JANUARY 15, 1936
9
ANALYTICAL EDITION
Summary The estimation of fluoride by titration with standard thorium solution in the presence of sodium alizarin sulfonate as indicator has been studied. The most favorable procedure includes the use of the indicator a t a concentration of 4 x lob6per cent, in a total volume of 50 cc., titration to match a blank in which the end point is taken a t a very light pink shade, and careful regulation of the p H in both blank and sample, the most favorable p H being 3.5. This latter condition is readily met by the use of the buffer system of sodium hydroxide and chloroacetic acid at a ratio of 0.5 and total concentration of 0.02 M . The dissociation constant of chloroacetic acid in 50 per cent commercial alcohol has been found Sodium alizarin sulfonate in this alcoholic to be 2.8 X solution acts as an indicator for hydrogen ion over the pH range 4.8 to 7.2 instead of 3.7 to 5.2 as in water. In a volume of 50 cc. an average accuracy of 99 per cent has been secured with known amounts of fluoride ranging from 57 to 760 y of fluorine. With 5-cc. volumes approximately the same accuracy is possible with 6 to 90 y of fluorine. Data are given regarding the concentrations a t which several interfering ions have an effect. The most serious of these are sulfate, arsenate, and phosphate, which fortunately are left behind when the fluoride is distilled as hydrofluosilicic acid. Sulfuric acid is entirely suitable for the distillation and a volume of 200 cc. distilled a t 140" C. accounts for all but a trace of the fluoride. I n the ashing of fruit samples containing fluoride for distillation it is very important not to allow the temperature to reach above 800" C., for loss is then excessive. Allowance must be made for the fluoride contained in the lime which is added to the sample before ashing. Re-
covery of fluoride added to apple pulp was 97.3 per cent under the most favorable conditions.
Acknowledgment Acknowledgment is gratefully made of the assistance given by Merle Randall of the Chemistry Department during the writing of this paper, and by R. Craig of this division in the study of the buffering properties of sodium chloroacetate in alcoholic solution,
Literature Cited (1) Armstrong, W. D., J . Am. Chem. Soc., 55, 1741-2 (1933). (2) Barre, M.,Compt. rend., 151,231-4 (1910). (3) Bowes, J. H.,and Murray, M. M., Biochem. J . , 29,102-7 (1935). (4) Dahle, D.,J. Assoc. O$ciaZ Agr. Chem., 18, 194-7 (1935). and Schmidt, C. L. A., J. Bid. Chem., 105,359-71 (5) Jukes, T.H., (1934). (6) Kolthoff, I. M., and Furman, N. H., "Indicators," p. 97, New York, John Wiley & Sons, 1926. (7) Reynolds, D.S.,J.Assoc. O$ciaZ. Agr. Chem., 18,108-13 (1935). (8) Reynolds, D. S., Ross, W. H., and Jacobs, K. D., Ibid., 11, 225-36 (1928). (9) Sanchis, J. M., IND. ENQ.CHEM.,Anal. Ed., 6,134-5 (1934). (10) Scott, W. W., "Standard Methods of Chemical Analysis," 4th ed., Vol. I, p. 226,New York, D. Van Nostrand Co., 1925. (11) Shuey, G.A., J. Assoc. Oflcial Agr. Chem., 18,156-7 (1935). ENQ.CHEM.,Anal. (12) Thompson, T. G.,and Taylor, H. J., IND. Ed., 5, 87-9 (1933). (13) Vanselow. A. P..arivate communication. (14) Willard, H. H., and Winter, 0. B., IND. ENG.CHEM.,Anal. Ed., 5, 7-10 (1933). (15) Winter, J. H., and Butler, L., J. Assoc. Oflcial Agr. Chem., 16, 105-7 (1933). REO~I~F October ID 5, 1935. Presented before the Division of Agricultural and Food Chemistry at the 90th Meeting of the American Chemical Society, San Francisco, Calif., Auguat 19 to 23, 1935.
