THE FLUORIDES OF LAETHA'NUM. CERIUM AND THORIUM WITH SPECIAL REFERENCE TO THE DETERMINATION OF FLUORINE - THESIS FUR THE IIIGEIIIIQUI III. ' ' Cecil Chester Lang‘hauI ~ 1934 0/ 1-: Nil)! ‘ f. ht --. ....... u <~I if )3 I“ y . : .6. :311’4’9 12:? at; ' _?,< _»-:. J.: g. 4?! J v t. I} I ‘ % '1 2‘ \- 1' b: . ~M $ ‘ ! Fifi? z " K - ‘v A"! a} 7*. " 3 i. 9'"; I v .3‘. ‘ :.'Q';'/ L, \ v ' , ' ‘ . H? .E.“ , “' _ ‘uy‘. ‘ . . ‘v ' ,' u a a. 2“ . ~ I: I ~ .» i '4‘“ fi 1‘ - U. 4 2 . .__.;:; I .'..,:,_F:"- ’1 .. ' Ii *. o 34* 1Q“! 11%: ‘6 ’k ’ . ‘-'..'\’ .. 15‘ ‘67 A ' :A i V I! é. . :3 IQ ‘51." C~ 1% 1; 97 . r J y 7.. l , -.,.’;, . 1&3". .. ‘Jojyk. ' is 0" egg“ 4; 3. fr. - :1LIn“"' o ‘ I -.'o'n~ h’t‘: 3‘ .' 1 V ‘ ,. , _ f ‘ v 9’ "1‘ 9““! ' ‘ . Afr" ' ‘2 f ‘. ' - I; ...( L A f » I. ’~. .3 ‘I $. . . .Iéiti" ._ I: II It“. 9!. I" F I 4,... I. :- ~. m. 1?. EaghI‘i-i I" I, .vI‘.3-%II.I".. ‘IE‘I..I...S‘ I';«‘?»II.“I5.:”-:>II I»; “I A”: ._.--,-. \l‘, V :5." 5 I". \“ ",w . ' ‘ 1:,”"k'. . .‘cil ‘ _ 2.. .. I‘sar vL-II'TI-vII-m-H I... kw“ .. ~I1I mm , . ' . “ I d“‘ 1"} "rl . :5— ' o w: I l P‘l (s :B-ui‘g' ' "I _ 191% . ‘. fi‘ “i;:‘.\-1" ' X - 1 , I'n, 34 $4 :L'II w- - U 1 . {h : A..- . .- I - ,~ . '. IN .I ‘l' 11”.} r". 2‘52: 'h-IR‘ -n \‘Z S THE FLUORIDES OF LANTHANUM, CERIIM AND THORIUM WITH SPECIAL REFERENCE TO m DETERIIHNATION OF FLUORINE A THESIS Submitted to the Faculty of'Michigan State 001103. of Agriculturo and Applied Science 1n.Part1a1 rulrillnnnt of tho Roquiramonto for tho Degree of’Mhator or Soionco by Cecil Choltcr Langhan May 26, 1934. T5%bu L742: "1 q ' ACKNOWLEDGEMENT - Grateful appreciation is expressed to Professor E. Leininger under those kind, efficient direction this work was carried out. Cecil C. Langham. 1(’)23_8 ‘5- INTRODUCTIOH Only within the last few years has fluorine assumed a place of sufficient importance in the scientific and economic world to warrant the general attention of investigators. Recently, hovever, a great deal of uork has been done in connection with the detection and estimation of small amounts of fluorine in such materials as sprays, plant ash, sea.Iater, etc. Yet, in spite of the numerous methods and modifications of methods proposed, there as yet remains to be found an accurate, rapid, and easily carried out method for the determination of fluorine. This work comprises a eeriee of observations and experiments conducted on the fluorides of lanthanum, cerium, and thorium.in their relation to the fluorine determination. HISTORICALIINTRODUCTION Pieani (1) found that the addition of a solution of Th1R03)4 to the solution of an alkali fluoride slightly acidified with.LoOH or HNO:5 -2- produccs a heavy gelatinous precipitate of Thr‘4fizd which may be dried at 100° and seighed, or ignited and weighed as Thoz. .Adolph (2) reported unsatis- factory results from.the method and advanced the idea that simple Th2?4 might not be precipitated from a solution of sodium.fluoride without being accom- panied at the same time by'Nathrs. To prove this he treated a neutral solution of sodium.fluoride with an excess of thorium.nitrate, dissolved the washed precipitate with acid, precipitated the thorium.Iith ammonia, and fromIthe filtrate obtained a small amount of sodium.chloride. Gooch and Kobayashi (5) modified Pisani'e method to include both a gravimetric and a volumetric method in thich the gravimetric process serves as a preliminary step to the volumetric procedure. They precipitated the fluoride with a measured excess of thiNOs)‘ and ignited the IhF‘ ° 4320 to Thoa. They then determined the excess thorium present in the filtrate by converting it to the oxalate and sub- sequent oxidation by KMhO4. meyer and Schulz (4) carried out many experiments utilizing calcium, lanthanum, and thorium -3- salts as precipitants. They found that even.in the case of the Care method shieh is the oldest and best known method for the precipitation of fluorine, two opposing errors are involved. First, an unavoidable loss is suffered because of the considerable solu- bility of the fluoride, and second, a proportionate adsorption of calcium.aeetate on the fluoride molecule occurs. When they attempted the use of thorium salts as precipitating reagents, they found that an excess of the reagent caused the precipitate to apparently dissolve Iith the formation of a complex. Hosever, they considered it questionable as to whether a come plex formation actually took place or e peptization secured, but in either event, complete precipitation is not achieved. This led to the study of lanthanum. salts as precipitants. The character of the precipitate is the same as that of thorium, but shereas, the fluoride dissolves in three times the equivalent amount of lanthanum nitrate or chloride, they found that it would not dissolve in an excess of lanthanum acetate. Upon the addition of the acetate ion, acetic acid or ammonium.acetate, to the solution, the fluoride again separates as a gel.