-.- ._-- . V- .1. 3 f .' L. . !- . 3“: .. . ‘ “A .' - . ' i ., :“‘= ‘1 .3 W" " b‘ v A I ,‘I‘ W ' "‘ §}i WM ..'_ ‘ _ ‘ . '_ .‘ " ”a. " " "t . L. ' (I "t I ' ‘F I 2" ' PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE ’ DATE DUE 6/01 c:lClRC/DateDuo.p65—p.15 THE COEDUCTIVITY OF CHRONIC ACID SOLUTIONa. A Thesis Submittod to the Faculty of HICHIGAH STATE COLLEGE In Partial Fulfillment of the Requirements for the negro- of Easter of Science \ -.‘ 5" Department of Chewing-”A -f. f ,r' > t -: '. T 4"; Edgar CK: Hanson 19287 -n=_-4.‘.,d_‘-4 fl‘ COMES Page uncommon 1 03m! 4 mmnmnm. 5 cm consume mmmmumx a mu mm.) 1 9 mm 11 12 mm: III u an: I" 116 mu 7 13 man u. so mm: 711 22 DISCUSS 10m 29 conomsxons 51 100250 INTRODUCTION. A great interest and activity in the new field of. chromdnm plating has developed within the past_fel years. A1though.the electro deposition c1.chrcmium has been accomplished for many years, its successful commercial application has only recently been made. The unusual properties and advantages of chromium plating has resulted in numerous patents being granted.on this process. Bunsen (Poggendorrf's Ann.._g;, . 619) was proh- ahly the first to accomplish the electro deposition of chromium but Guenther (Liebig's Ann.,.g2, . 31‘) was the first to obtain it from solutions or chromic acid. Garveth and Curry (Jor. of Phy. Chem._g, . 353, “(1965) proved that chromium could be deposited readily tram solutions of chromic acid. provided that the bath contained some impurity such as a sulfate in an amount up to one per cent. In 1915 the successfulrapplication of electro- deposited chromium to printing plates at the U. 8. Bureau of Engraving was announced and the method used fully deecréhgd. (H. E. Haring, Chem. and.Met. Eng., 35;, 592-75%). In the electroplating of metals the influence of conductivity upon the power used is or real importance. Good conductivity is useful in reducing power costs. the usual means of raising the conductivity of an electrolyte are; by using a more concentrated solution; adding highly ioniaed salts: heating; and preventing the accumulation in the electrolyte of materials which reduce the conductivity. In electrOplating from chromic acid solutions a certain amount of chromium dichromate is famed by reduction. For some time it use believed that this compound of trivalent and hemavalent chromium had a beneficial effect on the efficiency of the bath. It has new been shown as the result of experiments, (U. 8. Bureau of Standards. Vol. 21. no. 346) that this can- peund serves no useful purpose in a plating bath and is in fact undesirable since its presence results in an increase of resistivity in the bath. The resistivity of chronic acid baths is very low in comparison with other baths, but because of the relatively great current densities required it is still of real importance, and it seems unavoidable that chromium plating baths attain an increased resistance after use. to what extent this increased resistance is due to the formation of chromium dichromate within the bath, and to what extent it is due to common additions to the bath, is not known. no work seems to have been done along this particular line. Related work on nickel depositing solutions has been done by Hammond, (Trans. 3. Al. Electchem. Soc. Vol. 45,0924) who studied effects on conductivity of different additions. and Kern and Chang (franc. Am. Electrochem. 800.. Vol. dl.(1922) who did similar work on copper refining electrolytes. 