Iodometric Determination of Copper Adjustment of Hydrogen-Ion Concentration WILLIAM R. CROWELL, THOMAS E. HILLIS, SIDNEY C. RITTENBERG, AND RAYMOND F. EVENSON University of California a t Los Angeles, Los Angeles, Calif.
P
ARK (3) has recently described a method of determining copper in the presence of ferric iron and arsenic acid. By his method an iodometric titration is carried out in a solution containing ammonium bifluoride and potassium biphthalate. He states that the purpose of the bifluoride is to suppress the action of the ferric iron on potassium iodide, and that the biphthalate forms a buffer solution in which the pH is about 4.0, a value sufficiently high to cause no appreciable oxidation of iodide by the arsenic acid. The problem of pH adjustment can be best understood if we consider the technic involved in the process. After decomposition of the ore with mineral acid, sufficient ammonium hydroxide is added to neutralize the excess mineral acid, arsenic acid, etc., precipitate the i r p , convert the copper to the cupric ammonia complex, and produce a slight odor of ammonia. The bifluoride and biphthalate are then added. It is assumed that if the proper amount of biphthalate is added, the concentrations of phthalate and of biphthalate will be such as to produce an effective buffer solution a t the pH desired. It is evident that an effective buffer action is necessary to allow for a reasonable variation in the amounts of excess ammonium hydroxide and of the other substances which react with the weak acid. In this process the possibility of the bifluoride's acting as a buffer evidently has been overlooked. Hudleston and his co-workers (1) have shown
that hydrogen fluoride in its aqueous solutions undergoes the following equilibria: KI = ____ [H+l[F-J = 6.9 x 10-4 (250 c.) (1) [HFI HF2- Kz = [HF2-1 = 4.7 (25" C.) (2) LHFl[F-l
H+ +F-$HF HF
+ F-
The ionization constant of biphthalate a t 25" C. is 3.1 X To obtain a p H of 4.0, the ratio of concentrations of biphthalate to phthalate should be about 32 to 1, while the ratio of concentrations of hydrofluoric acid to fluoride should be about 1 to 7. At this pH the hydrofluoric acid should be a much more effective buffer than the biphthalate, and the p H a t which i t has maximum buffer efficiency is about 3.2. It seems reasonable to suppose, therefore, that it should be practicable to add such an amount of bifluoride that the concentrations of fluoride and of hydrofluoric acid formed would be sufficient not only to produce the ferric complex but also to yield simultaneously a buffer solution a t a p H between 3 and 4. It is the purpose of the present paper to show that under the conditions described by Park the p H a t the end point is nearer 3.3 than 4.0, that this pH is high enough t o insure no appreciable reaction between iodide and arsenic acid, that the biphthalate plays practically no part in the adjustment
INDUSTRIAL AND ENGINEERING CHEMISTRY
10
of the pH, and that it may be omitted without any material effect on the accuracy or precision of the method.
Reagents The cupric sulfate solution was prepared from the salt which had been twice recrystallized from a solution of the c. p. pentahydrate, and was made 0.1152 M in copper sulfate. The concentration of this solution was determined electrolytically, and iodometrically by titration with thiosulfate solution which had been standardized against pure copper foil. Results by the two methods agreed within less than 0.2 per cent. The sulfuric acid, ammonium hydroxide, potassium iodide, sodium thiosulfate, and potassium biphthalate consisted of c. P. reagents which conformed to the standards of Murray (8). The iron and arsenic added as impurities were supplied from solutions of c. P. ferric nitrate and arsenic acid. These solutions contained approximately 0.1 gram each of iron and of arsenic in each cubic centimeter of reagent. The ammoninm bifluoride was Merck’s Purified grade, The starch solution was made from soluble starch prepared “according to Lintner.”
