Iodometric Determination of Copper Selection of a Suitable Buffer Solution WILLIAM R. CROWELL, University of California at Los Angeles, Calif.
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N THE iodometric determination of copper the titration is usually performed in a buffer solution: (1) to keep the p H around 3 or less but high enough to cause the oxygen-iodide reaction to have a negligible rate; (2) if arsenic is present, t o adjust the p H to a value between 3.2 and 4.0 to prevent an appreciable reaction between iodide and pentavalent arsenic. The buffer solutions used for such purposes should be those of acids with suitable ionization constants and those in which the cupric ions do not form insoluble, weakly ionized, or complex compounds to too great an extent. In determining copper in solutions containing iron and arsenic, Park (4) used a buffer solution of potassium biphthalate containing ammonium bifluoride. Foote and Vance (3) employed a buffer solution of acetic acid and ammonium acetate containing sodium fluoride. The purpose of the fluoride in each case was to cut down the activity of the ferric ions so that they would not react with iodide. Crowell and associates (1) have shown that the biphthalate used by Park has no value as a buffer and that the bifluoride is in reality the effective buffer in that method. Foote and Vance in their procedure introduced the use of thiocyanate toward the end point of the titration. The effect of this addition is to cause the reaction between iodide and cupric ions to go further toward completion and to give end points that are whiter and somewhat sharper than when thiocyanate is omitted. As a result of this behavior the presence of cupric complexes or weakly ionized compounds interferes with the titration less than when thiocyanate is absent. In 21 analyses of cupric sulfide samples containing iron and arsenic in considerable quantities, Foote and Vance obtained an average error of -0.05 per cent and a maximum error of -0.11 per cent. The author of the present paper and associates (1, 8 ) carried out similar copper sulfide analyses, using bifluoride buffer solutions both with and without thiocyanate, and in both cases obtained practically the same percentage errors as Foote and Vance. It has been their experience that bifluoride buffers give highly satisfactory end points without the use of thiocyanate. However, when analyses are made without thiocyanate the thiosulfate solution should be standardized against copper metal under the same conditions. If thiocyanate is used in the analysis, the thiosulfate may be standardized against iodine or by any accepted iodometric method. To seek a more satisfactory basis of comparison of the behavior of buffer solutions in order to be able to select those most suitable for the purposes described, the present investigation was undertaken. While the use of thiocyanate tends to extend the choice of satisfactory buffer solutions somewhat, such a study should be not only of theoretical interest but also of practical value in showing that certain buffers have advantages over others in certain cases and what buffers can be used and under what working conditions if the thiocyanate is omitted. The work was confined to solutions which have the proper ionization constants and the salts of which a t moderate concentrations do not form objectionable precipitates with cupric ions. The acids selected were acetic, propionic, formic, and hydrofluoric. In such a study the main reactions to be considered are:
+ 51-=2cuI + + 2SzOa---t 31- + SaOa--
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(1) (2)
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To make the behavior more apparent, Reaction 1 was studied separately by mixing cupric sulfate, potassium iodide, and the buffer solutions in a suitable flask, allowing them to come to equilibrium in a thermostat, and determining the unreduced copper. In order to observe the effects of the various buffer constituents on the iodometric end points, titrations in these buffer solutions with thiosulfate were also made in the usual manner, whereby Reactions 1 and 2 determine the per cent of copper reduced. As a result of this investigation it was hoped to reach conclusions regarding the following points: 1. The effects of pH, concentration of acid, and the salt of the acid on the per cent of copper reduced in fixed initial concentrations of cupric and iodide ions. 2. What solutions can be used and under what conditions in order to work at a pH between 3.2 and 4.0. 3. What solutions are the most suitable in the most general case in which iron is present alone or with arsenic; when arsenic is the only interfering impurity; and when arsenic and iron are absent and the pH may be 3 or less but sufficiently high to cause the oxygen-iodide reaction t o be inappreciable.
