Estimation of Acetate in Zinc Plating Baths T. H. WHITEHEAD and HENRY W. WRIGHT’ Department o f Chemistry, University
o f Georgia,
Athens, G a .
A method is described for the estimation of acetate in zinc plating baths. Acetate is separated by a double distillation procedure and estimated in the distillate by a colorimetric method using lanthanum nitrate, iodine, and ammonia as reagents. The final solution is carefully buffered to pH 9.0 with ammonium chlorideammonia solution. The procedure is very sensitive and can estimate milligram quantities of acetate, but the accuracy and precision vary by 10% of the amount of acetate present on the average.
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UAXTITATIVE analyses of solutions from which metals are electroplated are highly important in the control of such processes. Methods for the estimation of all cations and anions in typical zinc-plating baths, except acetate ion, have been reported in the literature ( 1 ) . Acetate ion is difficult to estimate quantitatively, because of the interfering substances which are normally present in a zinc-plating bath. A typical solution for zinc plating is: Zinc sulfate, ZnS04.7HzO Ammonium chloride, “aC1 Sodium acetate, SaCzHaOz. 3Hz0 Glucose, C ~ H ~ Z O E
360 grams 30 grams 15 grams 120 grams
“-1drop of 5 % solution of lanthanum nitrate is mixed with a drop of 0.02S iodine on a spot plate. One drop of the solution containing acetate ion is added. The test is positive, if, when this mixture is alkalized with ammonia, a blue color is formed or a blue ring is formed around the edge of the mixture.” BASIS OF PROCEDURE
Kruger and Tschirch ( 7 ) stated that the formation of the blue color seems to be due to a basic lanthanum-iodo complex with acetate ion rather than to adsorption phenomena. The results of this study are in agreement with this assumption. This eystem is far from ideal for a quantitative method, because it is timesensitive, pH-sensitive, and varies \T ith dilution. Consequently all of these factors must be controlled carefully. Reproducible color formation takes place between pH values of 8.6 and 9.4 a t 20” C. At lower pH values, the color is less intense, and at pH values above 9.4 a precipitate forms. The optimum value is p H 9.0 at 20” c.
per liter per liter per liter per liter
Scott reported (10)that acetate may be separated from such a bath by distillation and titrated as acetic acid with a standard solution of strong base. In cases where other acids also distill, a conductometric titration has been reported ( 3 ) . The first method must be carefully standardized, because hydrochloric acid usually distills with the acetic acid, and the second method requires a considerable period of time to perform it, plot the results, and calculate the final concentration.
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Figure 2.
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2 4 6 CONCENTRATION
I
8
IOxIO-4N
OF ACETATE
Calibration curve for acetate
5 s 16‘N a.050
,
400
Figure 1.
450
‘
o
500 550 600 6 5 0 7 0 0 750 WAVE LENGTH IN MILLIMICRONS
Absorbance v s . concentration of acetate
The present paper presents a colorimetric method, which is based upon methods for the qualitative detection of acetate in water solutions (2-9). Feigl ( 2 ) describes this test as follows: 1
Present address, Carbide and Carbon Chemicals Co., Oak Ridge, Tenn.
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A spectrophotometric study of this system was made with a Coleman Universal spectrophotometer a t intervals of 5 mp over a range of 350 to 800 mp. These data are plotted in Figure 1 and show that maximum absorbance occurs in the range 600 to 625 mu with a slight shift toward the longer wave length as the concentration of acetate ion is increased. hccording to the procedure, measurements were made on solutions of pure acetic acid varying from to 10-dM with a Fisher Electrophotometer using color filter A-650. Typical data are plotted in Figure 2, which show the divergence from Beer’s law
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1835
V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5 and the necessity for preparing a calibration curve before attenipting to use this method for any given type of zinc-plating bath. Sulfate ion interferes seriously with the color formation as does furfural, which is formed by decomposition of glucose during distillation. However, the addition of phosphoric acid folloit ed by careful distillation, precipitation of any sulfate with barium chloride, and a second distillation removed the interference from sulfate and furfural. With this method small amounts of chloride ion do not interfere in the estimation. DISTILLATION APPARATUS
The distillation train required uses conventional apparatus with some modification. The first distilling flask is a 500-ml. Kieldahl flask closed with a three-hole rubber stopper through which pass a thermometer, a glass delivery tube, and the stem of a 60-ml. separatory funnel. This flask is connected to the second distilling flask by the glass delivery tube that dips almost to the bottom of a 250-ml. Claisen distilling flask. The delivery tube between the two flasks is joined by a short length of rubber tubing over which a pinch clamp is fitted. The Claisen flask is closed with a two-hole rubber stopper through which pass a delivery tube and the stem of a 60-ml. separatory funnel; the second neck of the flask is closed with a one-hole rubber stopper carrying a thermometer. The stem of the Claisen flask is connected to a Liebig condenser in the usual manner. The final distillate is caught from the condenser in an Erlenmeyer flask of 200-ml. capacity, dipping in an ice bath. OTHER APPARATUS
Fisher Electrophotometer, AC Model, No. 1697 with color filter A-650 and 23-ml. cuvettes (or an equivalent instrument). Beckman pH meter, Model G, No. 10074 with shielded glass electrode and saturated calomel half-cell (or equivalent instrument).