Experimental Procedure and Results Table I shows results of titrations of solutions of copper sulfate containing iron and arsenic as impurities. In series 1, 2, 4, 5, 7, 8, and 11 ammonium bifluoride, NHaFH2,alone was used, and in series 3, 6, 9, and 10 potassium biphthalate was also present. The procedure was as follows: To 25.00 cc. of copper sulfate solution containing 0.1831 gram of copper were added 5 cc. of a solution containing the impurities as designated in Table I and 5 cc. of concentrated sulfuric acid. This was followed by concentrated ammonium hydroxide until the blue copper complex began to appear. Then 6 N ammonium h droxide was added dropwise until the solution smelled faintly ofammonia, and 2.0 grams of ammonium bifluoride and, in the runs indicated, 1.0 gram of potassium biphthalate were dissolved in the mixture. As soon as solution was complete, 10 cc. of 3 M potassium iodide were added and titration with thiosulfate was carried out. This procedure is essentially the same as that described by Park except in the cases in which the biphthalate was omitted. The amount of ammonium bifluoride added corresponded to 1 gram for each 0.1 gram of iron. pH measurements were made by means of the quinhydrone electrode on separate solutions containing in 100 cc. (the approximate end-point volume) the same amounts of the same constituents that were present before the addition of the iodide. The iodide was omitted because it reduces the oxidized form of the quinhydrone and causes the pH determinations to be too high. Park in describing his procedure states that “the pH values of the solutions a t the end point were determined by means of the quinhydrone electrode.” If this means that potassium iodide was present, that condition alone might account for the fact that the authors’ values are so much lower than his. In the cases in which ammonium hydroxide was added until a faint odor was produced, the p H of the solution was approximately 3.3. In order to obtain a pH of 4.0 it was found necessary to add about 2 cc. of G N ammonium hydroxide more than that required to produce a faint odor of ammonia. This was introduced after the addition of the bifluoride. The quinhydrone potential was always read after the addition of the bifluoride. When the biphthalate was used, it was added after the bifluoride and its addition caused no recognizable change in the potential of the solution. In the cases in which the p H was 3.3, as well as those in which it was 4.0, the end points were sharp and after the titration was completed there was no further liberation of iodine for 15 minutes or more. The values designated as “average per cent error” represent
VOL. 8, NO. 1
the percentage differences between the iodometric titration on the blank (performed as described in the first paragraph under “Reagents”) and the average of the titrations in question. The value of the blank was 25.37 CC. TABLEI. EFFECTOF IMPURITIES ON TITRATION OF COPPER IN BUFFERSOLUTIONS OF AMMONIUMBIFLUORIDE WITH AND WITHOUT POTASSIUM BIPHTHALATE (Copper taken, 0.1831 gram. Volume of thiosulfate required, 25.37 cc.) pH of Solution Weight of Impurity Series Iron Arsenic NHdHFn Gram Gram Grams 1 0.2 0.0 2.0 2 0.2 0.0 2.0 3 0.2 0.0 2.0 4 0.0 0.2 2.0 5 0.0 0.2 2.0 6 0.0 0.2 2.0 7 0.2 0.2 2.0 8 0.2 0.2 2.0 9 0.2 0.2 2.0 10 0.2 0.2 2.0 11 0.3 0.2 3.0 Average per cent error without -0.02. Average deviatjon of each =t0.06 per cent; with biphthalate,
ThioKHCsHcOc sulfate Grama Cc. 0.0
NHiOH o?;’N added NHlOH t o faint added odor in excess
...
25.35 3.3 25.39 4.0 25.39 4.0 25.34 3.3 0.0 25.36 3.9 1.0 25.35 3.9 0.0 25.37 3.3 0.0 25.37 4:o 1.0 25.35 3.4 1.0 25.37 3.9 0.0 25.35 3.3 biphthalate -0.04: with biphthalate, result from’ mean without biphthalate, k0.06 per cent. 0.0 1 .o 0.0
... ... ... ... ...
...
...
...
TABLE11. ANALYSISOF A MIXTURECONTAINING CUPRIC SULFIDE AND ARSENOPYRITE BY THE BIFLUORIDE METHOD (Per cent of copper present, 14.72. Bifluoride added 2.0 grams. Conoentration of thiosulfate. 0.1022 N . j Weight Volume of pH at Run of Sample Thiosulfate End Point Copper Grams
cc.