Equilibrium Experiments and Iodometric Titrations in Buffer Solutions REAGENTS.In preparing buffer solutions of the acids and their salts, ammonium salts were used in all cases except that of acetic acid, when the sodium salt was employed. Glass apparatus lined with paraffinwas used in handling the hydrofluoric acid solutions. The 1.0 iM hydrofluoric acid buffer solutions consisted of mixtures of the acid and ammonium fluoride, but the 0.10 M acid solutions consisted of mixtures of ammonium fluoride and ammonium bifluoride. In the case of bifluoride the "purified grade" was used. EXPERIMENTAL PROCEDURES. In the equilibrium experiments the procedure was as follows: The air in a 200-cc. conical flask was displaced with nitrogen and the following solutions added in the order stated: 25.0 cc. of potassium iodide solution, 50.0 cc. of buffer solution, and 25.00 cc. of 0.1152 M copper sulfate. The flask was sealed and rotated in a thermostat at 25" C. until equilibrium was reached. Samples were then removed through a filter and treated with nitric and sulfuric acids. The sulfuric acid solution of cupric sulfate remaining was neutralized with ammonia, acidified with formic'acid, treated with iodide, and titrated with thiosulfate. pH measurements were made by means of the quinhydrone electrode on separate solutions containing the same constituents a t the same initial concentrations as those used in the regular runs except that potassium chloride was substituted for the iodide. In the end-point study, titrations of the iodine in the buffer solutions with thiosulfate were made immediately after adding iodide and copper sulfate. In these experiments the titration during the addition of the last 0.5 cc. took no longer than about 2 minutes. RESULTSOF EQUILIBRIUM RUNS. Results are recorded in Table I and plotted in Figures 1 and 2. Equilibria were established in three types of solutions containing different concentrations of the ammonium or sodium salt of the acids: one in which the ammonium salt alone was used, a second in which the acid concentration was maintained a t 1.0 M , and a third in which the acid concentration was 0.10 M . In Table I are tabulated the percentages of reduced copper and the corresponding pH values. I n Figure 1 percentage reduction of the copper is plotted against corresponding salt concentration. I n Figure 2 percentage reduction is plotted against pH for the runs in which the acid concentrations are maintained a t 1.0 M and 0.10 M , respectively. I n order to observe the effect of salt concentration as well as pH, the numbers corre-
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OF COPPERREDUCED AND CORRESPONDINQ PH VALUESIN CERTAINBUFFERSOLUTION^ TABLEI. PERCENTAGES
(Initial concentration of copper sulfate = 0.02880 M . Initial Concentration of potassium iodide = 0.200 M ) Composition of Equilibrium Conoentration of Salt Solutions 0,000 0.0500 0.100 0.200 0.400 0.600 0.800 1.00 1.20 Moles p e r liter 1.00 M acetio acid and 92.4 85.4 70.4 56.5 43.0 30.6 ... sodium acetate P '27:!5 3.33 3.69 4.01 4.23 4.38 4.50 ... 89.6 79.8 61.0 43.9 30.4 19.2 0.100 ,M acetic acid and '27:634 ... 4.27 4.64 4.94 6.16 5.27 5.37 sodium acetate P Ammonium propi0nat.e % 89.6 76.7 52.3 32.1 18.3 9.5 1.00 M pSopionic acid and % 98.5 96.9 93.8 87.6 73.8 60.2 48.1 35.9 .... ammonium proponate pH 2.26 3.15 3.46 3.84 4.21 4.39 4.58 4.65 ... 0.100 M p ~ o p i o n i c a c i d a n d % 97.8 94.7 90.4 80.9 61.5 47.2 32.9 23.0 ... ammonium propionate pH 2.72 4.13 4.42 4.76 5.05 5.19 5.33 5.42 ... Ammonium formate % 99.5 , 98.0 95.0 89.3 82.4 70.5 55.3 ... 98.9 . . . 97.5 95.4 93.3 87.9 83.5 76.1 ... 1.00 M f,ormic acid and ammonium formate pa 1.79 ... 2.17 2.57 2.91 3.14 3.31 3.44 ... 0.100 M formic acid and % 97.4 ,.. 96.5 94.6 90.7 85.6 77.5 63.6 ... ammonium formate PH 2.19 ... 3.37 3.72 4.09 4.32 4.46 4.79 ... ... 95.5 91.5 , .. 73.8 63.0 51.0 Ammonium fluoride ... 98.2 ... ... 97.3 .,. Ammonium bifluoride 2.87 2.84 2.81 2.72 ... PH % 99.5 ... 99.3 99.0 98.8 98.0 97.3 93.6 1.00 M H F and NHaF PH 1.18 ... 1.50 1.72 2.12 ... 2.57 2.80 2.95 . . . , . . 9 8 . 5 9 7 . 0 9 4 . 0 9 2 . 4 8 7 . 9 8 3 . 4 0.100 M H F and NHaF % ... ... 2.87 3.28 3.71 3.96 4.16 4.33 PH