} Other electrolytes cause only -4- incomplete flocculation. They found that Lars adsorbs lanthanum.acetate from.solution, but not the nitrate or chloride, therefore, their method involved the precipitation of the fluoride and the use of a cor- rection factor for the amount of adsorbed lanthanum acetate. The above described work of Meyer and Schulz in.which they used lanthanum.acetate as a precipitant for fluoride ions suggested to Batchelder and Meloche (5) the possibility of the use of cerium. Their attempts to make a gravimetric separation of serous fluoride were unsuccessful, however, because of the gelatinous character of the precipitate. The results obtained indicated a great amount of adsorption on the fluoride molecule. This difficulty led them to investigate a volumetric modification in the hopes that the error included in the gravimetric method due to adsorption might be eliminated. Accordingly, an attempt was made to precipitate the fluoride by adding an excess of serous nitrate solution and titrate the excess cerium by means of potassium permanganate according to the method of Lenher and - 5 - c. Heloche (6). This method depends upon the equations dCe(N03)3 + zmmo,+ 4320 3 4C.(N03)4+ zCe(0H)4+ em03+ smog ' and Ce(N03)4 + 4320 = Ce(OH)‘+ canes Batchelder and Meloche were able to obtain results by this method with amounts of fluorine ranging from .1 to .001 g. with an accuracy of about .0005 3. They found the procedure rather cumbersome and difficult however, for one not accustomed to the method since it was necessary to filter off the pres ipitated MnOz, Cars and some), at a point Just before the equivalent point was reached. Pi ltration at this point was necessary to prevent an over-titration due to the adsorbed mo; ion. Ions such as phosphate and oxalate which precipitate cerium interfere with the determination. In an attempt to find a more suitable scheme of analysis they investigated the direct titration of fluorine with serous nitrate using methyl red as the indicator after the method of Kurtenacker and Jurenka ('I). Batchelder and Meloche obtained satisfactory results by this method with quantities of fluorine C -6- amounting to less than a miligram. This was not con-f sistent with the findings of Kurtenacker and Jurenka, however, who reported a deviation of results amount- ing to as much as 4% in.some cases. Batchelder and 'meloche explained the anomalous results obtained by Kurtenacker and Jurenka in the following manner: The color change of methyl red at the end point was attributed by them to be due to a change in.pH caused by hydrolysis of the corona nitrate. Batchelder and ‘Meloche attempted to check this theory by running a blank determination to find out how much of the standard serous nitrate solution was required to change the color of the indicator. They found that 2.6 cc. of the standard solution was required to change the color of an aqcous solution of methyl red, whereas, if fluorine were present, the color change occuredwhen an equivalent amount of cerous nitrate was added. They also found that when amounts of cerous nitrate which were insufficient to change the color of the indicator were titrated with sodiua.fluoride solution, the color changed gradually from yellow to red and at the equivalent point changed back to yellow. new, if an excess of either the cerous nitrate or sodium fluoride were added the color change -7- was reversible. Hence, they concluded that in the ’presence of cerous cerium.ions the precipitated cerous fluoride preferentially adsorbs indicator, while in the presence of fluoride ions, the corona fluoride precipitate preferentially adsorbs fluoride. They reported the interference of sulfate or, in general, any ions such as phosphates or oxalates which precipitate cerium. Willard and Winter (8) have developed a volumetric method for the determination.of fluorine based upon the decolorizing effect of the fluorine ion on zirconiumralizarine dyo. The fluoride