4. - 033309. the conductivity of solutions of chromic acid at various concentrations uas measured. and the effect upon the conductivity of common additions to and. canon formations in chromic acid plating baths was determined. IEIEBIHEWILL As the primary object of this study was to obtain information in the interest of commercial chromium plating, no attempt at precision measurements was made. All measurements were made at 25‘0, using the slide wire of a floods and Northrup student potentiometer as the conductivity bridge. A four dial resistance box with a range ofgl to 1000 ohms was used. The high frequency current was supplied by an electron tube arrangement as shown in the diagram. It was found by trial that this source of high frequency current was the most desirable. A high frequency generator, a aichrophone hummer, and a buzzer were all tried out, but the electron tube proved the most suitable. Its greatest advantages are/,- its noisjlessness and the easy control of its frequency. The direct current for operating the tube was supplied by two d-volt lead storage batteries. The tube was type 216A, lestern Electric. It was not necessary to use such a large tube, but as it was part of a setup at hand it was made use of. is seen in the diagram, there are no condensers in the circuit. The capacity which is necessary for oper- ation of the tube is in the form of distributive capaci- tance within or between the coils of the iron ring core. These coils were inclosed under an iron cover and the free space within filled with rosin which acted as the dielectric. 6. rho B—battery had an EAL}. of about '70 volts. The A-battery was at all times kept well charged, not vary- ing more than one half volt. It was found that a change in the current from the A-battery would cause a change in frequency which resulted in a change in the resistance of the cell being measured. According. diam? of Am. chem. Soc. 351,315, 1916) there is no measurable change in the resistance of a solution when platinized electrodes one inch in diameter are used at a frequency of 500-1000 cycles. However, the electrodes used in this work were about oneohalf inch in diameter which may account for the error introduced, or it may be that Washburn's statement was not meant to hold with solutions of high concentration. By experiment it was found that the change caused in the resistance of the cell, due to a change of one half volt in the 1-battery, would cause an error of about0.l per cent. It was necessary to attach a ground to each end of the bridge to eliminate harmonics. A very good end point could then be obtained. thing to the high concentrations of the solutions used in this work, ordinary types of conductivity cells could not be used. Several different kinds were tried out preliminary to beginning the work, in order to determine which would be the most suitable. .The first one tried was made from glass tubing 1/4 inch in diameter 7. and bent into a U shape. Platinum wires for electrodes were sealed into the tube through the sides with a dis- tance of about 20 centimeters between them. The leads from the electrodes were led up along the outsidcs of the tube and insulated by covering with rubber tubing. The electrodes were platinised before using. This type of’cell proved unsatisfactory. The area of the electrodes was eVidently too small. A change in frequency caused too great a change in the resistance of’the cell. An immersion type of cell was tried, but the resis- tance was altogether too small fer measurements of solutions of high concentration. The cell that was finally found to be satisfactory was a Leeds and Horthrup, Students U type. The U tube was a little more thano.5 inches'in diameter and 18 inches in length, tThe sides being graduated so that the electrodes might be set at any desired point. The electrodes were platinum discs that Just fitted inside the U tube, and were sealed to the ends of glass tubes, contact being made with mercury inside these tubes. They were fitted and adjustable through hard rubber caps which fitted over the ends of the U tube. Adjustment having once been made, the electrodes were tightly sealed into these caps to prevent any necessity of readjustment of them. All measurements were made at 25‘0 by keeping the cell immersed in a water bath constant toCtl'. The bath ‘was provided.with a small electric stifidng motor. CELL COESTAHT DETERMINATION The cell constant was determined with a 0.1 normal solution of potassium chloride using Kohlranch's value of 0.01288 reciprocal ohms-cm. as the specific conduc- tivity at 25'0. Box Bzmtance Bridge Rdg . Sale Big: . Cell 1000 840 1146.4 1140 510 1144.5 1150 4 90 1145 .4 Average .