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sponding to salt concentration in moles per liter are indicated a t the various points plotted: CONCLUSIONS FROM EQUILIBRIA DATA. A study of Table I and of Figures 1 and 2 considered in conjunction with results of the iodometric titrations of the buffer solutions immediately after the addition of iodide and copper sulfate leads to the following general conclusions: 1. In buffer solutions of propionic, formic, and hydrofluoric acids evidently cupric complexes are formed similar to those in buffer solutions of acetic acid. The lowering of the
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per cent reduction of copper a t a given initial concentration of cupric ion, iodide, salt, and acid roughly seems to depend upon the ionization constant of the acid. Acetic and propionic acids, whose ionization constants and 1.4 X a t 25" C. are 1.8 X respectively, show the lowest per cent reduction. Formic acid with a constant of 2.4 X has the next higher per cent and hydrofluoric acid with a constant of 6.9 X has the highest per cent. The last named no doubt has its value raised somewhat by reason of the bifluoride equilibrium. 2. The per cent reduction in buffer solutions of a given acid does not depend so much upon p H as upon actual concentrations of acid and of salt, especially of salt. A tenfold change can be made in the hydrogenion concentration without materially affecting the per cent reduction. At a fixed acid concentration a tenfold increase in salt concentration may lower the reduction 3 to 15 per cent in hydrofluoric acid, 20 to 30 per cent in formic acid, and 60 to 70 per cent in acetic and propionic acids. For a given salt concentration a tenfold increase in acid concentration may raise the percentages 3 to 15 per cent. 3. Results of titrations of iodine in the buffer solutions immediately after the addition of iodide and copper sulfate in general agreed with those obtained with copper sulfate containing no buffer constituents within a few hundredths of a per cent if the equilibrium per cent reduction was no lower than 85 to 90. On this basis satisfactory end points without the use of thiocyanate can be obtained with the various buffers a t a pH between 3.2 and 4.0 if, a t the concentrations of iodide and cupric ions commonly employed, the concentrations of the salt of the acid are not much greater than the following values: fluoride, 1.0 to 1.6 M ; formate, 0.6 to 0.7 M ; and acetate and propionate, 0.1 to 0.2 M . If thiocyanate is added, probably these values can be materially increased. From the standpoint of salt effect the superiority of hydrofluoric acid buffers over the others is evident, and formic acid
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buffers have an advantage over those of acetic and propionic acids. Since hydrofluoric acid buffers, prepared from ammonium bifluoride, can serve the double purpose of acting as a n effective buffer and of forming a complex with ferric ions, there is little doubt of their superiority when iron is present alone or with arsenic. Since hydrofluoric acid solutions because of their effect on glassware probably would not be selected for their buffer action alone, formic acid solutions should show themselves the most satisfactory in iron-free solutions containing arsenic and in iron- and arsenic-free solutions in which it is desired to work a t a pH between 2 and 3, a region within which the rate of the oxygen-iodide reaction is inappreciable. As an example of the latter case, this laboratory now employs formic acid instead of acetic in the standardization of thiosulfate solutions with copper metal.
