acetic acid gives neither pre-
cise nor accurate results. It was very difficult to obtain
good reproducibility, eSpecially when sodium was present in
concentrations lower than 1150 ppm. The recovery of added
sodium obtained when this method was applied, decreased with
decreasing sodium concentration, although not in linear prOp-
ortion. The fact that milk and buttermilk presented diffi-
culties in the precipitation of the salt might be due, not
only to the low sodium content, but also to the high calcium
content of the samples.

The use of the sodium electrode is a fast, convenient,

and ineXpensive method. The average recovery of sodium

40

41
added as sodium chloride was quite acceptable (100 t 1.7).
The maximum equilibrium time, except when suspended material
was present in the sample, showed to be 3 minutes. The longer
time required to reach the equilibrium may be eXplained as
polarization of the electrode caused by suSpended material.
In the prOportions usually found in foods, fat, carbohydrates,
and proteins do not affect the response of the electrode.
0f the most common ions present in foods, phOSphate seems
to be the one reSponsible for the polarization effect. Ca1-
cium did not show any effect in the reading: sulfate ion was
reported not to affect it: and potassium was precipitated
before reading. Canning and freezing Operations affect the
sodium response, although not to a very great extent (-6%).

In general, it may be said that no success was ob-
tained in the application of the MOPn method to the sodium
analysis in foods. The sodium electrode presents a good
possibility as standard method of analysis in foods because
of its simplicity and economy. A new procedure for meats is,
however, necessary, since in this case the biggest deviation

from the standard method was found.

“FEELDIK
Tables I to VII
Variation in the reading with increasing the time allowed

for sodium ion electrode to reach equilibrium.

Table I

Sample: 30 g. skim milk.
Dilution: 100 ml.

Time (minutes) Reading (mv.) Av. Sodium ReSponse
in 100 ppm

0 65 4.50
2 67 4.10
3 68 3.95
4 68 3.95
6 68 3.95
8 69 3.95
10 68 3.95

Sample: 30 g. Half 5: Half
Dilution: 100 ml.

Time (minutes) Reading (mv.) av.Sodium Response
in 100 ppm

0 65 4.50
2 67 4.10
4 67.5 4.00
6 68 ' 3.95
8 68 3.95

10 68 3.95

 

43

Table III

 

Sample: 30 g. Whole milk.
Dilution: 100 ml.

Time (minutes) fieading (mv.) Sodium ReSponse
in 100 ppm

0 69 3.80

2 70 3.65

4 70 3.65

6 70 3.65
Table IV

 

Sample: 30 g. Buttermilk
Dilution: 100 ml.

Time (minutes) Reading (mv.) Sodium ReSponse
in 100 ppm
0 74 7.65
2 74 7.65
4 74 7.65
6 74 7.65
Table V

Sample: 25 ml whole milk in acid media so that protein
is precipitated. do filtration effected.
Dilution: 100 ml.

Time (minutes) Reading (mv.) AV. Sodium Response
in 100 ppm
0 64.5 5.50
2 65.5 5.25
4 66.5 5.05
6 67.5 4.85
8 67.5 4.85

10 68.0 4.80

44
Table VI
Sample: 25 ml Supernatant from centrifuged buttermilk

(Buttermilk serum).
Dilution: 100 ml.

 

Time (minutes) Reading (mv.) Av. Sodium Response
in 100 ppm
0 52 9.50
2 53 9.10 Fl
3 54 8.50 1
4 54 8.50
6 54 8. 50 E '
Table VII

 

 

Sample: 30 9. milk (precipitating and filtrating the
casein off).
Dilution: 200 ml.

Time (minutes) Reading (mv.) Sodium ReSponse

in 100 ppm
0 89.0 3.32
2 87.5 3.30
4 87.5 3.30
6 87.5 3.30
8 87.5 3.30

Table VIII

 

Effect of the_protein on the resoonse of the electrode:
Calcium caseinate, added in different amounts, was used as
protein source. Bower's technique for Sodium Electrode

method was followed.

43

Sample: 5 ml 0.1M sodium chloride solution.
Protein source: 5% calcium caseinate.
Dilution: 100 ml.

% Calcium caseinate Av. Sodium ReSponse

in 100 ppm
0 19.6
0.1 19.6
0.25 19.2
0.50 19.2
1.25 19.2

Table IX
Effect of Carbohydrate Content on the Electrode Resppnse
Sample: 5 ml 0.1 N Sodium chloride.

Source of Carbohydrate: 10% potato starch.
Dilution: 100 ml.

71.5

Percent Starch Added Reading Av.Sodium ReSponse
(mv.) in 100 ppm
0 71.5 21.2
0.2 70.5 21.6
0.5 71.5 21.2
1.0 71.5 2l.2

21.2

46
Table X
Influence of mineral mixtures on the electrode response when
following Bower's procedure (9).
Sample: 2.5 ml 0.1N sodium chloride solution.
Sources of minerals: 10 ml 5% salt mixture solution

(From N & B Laboratories).
Dilution: 100 ml.

Varying the amount of salt mixture.

% Salt Mixture Reading Av.Sodium ReSponse Relative %
(ml) (mv.) in 100 ppm Sodium
Recovered

0 88 .50
1.0 83 .63 100
2.5 74.5 .90 118
5.0 66.0 1.34 125

12.5 51.0 2.52 120

Table XI
The same conditions as in Table K were used, but in this case
the amount of salt mixture was kept constant, and different
amounts of sodium chloride solution were added.

Sample: 5 m1 % salt mixture solution.

0.1 N Sodium Concentration Na Reading % Na
Chloride (m1) added (mv.) Recovered
in 100 ppm
0 . 76
1.0 .23 76 Rot recovered
2.0 .46 71 52
5.0 1.15 61 77
10.0 2.30 51 86

(.0
O\

50.0 11.5 18

 

47
598131 e XI I

Influence of the sample dilution on the Sodium Electrode

Sample: Sodium chloride at different concentrations
(10 m1).
Dilution: 100 m1.

 

haCl Concentration Reading av.Sodium ReSponse % Recovery*
(ppm) (mv.) in 100 ppm
114.5 128 1.10 98.4
229.0 111 2.30 99.5
572.0 88 5.80 100.9
1145.0 73 11.50 100.7

*Correction factor applied.

43

Table 3111

 

Effect of processing operations over the reSponse for sodium,

when following procedure described by Bower (9) for
electrode method.

Sample: Beef Broth
Treatment given Reading Total Na Na Recovered

to the sample (mv.) Response from AaCl
in 100 ppm in 100 ppm

Ereezing 99.5 3.50 --
Canning 100.5 3.45 -—
Addition of 2342

ppm. for blank 53.0 26.80 23.3
Addition of 2342

ppm of sodium,

then canning 52.0 25.80 22.4
Addition of 2342

ppm sodium, then V
freezing 54.0 25.40 21.8
Addition of .058

5"..5.\J. and 0.4/U

sugar, then canning 92.5 4.85 --
Addition of 0.055

$1.3.G.’ 0.45 sugar

and 2342 ppm Ha,

canning 50.0 30.60 25.7

*Correction factor applied.

the

% Na
Recovered*

 

110.0

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