is titrated with a standard thorium nitrate solution, using a mixture of zirconium.nitrate and sodium alizarine sulfonate as the indicator. In case inter- fering elements are present in the soluble fluoride solutions, the fluorine is separated by volatilizing it as hydrofluosilic acid and titration of the fluorine in the distillate. Armstrong (9) found that a dilute solution of sodium.alizarine sulfonate alone could be used as the indicator. Shuey (10) found the method to be accurate for the direct titration of pure fluoride solutions, but he was able to recover only -8- about 90 per cent of the added fluorine from.plant ash. This was attributed to loss of fluorine dur- ing ignition of the plant material. Allen and Furman (ll) titrated fluorine potentiometrically by the application of a ferri- ferrocyanide electrode. Good results are reported for amounts of fluorine ranging from 0.1 mg. to 50 mg. Large amounts of foreign salts mask the equivalent point, and ions which precipitate serous cerium.must be absent. EXPERMNTAL RESULTS In attempting to find a suitable method for the volumetric determination of fluorine in plating solutions the Willard, Winter method was first tried out using thorima nitrate solution as the titrating reagent with a zirconium nitrate and alizarine dye mixture as indicator. Before beginning this work, however, it was necessary to prepare some pure sodium fluoride. This was done in the following way: Baker's c.P. sodium carbonate was treated with an excess of Baker's C.P. hydrofluoric acid in a large platinum dish. The product was allowed to stand sev- eral hours and then heated to drive off the excess acid. After cooling, an excess of hydrofluoric acid was again added and thoroughly mixed. The resulting product was then heated gently at first and then in- tensely until the sodium fluoride was entirely fused. After cooling the fused sodim fluoride in a dessicator and pulverizing it in an agate mortar, it was dried in a platinum dish at 110° for three days. The product was then stored in platinum over calcium chloride. -10.. The Willard, Winter method is promising inasmuch as neither sulfates nor borates apparently interfere, but from the writer's experience the fact that the end- point of the titration is hard to determine because of the fading of the color, it seems evident that consid- erable experience would be necessary in order to con- sistently duplicate results. An attempt was then made to apply the Kurtenacker and Jurenke method which consists of the determination of fluorine by means of cerous nitrate solution using methyl red as the indicator. This method, however, was not applicable because of the fact that sulfates in appreciable quantities and borates interfered. In the absence of sulfates and borates though, very satis- factory results were obtained as shown in Table I. -11- Aswan sues so cessapmoov aaa.a oa.es o..oa aa.n an.s ..a£ II. on.oa ea.o o ne.oa no.» oo.s a oe.oa as.a on.» a oa.ea e¢.oa pa.a ,so.» a ma.oa on.oa «a.» ne.n a oa.on ov.oa as.a oo.s a ..ao. amnozvoo x.ao. n.eog..e ..aav n.nozvoo ..aa. execs..o emanosah emanosah enqu056h emaaosAh .ms_oo .ms on .es ca .ms 0 l assem— H mummy -lla- sweeps Hoaoo oz nflflflflflflflflflflflflflflflflflflTHHHHHHHHHflHHHHHHHHHHHHAflflHHHflHHHflflflflflflflflflflflflfldflflflflflflflflfi OH ON A ..aav n.»o2.oo x.wsv eoeee assesses A .3. unseen." non Adana no.9 named as eeneaeum an OH on no .m o." On a A . 7.: 91°58 ..wsl ease. .aasaeaa Twas anemone vom ill am «an. vommsz no 23395 3.. ..paoov H mamas The method works especially well with small amounts of fluorine present. In the case of the 50 mg. samples the color change was more gradual and the end-point hard to detect. With the smaller amounts of fluorine, however, the color change was very sharp and distinct. Very unsatisfactory results were obtained in the presence of sulfates or borates. The sulfate ion caused high results, while no color change occured in the presence of borates. It was attempted to substitute perchloric acid for sulfuric acid in the distillation of the fluorine, but the large amounts of acid carried over during the distillation rendered the end-point somewhat indistinct and harder to detect. Lanthanum.nitrate was also substituted for serous nitrate, using methyl red as the indicator. The results of this are shown in Table II. TABLE II - 13 - I }11189 or r = Trial Fluorine present (mg.) La(N03)3(cc.) lcc. La(N03)5 1 5 4.60 .920 