Pipet 5 ml. of lanthanum nitrate solution into a 50-ml. beaker, which also contains the electrodes of the Beckman pH meter. Pipet 3 ml. of iodine solution and the proper aliquot of the distillate (normally 5 ml.). Add distilled water to make a total volume of approximately 1.5 ml., and rapidly adjust to p H 9.0 a t 20” C. with 7.5M ammonia. Allow 5 minutes for the color to develop, then transfer the solution from the beaker to a 100-ml. volumetric flask. Rinse with the ammonium chloride buffer solution and make to 100-ml. volume at 20’ C. with the buffer solution. Shake well and measure the light absorbance of the solution with the Fisher Electrophotometer using the -4-650 color filter. From the calibration curve, calculate the amount of acetate ion in the solution, and, knowing the dilutions which have been made and the size of the aliquots taken, calculate the acetate concentration of the original plating solution. The calibration curve should be obtained from different aliquots of the distillates. DISCUSSION OF RESULTS
Provided the calibration curve has already been obtained, the procedure described requires about 1.5 hours to make a complete estimation. The precision and accuracy leave much to be desired as deviations as high as 15% of the amount of acetate present were obtained in some cases. On the average, results can be reproduced on similar plating baths within 10% of the amount of acetate present. For example, three analyses run on the plating bath cited in this paper and using the dilutions described gave results of 6.00, 5.61, and 5.92 mg. of acetate when the correct amount was 6.51 mg. of acetate. If greater accuracy than this is required, this method is not satisfactory; however, for most purposes in electroplating this is sufficiently exact. It eliminates interference from furfural, sulfate ion, and hydrochloric acid. These impurities seriously interfere with procedures which depend upon titration with standard base. The order of adding reagents is important and must be followed exactly as described or wide variations in color will result.
REAGENTS
Lanthanum Nitrate Solution. Dissolve 50 grams of reagent grade lanthanum nitrate in distilled water to make 1 liter. Iodine Solution. Dissolve 1.2692 grams of reagent grade iodine in 95% ethyl alcohol to make 500 ml. of solution. Buffer Solution. Dissolve 2.70 grams of reagent grade ammonium chloride in 1 liter of distilled water, immerse the electrodes of the Beckman pH meter in this solution a t 20’ C. adjust the pH of the solution to 9.0 with 7.5M ammonia by adding it dropwise and stirring. After several hours check the pH and adjust to pH 9.0 if necessary. EXPERIMEYTAL PROCEDURE
Pipet into the Kjeldahl flask 50 ml. of the plating solution and 5 ml. of 85% phosphoric acid. Pipet into the Claisen flask 50 ml. of a saturated aqueous solution of barium chloride and 5 ml. of 85% phosphoric acid. Pour 50 ml. of distilled water in each of the separatory funnels with their stopcocks closed. Connect the distillation train tightly and heat the Kjeldahl flask with a Bunsen burner or electric hot plate. Continue distillation until the thermometer in the Kjeldah1 flask reads 105’ C. Remove the heat then, and as soon as active boiling ceases, open the stopcock on the separatory funnel and allow the water to run into the flask. Next quickly close the stopcork of the separatory funnel and resume heating the flask. Again distill until the temperature reaches 105” C. Remove the heat, close the rubber tubing of the delivery tube with the pinch clamp, and open the stopcock of the separatory funnel. Apply heat to the Claisen flask and repeat the operations just performed on the Kjeldahl flask-that is, conduct two distillations. Transfer the distillate from the Erlenmeyer flask to a 400-ml. beaker, wash the flask with several 10-ml. portions of distilled water, and add them to the beaker. Immerse the electrodes of the Beckman pH meter in the distillate and adiust the pH value to 8.0 at 20” C. n.ith 7.5.11 ammonia. Remove the electrodes, rinse them with distilled water, and allow the rinsings to go into the beaker. Transfer the solution from the beaker to a 250-ml. volumetric flask, rinse the beaker, and add the rinsings to the flask. Then make up to 250 ml. with distilled water a t 20” C.
LITERATURE CITED
Blum, W., and Hogaboom, G. B., “Principles of Electroplating and Electroforming,” hIcGraw-Hill, New York, 1930. ( 2 ) Feigl, F., “Qualitative Bnalyse mit Hilfe von Tupfel-Reaktionen,” Bkademische Verlagsgesellschaft, Leipsig, 1935. (3) Kolthoff, I. AI., and Laitinen, H. A . , “pH and Electro Titrations,” Wiley, London, 1947. (4) Kruger, D., and Tschirch, E., Ber. deut. ehem. Ges., 62-B, 2776 (1)
(1929). (5) Ibid.,63-B, 826 (1930).
(6) Kruger, D., and Tschirch, E., Chem.-Ztg., 54, 42 (1930). (7) Kruger, D., and Tschirch, E., Mikrochemie, 2, 337 (1930). (8) Kruger, D., and Tschirch, E., Pharm. Acta Hela., 5, 25 (1930). (9) Kruger, D., and Tschirch, E., Pharm. Zentralhalle, 71, 145 (1930). (lo) Scott, W. W., “Standard Methods of Chemical Analysis,” 5th ed., p. 2251, Van Nostrand, Xew York, 1939. RECEIVED for review March 17, 1955. Accepted August 2, 1955. Presented in part before the Southeastern Regional Meeting of ACS, Birmingham, Ala. 1954.
Colorimetric Determination of Sulfate Ion-Correction I n the article on “Colorimetric Determination of Sulfate Ion” [Lambert, J. L., Yasuda, S. K., and Grotheer, AI. P., ANAL, CHEY.,27, 800 (1955)], the first sentence under “Preparation of Reagent” should read: Thorium borate is obtained by the reaction of 1 liter of 0.01M thorium nitrate solution and 1 liter of 0.05M sodium tetraborate solution, the latter being added dropwise with constant stirring. JACKL. LAMBERT