- 1145.4 1145.4 2 .01288 .- l4.76 .- cell constant. "1: skflh‘gb tag QKLSS a} KO {ataa «can . 0 wahct . k fleet. . Q assassin. . k anguish . 5 he 8‘59th .- h 4r #7 . a». 9. The conductivity of chromic acid solutions at different concentrations was measured. ‘A solution of chromic acid containing 600 grams of Gr 05 per liter of solution, was prepared. The concen- trations as listed in Table 1 below, were made by measur- ing out a certain volume of the chromic acid solution ‘with a burrette, and then adding water from a second burr’ette until the required dilution was obtained. Two checks on each measurement were made by varying the resistance in the box by a few tenths ohmfi, and thus obtaining other bridge readings. The average value of the calculated all resistance was used in determining the specific conductivity. TABLE I Effect of concentration on the Conductivity of 0hrcmic Acid Solutions Gone. Box Res. Bridge Res. (calc.) Gms’. ores/1.. om Reg. Ohms 500 22.6 457 22.30 " 22.4 ' 489 22.30 ' 22.1 523 22.30 Arg. 22.30 450 22.6 458 22.22 ' 22.4 480 22.22 ' 22.1 513 22.21 Avg. 22.22 400 22.6 481 22.42 Thble I (continued) Ocnc. one. 0r03/L. 400 100 Box Res. Ohms 22.4 22.1 23.0 22.8 22.6 24.0 23.8 23.6 25.8 25.6 25.4 28.8 28.6 28.4 34.4 34.1 34.0 46.8 46.2 Bridge Rdg e 503 535 .84. 505,, 525 482 502 525 480 520 484 503 520 490 512 520 502 535 Avg. A78 e A78 a Avg s ‘78 e Avg 0 10. Res. (0alc.) Ohms 22.42 22.41 22.42 22.85 22.84 22.83 22.84 23.84 23.82 23.83 23.83 25.59 25.60 25.60 25.60 28.61 28.63 28.63 28.63 34.26 34.26 34.28 34.26 46.84 46.86 Table I (continued) Com.. ena.crcg/L. 100 50 00110. cm. 0r03/L 400 350 300 250 150 100 Box Res. Summary 30‘s (0810 e) Ohm. 22.30 22.22 22.42 22.84 23.83' 25.60 28.63 34.27 46.85 85.13 11. Bridge Res..(0alc.) Rdg. Ohms 545 46.84 Avg. - 46.84 504 . 85.13 510 85.13 522 85.15 Arg. - 85.13 Sp. ACond. Rhos. ().666 i).663 0.657 0.645 0.618 “.575 0.504 (L431 0.316 0.173 12. The effect of 5a2304'upon the conductivity of chromic acid solutions of different concentrations. A solution containing 500 grams of 0r03 and 7.396 grams of anhydrous nazso‘ per liter was made. Dilutions as in Table I were then made. The amount of 80‘ then was always l%>of’the amount of 0r03 present. TABLE 11 Gone. B 1 Res. Bridge Res. (calc.) Gms. 0r03/L. Ohms nag. Ohms 500 23.0 475 22.77 " 22.8 497 22.77 ' 22.6 520 22.78 Avg. 22.77 450 23.0 465 22.68 ' 22.8 485 22.66 ' 22.6 510 22.65 Avg. 22.66 360 23.4 475 23.16 ' 23.2 497 23.17 " 23.0 520 23.18 Avg. 23.17 252 26.2. 460 25.78 ' 26.0 479 25.78 ' 25.6 517 25.78 Avg, 25.78 157.20 34.6 477 34.28 " 34.4 491 34.28 Table 11 (continued) 13. Gone . Box Res. Bridge Res. Gale . one. croz/L. chm. Bdg . Ohms 157.20 34.2 505 34.27 Avg. . 34.28 78 .60 58 .4 505 58 .52 " 58 .2 115 58 .55 " 58 .0 523 58 .54 Avg . .. 58 .54 Summary Gone. Res. (0a1c.) Sp. 00nd. or. arcs/L. _ Ohms . iihos 500.00 22 .77 0.648 450 .00 22 . 66 . 