%
Average percentage error $0.01. Average deviation of eac; result from mean, A0.07 per cent,
Table I1 shows results of analyses of a mixture containing approximately 22 per cent of c. P. cupric sulfide and 10 per cent of copper-free arsenopyrite mixed with finely ground unglazed porcelain which gave no test for iron when treated with hydrochloric and nitric acids. The cupric sulfide was thoroughly mixed with the unglazed porcelain powder and a number of samples accurately weighed out. Some of the samples were analyzed for copper and the values thus obtained were used as a basis on which to determine the error of the method. To each of the other samples was added about 0.1 gram of arsenopyrite, the whole thoroughly mixed, and the copper determined by the bifluoride method. The procedure used on the blanks was essentially the same as that employed in the standardization of the thiosuhte by copper foil. Runs were also made on the blank samples, using the procedure described below for the mixture after the addition of the arsenopyrite. The two sets of values agreed within 0.1 per cent. The procedure used on the cupric sulfide arsenopyrite mixture was as follows: Fifteen cubic centimeters of concentrated nitric acid were added to the sample, evaporated to 5 cc., 10 cc. of concentrated hydrochloric acid and 10 cc. of 18 N sulfuric acid added, and the whole was evaporated to dense white fumes. In runs 1, 2, and 3, 10 cc. of concentrated nitric acid and 10 cc. of concentrated hydrochloric acid were then added, and the solution was again evaporated to dense white fumes and diluted with 20 cc. of water. In runs 4, 5, and 6 the fuming was followed by the addition of 20 cc. of water and 10 cc. of saturated bromine water, and the solution boiled until all bromine fumes were removed. These two types of procedure were found to be necessary to insure complete oxidation of the arsenic. After the addition of the water and boiling, the procedure was the same as in the titration
JANUARY 15, 1936
11
ANALYTICAL EDITION
of the copper sulfate solutions previously described. In the present case it was not necessary to filter off the insoluble residue before making the copper titration.
Summary and Conclusions I n the Park method of determining copper in the presence of as much as 0.3 gram of iron and 0.2 gram of arsenic, the potassium biphthalate may be omitted without any appreciable effect on the accuracy or precision of the results. The addition of biphthalate has no material effect on the pH of the solution. The p H at the end point is nearer 3.3 than 4.0 and yet the end point is practically permanent.
To insure complete oxidation of an ore containing sulfide, iron, and arsenic, treatment with nitric acid alone is not sufficient. A double treatment with nitric and hydrochloric acids or a single treatment with the two acids followed by one with saturated bromine water is found necessary.
Literature Cited (1)
Hudleston, J. Chem. Soc., 125, 260, 1122, 1451 (1924); 127, 1122
(2)
Murray, “Standards and Tests for Reagent Chemicals,” New York, D. Van Nostrand Co., 1920.
(1926).
(3) Park, IND.ENG.CHEM.,Anal. Ed., 3, 77 (1931). RECEIVED October 15, 1935.
The Determination of Rhenium I.
Qualitative
LOREN C. HURD, University of Wisconsin, Madison, Wis.