2 5 5.15 1.026 5 5 5.15 1.026 4 10 9.85 .985 5 10 9.85 .985 6 10 9.80 .980 7 10 9.83 .985 8 10 9.85 .985 9 10 9.87 .987 10 20 20.00 1.000 11 20 19.92 .996 12 20 20.85 1.045 13 20 20.70 1.055 14 40 41.15 1.028 15 40 41.20 1.030 16 40 41.20 1.030 - 14 - While some of the results obtained from.this procedure seemed promising, the titrating itself was much less satisfactory than was the case with the 'cerous nitrate solution, due to the fact that the color change was so much more gradual and the end- point harder to establish. This probably accounts for the wide variation of results in many cases. It was next deemed advisable to try out a number of indicators selected more or less at random. Accordingly, a series of dyes and indicators was made up and tried both with lanthanum nitrate and thorium nitrate as titrating reagents. Those which were tried are listed below with their pH range e (see next page) -15- .6 .. o.» ago .333 doom «.0 as. 1.83.8 o3 .. a.» 3.8 Hogans scam «6 «6 33323.88 a6 .. or. 533233 0.0 o6 aosoem Baez-..“ o6 .. to 33 393m «6 a... cam 3.98 Tea. Tea sod.» .3933 , finesse. o4 .. to «33> assoc: £8838: .m esgomm mason scene 3.33:! a6 0.0 saga Honcho loam To .. to 88 Hashes o; is .on Hashes scam «.0 - a... 3053 ed in com «season 0.“ .. «A 8 afiooeoofl. .4. or. com assess n a... .. .J .398 H55... o3 n6 com sauce .33 mm noveeaesH ewsem mm nopeeueeH $0840 HPZH ho Baa Both brom phenol blue and brom cresol green gave someWhat promising results when used as the indicator in the titration of fluorine Iith thorium nitrate solution. The color change seemed to be sharp, but upon standing a fading occured and further addition of the thorium nitrate solution caused a similar color change to take place. The point at which the change occured seem- ed to be dependent upon the rate at which the titration was carried out. It was thought that it should be possible to finally reach a point in the titration where no further fading would occur and subsequent addition of the thorium.nitrate solution would only tend to cause a deepening of the color, but all attempts to consistently duplicate results were more or less unsatis- factory. The titration Ias carried out both in aqueous and alcoholic solution and in the presence of dextrine as a protective colloid, but even though some of the series run vere in fairly close agreement, in the majority of cases there was such a side variation in results that little reliance could be put in them. It has been shovn later in - 17 - this work that in the titration of fluorine with cerous nitrate solution that there is an abrupt change in pH at practically the same point in the titration as where the end-point was determdned with methyl red indicator, but whereas, the pH of the solution at that point was about 8.2, the pH range of methyl red is only from.4.8 - 5.4. Fur- thermore, some of the other indicators tried out such as lacmoid, p-nitro phenol, and cochineal which show color changes within the pH range of methyl red did not exhibit color changes in these titrations. Hence, it seems plausible that the change in color observed with brom.phenol blue and brom.cresol green is due largely, as suggested by Batchelder and Meloche, to preferential adsorption by the fluoride precipitate. It was next decided to attempt some potentiometric titrations of the fluoride with the nitrate solutions of serous cerium, thorium, and lanthanum. Batchelder and Meloche reported an unsuccessful attempt to carry out the titration potentiometrically with cerous nitrate. With a hydrogen electrode their results were corroborated, -18- however, with the use of a quinhydrone electrode it was found possible to carry out the titration. For this purpose a small, commercial set of the type commonly used in making pH measurements was employed. The results obtained are graphically shown in figures 1, 2, and 3. When the pH values were plotted against the volume of nitrate solution used during the titration the curves for both cerous nitrate and thorium nitrate exhibit a rather interesting phenomenon. The first additions of the nitrate solution caused an increase in the pH values up to a certain point, whereupon, further addition of the reagent caused an abrupt drop in the pH of the resulting solution. The phenomenon was more pronounced in the case of the cerous nitrate solution, and was not observed at all with the lanthanum nitrate. 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