0.651 360 .00 23.17 0.637 262 .00 25 .78 0.573 157 .20 34 .28 0.436 78 .60 58 .53 0.252 14. Effect of’nazso4 on the conductivity of chromic acid solution. To 100 cc. of a stock solution of chromic acid containing 250 gr. Grog/L, was added 9.260 gr, of anhydrous Ha2804. By heating a few minutes just below the boiling point, enough water vaporized to bring the volume down to 100 cc. The solution then contained 250 ginning/L and so 5;. Ila/L. By dilution with more of the stock solution of chromic acid the concentrations as given in Table III were obtained. The same procedure was used for all the salts added in this work. Some of them required considerable heating before being brought into solution. This was especially true of 0r(OH)3; water then had to be added to bring it up to the required volume. TABLI III Gone. Bo: Res. Bridge Res. Cale. 0r. Ba/L. Ohms Rdg. Ohms 30 32.6 490 32.47 ' 32.4 506 32.48 7 32.2 522 32.48 Avg. - 32.48 21 30.4 470 30.04 ' 30.2 487 30.04 " 3O .0 504 30 .04 Avg. - 30.04 Table III (continued) 15. Gone. BOX‘ROB. Bridge Res. (Oalc.) Gms. Ha/L. Ohms Rdg. Ohms 12.60 28.4 473 28.09 ' 28.2 490 28.08 - 7 28.0 507 28.08 Avg. . 28.08 7.56 27.2 482 27.00 ' 27.0 500 27.00 * 26.8 ’ V 518 27.00 Avg. . 27.00 3.78 26.6 465 26.23 ' 26.4 485 26.24 ' 26.2 504 26.24 Avg. . 26.24 Summary Ocnc. Res. (0810.) Sp. 00nd. Gum. Ba/L. Ohms lhos. 50.00 '52 .48 0.454 21.00 30.04 0.481 12.60 28.08 0.526 7.56 27.00 0.543 3.78 26.24 0.552 16. TABLE IV The Effect of le3(80‘)3. 5 220 on the Conductivity of Chronic Acid Solutions. Gone. 801.288. Bridge Res. (0810.) can. re/L. Ohms Rdg. Ohms 35.00 39.2 490 39.04 " 39.0 503 39.05 " 38.8 515 39.04 Avg. 39.04 30.00 37.0 485 36.78 " 36.8 499 36.79 ' 36.6 512 36.78 Avg. 36.78 19.20 32.6 475 32.27 ' ' 52.4 490 52.27 ' 32.2 505 32.26 .Avg. 32.27 15.00 31.0 480 30.75 " 30.8 495 30.74 ' 30.6 510 30.72 Avg. 30.73 12.00 29.6 500 29.60 s ’ 29.4 517 29.60 7 29.2 534 29.60 Avg. 29.60 9.60 29.0 475 28.71 ' 28.8 494 28.73 ' 28.6 512 28.74 Avg. 28.73 Table 17 (cont inued) 17 . cons . Box Res. Bridge Res . ( calc .) Gms . Pe/L. Ohms Rdg . chins . 6.72 27 .8 495 27 .73 " 27 .6 512 27 .73 7 27.4. ' 550 27 .75 Avg . .- 27 .73 4.70 27.2 490 27 .09 " 27.0 598 27 .09 " 26 .8 525 27 .07 Avg . .- 27 .09 1 .17 26 .0 490 25.90 7 25 .8 510 25 .90 " 25 .6 528 25 .89 Avg . a 25 .90 SW Gmgen56/L. Res. ( Oalc .) 31:55:36.. ' Ohms 35.00 39 .04 0.377 30 .00 36.78 0.401 19 .20 32 .27 0.457 15 .OO 30 .74 0.480 12 .00 29 .60 0.4 98 9.60 28.73 0.51. 5.72 27.75 0.552 4 .70 27 .09 0.545 1 .17 25 .90 0.570 18. TABLE 7 ‘ The Effect of 0r3(80‘)3 on the Conductivity of Chronic Acid Solutions. Cone. Box Res. Bridge A78 e Gms. 0r./L. Ohms Rdg. Regfiméoalo ') 30 .00 5O .4 484 50 .08 " 5O .2 495 50 .10 " 50 .0 506 50 .10 Avg . 50 .10 24 .00 43 .4 486 43.16 " 43.2 498 43 .17 " 43.0 510 43 .17 Avg . 43.17 19.20 38 .6 493 38 .49 " 38 .4 506 38 .49 " 38 .2 519 38 .49 Avg . 38 .49 15 .00 35 .4 490 35 .25 " 35 .2 504 35 .25 “ 35 .0 518 35 .25 Avg . 35 .25 12 .00 33 .0 495 32 .93 " 32 .8 510 32 .93 " 32 .6 525 32 .93 Avg . 32 .93 9 .60 31 .2 500 31 .20 " 31 .0 517 31 .21 " 3O .8 53.4 31 .22 31.21 19. Table 7 (continued) » Oonc . Box Res. Bridge Res. (Cale .) one. 0: . [13. Ohms Bdg . Ohms 6.72 _ 29.6 490 29.48 ' 29.4 507 29.48 7 29.2 524 29.48 Avg . a 29 .48 2.35 27.2 475 26.91 " 27 .0 491 26 .91 " 26 .8 510 26.90 ’ Avg . ._ 26 .91 1 .17 26 .4 474 26 .12 - ' 26.2 .492 26.11 7 26 .O 510 25.10 Avg . .- 26 .11 Summary , Oonc . Bee. .(Calc .) Sp. 00nd . ms. 0r./L. Ohms llhos. ' 50.00 50.10 0.294 24 .00 43.17 0.342 19 .20 38 .49 0.384 15 .OO 35 .25 0.419 12 .00 32 .93 0.448 9 .60 51 .21 0.475 6 .72 29 .48 0.500 2.35 26.91 0.548 1 .17 26.11 4.565 20. TABLE VI The Effect of reSO‘ on the Conductivity oi’Chromio Acid Solutions Geno. Box Rea. Bridge Res. (0810.) 0mg. ro/L. chm. Rdg. Ohms 30.00 71.0 485 70.58 ' 70.6 500 70.59 ' 70.2 514 70.60 Avg. 70.59 24.00 55.8 486 ‘ 55.47 ' 55.4 503 55.47 7 55.0 520 55.45 2:3. 55.47 19.20 45.8 487 45.95 7 45.6 510 45.98 ' 45.4 530 45.95 Avg. 45.95 15.00 40.4 490 40.24 " 40.2 502 40.23 ' 40.0 515 40.24 Avg. 40.24 9.60 34.0 480 33.73 ' 33.8 495 33.73 7 33.6 510 33.73 Avg. 33.73 hble ‘71 (continued) 21. “20mg?! Box Ros. Bridge Rea. (0810.) . . [1. Ohms Rdg . Ohms 4.70 29 .2 489 29 .07 ' 29.0 505 29 .07 " 28.8 523 29 .07 Avg . .. 