Although a number of papers have apacid by weight it has been the peared dealing wholly or in part with reacauthor’s experience as well as nium in 1925 (28) there have that of others (9) that precipitaappeared a number of papers tions of rhenium of value in the qualitation is not quantitative, In light dealing wholly or in part with various qualitative tests for the t h e detection of the element, no mention of this rather peculiar condition identification of the element. has been made of its place in the convenit was thought desirable to ascertain where in the conventional tional scheme. Data are presented which Its place in the Noyes and Bray indicate that in the Prescott and Johnson scheme of analysis the element system was studied by Kao and would be c o n c e n t r a t e d and ( I 6 ) who found that system the element will concentrate with what precautions must be taken s i g n i f i c a n t concentration was arsenic* Early work in the has been in order to insure a clean-cut effected in the tellurium group. critically examined and evaluated and separation, These authors reported that during the course of the analysis of several new tests are reported. New conA stock solution was prepared to the group, tellurium and rhofirmatory tests are described. which were added 5 mg. Der 50 ml. dium could be precipitated in the of each of the follo&i metals: Hg(ic), Pb, Cu, Cd, As(oG), As(ic), presence of rhenium by reducing Sb, Sn(ic), Cr(ic), Fe(ic), AI, Ni, Co, Zn, Mn, Ca, Sr, Ba, Mg, K, with hydrazine hydrochloride and sodium bisulfite in hydroand Na. Fifty-milliliter portions of this solution were carried chloric acid solution. Rhenium was subsequently precipithrough the qualitative scheme of Prescott and Johnson (19). tated as the sulfide from the filtrate. Each group precipitate was washed thoroughly, digested with sodium hydroxide to expel all ammonia, oxidized with hydrogen The insolubility of rhenium heptasulfide was one of the peroxide, acidified with sulfuric acid, and treated with nitron characteristic properties early reported. Although this was acetate as described by Geilmann and Voigt (7). a controversial subject for several years (Id) it has been In the case of solutions containing chromium and manganese definitely established by several workers, notably Geilmann it was necessary to add a few drops of alcohol to the acid solution prior to the nitron precipitation in order to reduce compounds of and Weibke (9),that the sulfide is quantitatively precipitated higher valence which yield precipitates with the reagent. Alcowhen a perrhenate solution containing as high as 33 per cent hol in low concentrations is without influence in the nitron preof hydrochloric acid by weight is treated with hydrogen cipitation (SI). Certain normal constituents of complete group sulfide. This separation is now used extensively in the precipitates were excluded because it was found that they yielded insoluble nitron derivatives. For this reason tungstates, molybanalysis of rhenium. For reasons to be discussed below, the dates, palladium, gold, chloroplatinates, and germanium were not sulfide precipitation is not applicable to all rhenium-containincluded in the stock solution. ing samples. A series of blank determinations using the stock solution was carried through the Prescott and Johnson separation and each TABLEI. RHENIUM FOUND BY PRESCOTT AND JOHNSON group or subgroup analyzed according to the nitron method. SYSTEM No precipitate was obtained in any case. Fifty milligrams of (50 mg. of Re added) rhenium as KRe04 were then added to a 50-ml. portion of the Sample 1 Sample 2 Sample 3 Sample 4 stock solution and the separation and analysis repeated. Group 1 reagents yielded no precipitate. When hydrogen sulfide was Mg. % Mg. % Mg. % MQ. % passed into the 0.25 N hydrochloric acid solution for second 7.0 First analysis 1.5 3 . 0 4 . 3 8 . 6 4 . 7 9 . 4 3 . 5 group precipitation, some trouble was experienced in obtaining a Second analysis 1 . 4 2 . 8 6 . 4 12.8 1 1 . 9 2 3 . 8 16.1 3 2 . 2 Third analysis 44.7 8 9 . 4 2 9 . 9 5 9 . 8 2 9 . 1 5 8 . 2 2 8 . 8 5 7 . 6 flocculent precipitate. It filtered without difficulty, however, - - - - _ _ - ~ and was analyzed (SO) for its rhenium content (first analysis, Total 47.6 95.2 4 0 . 6 81.2 45.7 91.4 48.4 96.8 Table I). The filtrate from the first precipitation with hydrogen sulfide was evaporated to a volume of 8 ml. for the arsenic precipitation. During the course of the concentration sulfur and rhenium sulfide Prescott and Johnson System separated from the solution. This was removed by filtration and analyzed for rhenium (second analysis, Table I). Samples Although rhenium heptasulfide is quantitatively precipi2,3, and 4 were allowed to stand overnight before filtering. tated from solutions containing a relatively high concentraThe solutions were then acidified and saturated with hydrogen tion of hydrochloric acid, the precipitation takes place slowly. sulfide to precipitate the arsenic. The mixed sulfides were I n solutions containing less than 4 per cent of hydrochloric analyzed for rhenium (third analysis, Table I). INCE the discovery of rhe-
S