scribed for the bifluoride runs @) except that after the addition of ammonia, instead of adding bifluoride, sufficient 4 N formic acid was added just to dissolve the precipitate. The pH of the solutions was determined with quinhydrone before the addition of iodide. Results of seven titrations of copper sulfate solutions without the addition of thiocyanate showed an average error of -0.03 per cent and a maximum error of =t0.13 per cent. Results of analyses of seven samples of copper sulfide during which thiocyanate was added showed practically the same percentage errors as the solutions. The percentage errors were determined on the same basis as that used for bifluoride (1). The pH was between 3.5 and 3.7. As in the case of bifluoride buffers, these results indicate that satisfactory determinations can be made in formic acid buffers without thiocyanate.
Determination of Copper i n the Presence of Arsenic Using Formic Acid Buffer
Summary
Determinations of copper in solutions of copper sulfate and in cupric sulfide were made using buffer solutions of formic acid containing 0.2 gram of arsenic. I n the case of the solutions the procedure was as follows: To 40 cc. of a solution containing the arsenic and the same amount of copper as was employed in the equilibrium runs was added enough 6 N ammonium hydroxide to give the solution a distinctly recognizable odor of ammonia. Sufficient 4 N formic acid was then added just to dissolve the precipitate formed and the titration was completed with iodide and thiosulfate. The procedure in the case of cupric sulfide was the same as that de-
In a study of the reaction between copper sulfate and potassium iodide in buffer solutions of acetic, propionic, formic, and hydrofluoric acids, propionates, formates, and fluorides lower the activity of the cupric ions in much the same manner as acetates. At a given initial concentration of copper and of iodide the lowering of the per cent of copper reduced is not governed so much by pH as by the concentrations of acid and of salt, especially of salt. In the iodometric determination of copper in the presence of iron and arsenic a t the concentrations of copper and of iodide commonly used, satisfactory end points in buffer solutions of these acids with a pH between 3.2 and 4.0 can be obtained without the use of thiocyanate if the maximum concentrations of the salt of the acid do not exceed 0.1 to 0.2 M for acetic and propionic acids, 0.6 to 0.7 M for formic acid, and 1.0 to 1.7 M for hydrofluoric acid. In solutions in which iron is present alone or with arsenic, ammonium bifluoride solutions are the most suitable. In iron-free solutions containing arsenic and in solutions free from iron and arsenic in which it is desired to work a t a pH between 2 and 3, formic acid has an advantage over acetic and propionic acids. Results obtained without the use of thiocyanate in formic acid buffer solutions containing arsenic but free from iron showed practically the same accuracy and precision as those obtained with the addition of thiocyanate, and also those obtained in a previous work in which buffer solutions of bifluoride were employed with and without the addition of thiocyanate. If thiocyanate is not used in the analysis, the thiosulfate solution should be standardized with copper metal under the same conditions. Acknowledgment The author was assisted in the experimental work by Sidney C. Rittenberg, Harlan L. Baumbach, Lee S. Reed, R. H. Sweet, Fred J. Clark, and Howard M. Bollinger, research students a t the University of California a t Los Angeles.
Literature Cited
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RUNS FIGURE 2. RESULTSOF EQUILIBRIUM
(1) Crowell, W. R., Hillis, T. E., Rittenberg, S. C., and Evenson, R. F., IRTD. EXQ.CHEM.,Anal. Ed., 8, 9 (1936). (2) Crowell, W. R., Silver, S. H., and Spiher, A. T., Ibid., 10, 80 (1938). (3) Foote, H. W., and Vance, J. E., Ibid., 8, 119 (1936); 9, 205 (1937); J. Am. Chern. SOC.,57. 845 (1935). (4) Park, B., IND.EXG.CHEM.,Anal. Ed., 3,77 (1931). RECEIVED September 28, 1938.