29.07 2.35 27 .4 482 27 .20 " 27.2 500 2'7 .20 " 27 .0 519 27 .21 Avg. - 27.20 1.17 25.5 475 25 .33 " 25.9 495 25.54 " 25 .2 514 26 .55 Avg . . 26 .34 Summary 00110 . 8 Res. (0810.) 3). 00:16.. 8748. 7.11.. Ohms 10108. 30 .00 70 .59 0.209 24.00 55 .47 0.255 19 .20 45 .95 0.321 15 .00 40 .24 0.555 9.60 33.73 0.458 4.70 29 .07 0.508 2.35 27 .20 0.542 1 .17 25.54 0.550 22. 711. Effect of 0110833 on the Conductivity of Chronic Acid Solutions. The 0r(OH)3 was prepared by precipitating it from a hot solution of chromium sulfate with concentrated again hydroxide. The precipitate was'filtercd and washed free from sulfates. It was dried at room temper- ature for about two days and then finely pondered to make a uniform mixture. An analysis to determine its chromium content was made by igniting in a crucible to 01-203. It was found to contain 36.40% by weight of chromium. The hydroxide was than approximately of the formula mama . 2820. 1431.] 711 Gone . Box Res. Bridge Res . (Oslo . ) Bans. 0r/L. OhIIe Rdg. 01mm 34.12 82.6 A! 472 81.68 " 82 .2 484 81 .67 " 81 .6 - 503 81 .70 Avg. - 81.68 27.30 59.6 . 472 _ 59.19 7 59 .4 484 59 .19 " 59 .2 503 59 .20 Avg . .- 59.19 21.84 59 .6 472 47 .50 "- 59.4 484 47 .61 " 59.2 503 47 .63 Avg. I ‘7 e61 fable 711 (continued) 23. ' 0on0. Box Bee. Bridge Res. (Calm) 0a.. 02/1. on. Rdg. Ohms . 13.65 36.6 500 36.60 7 36.4 512 36.58 " 36.0 540 36.58 Avg. :- 36.58 8.73 32.0 476 33.75 " 31.8 492 33.75 " 31.6 507 33.76 Avg. - 33.75 2.51 27.0 491 26.90 7 . 26.8 510 26.91 7 26.6 530 26.92 Avg. - 26.91 1.25 26.4 480 26.18 " 26.2 500 26.20 7 26.0 520 26.21 Avg. . 26.20 Sanitary Conn. Res. (Calm) Sp. 00nd. Gus. (Br/1.. Ohms 111108. 34.12 81.68 0.180 27.84 59.19 0.249 21.84 47.61 0.310 13.65 36.58 0.403 8.73 31.69 (L466 2.51 26.91 ()548 1.25 26.20 0.563 (00 I00 90 90 e'0 To .60 6 0 Codi-”fiat z [a ”dam-mm” o l \ \ \\ , L. / \ / \ f \ / *‘ \ / )3. / \. 7 “\m / cwmnvrtfl r/ow- Grru'. 6r 0, /2. F13. /. Oil/VJ EISIJ‘TA f/C'f - /oo 90 .’a JO 3’0 ’10 //0 25. C'OIVCI'IV r21 now of #4, Fe, 6"; 6'»... /2 Fig.2. 1 C'r@% F95} / // f / // /{/ tilifid’ // / / // ) Fe, 43 / // f / + //// l/i//””M 3.25:5 ”f“ 5- ,0 20 2.5' 30 :a‘ 25. [.00 .90 e80 .70 ‘ 'e. o L 5p. (Odd. - ”Ana. "3 .40 .30 e10 'JO '\ ‘ \\§\'\ \ \ \\‘ \ \ \ \MoJSOQ, \\ \ \\ £1 2.). \\ \ Peso» ‘ 51”)” j J I. I: no 21' so 86" C'OIVCENT'RATIwV of Fe,/Va, fCI‘. Guam/1.. F190. 28. DISCUSSIOH fig. 1 shows the variation of the resistance and specific conductivity of chromic acid solution with changes in concentration. The resistivity of the solution becomes a minimum and the conductivity a max- imum at a concentration of about 450 grams of 0r03 per liter of solution. This is the general effect of con- centration of a solution upon specific conductivity. From Table II it is seen that an addition of a salt such as 6.330‘ to chromic acid solutions of varying concentrations only increases its res4stivity by a very smell amount. The amount of 80‘ added was always 1% of the 0r03 present, which is approximately the amount present in some plating baths. 113. 2 shows the effects of addition of varying amounts of several different salts to chromic acid solution of a concentration of 250 gms. Bros/L of solu- tion. It is seen that reflo‘ and GHOH)3 have the great- est relative effect in increasing the resistivity of the solution. Both these compounds are basic or reducing substances and the increase in resistivity is probably due to a combination of two things: (1) The decrease in concentration of free chromic acid due to its reduction by the 0r(03)3 and the 7680‘. (2) The formation of a reduction product of chromic acid, chromium dichromate, a colloid, whose presence always increases the resistance 29. of the solution. In the case of both these compounds an equivalent amount of that component that carries practically all of the current, (the first hydrogen of chromic acid), is neutralized and as a result the resistivity 1. proportionally imreased. The first factor mentioned is no doubt the one of greatest impor- tance. A calculation of the amount cf’ehromic acid re- duced by 17.50 grams of chromium in the form of 014011): shows that the resulting concentration of the original solution would he 151.60 grams of 0r03 per liter. Referring to Fig. l, the resistivity of chromic acid at that concentration is 32.00 ohms, while the actual measured resistance of the reduced solution was 40.70 ohms. The difference then must represent the resistivity" due to the chromium dichrcmatc formed, and in the case cf’Pe804. to 152(30‘)3 formed or any other compound.'7 The increasing slope of the 7630‘ and 0r(OH)3 curves as their concentration increases shows that the concen- tration of free chromic acid is approaching that point on the graph of Fig. l, where the resistance begins to change rapidly with a small change in concentration. lonpreducing salts like 86230, and Iez(80‘)3 have very little effect on the conductivity of’chrcmic acid solutions. It is evident that a very great amount of either of these would have to be present in a plating bath to cause any serious effect, or change in its resistivity. 29. of the solution. In the case of both these compounds an equivalent amount of that component that carries practically all of the current, (the first hydrogen of chronic acid). is neutralisedand as a result the resistivity is proportionally increased. The first factor mentioned is no doubt the one of greatest impor- tance. A calculation of the amount of chromic acid re- duced by 17.50 grams of clu‘omium in the form of anon), shows that the resulting concentration of the original solution would he 151.60 grams of 0103 per liter. Referring to Fig. l, the resistivity of chromic acid at that concentration is 32.00 ohms, while the actual measured resistance of the reduced solution was 40.70 clans. The difference then must represent the resistivity. ' due to the chromium dichromato forned, and in the case of 2.804, to le2(80‘)3 formed or any other compound. 1’ The increasing slope of the 2930‘ and 0r(OH)3 curves as their concentration increases shows that the concen- tration of free chromic acid is approaching that point on the graph of Fig. l, where the resistance begins to change rapidly with a small change in concentration. Ion-reducing salts like 86230, and le2(80‘)3 have very little effect on the conductivity of chromic acid solutions. It is evident that a very great amount of either of these would have to be present in a plating bath to cause any serious effect, or change in its resistivity. Under ordinary plating conditions, it is practically impossible to prevent the formation of a limited amount of chromium dichromate as a lay-product of chromium de- position but as all ready stated the change in resistivity of the bath is not due merely to its presence, but also to the result of its formation. Fig. 3 represents resistances expressed as specific conductivity, or mhcs (reciprocal chm centimeters). 31. COHCLUSIDNS 1 - The conductivity of chromic acid solution be- comes a maximum and the resistance a minimum at a con- centration of approximately 450 grams of Grog per liter of solution. 2 - Basic or reducing substances such as 3.80; and Cr(0H)3 increase the resistance of chromic acid solutions due to the reduction of the free chromic acid present and also to the resultant formation of the colloid (chromium dichromate) and other compounds. 3 - Salts like nazso‘ and Pez(30‘)3 which are non- reducing, unless present in large amounts, have very little effect on the resistance of chromic acid solutions. 53 0404 3499031