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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 13, No. 8
The Determination of Small Quantities of Zinc’ By M. Bodansky LABORATORY OF BIOLOGICAL CHEMISTRY OF THE SCHOOL OF MEDICINE, UNIVERSITY
Quantities of zinc too small to weigh cannot be determined gravimetrically or electrolytically. In dealing with quantities ranging between 0.1 and 5 mg. the volumetric procedures are equally inadequate. A turbidimetric method based upon the precipitation of zinc by potassium ferrocyanide has been described recently by Breyer.2 This method has been applied by Meldrum3in the quantitative determination of zinc in water, and by Birckner4 in determining the zinc content of various food products. According to Birckner’s procedure, the zinc is precipitated as the sulfide from a formic acid solution, filtered, and dissolved in hydrochloric acid. The turbidimetric comparison is performed on this solution in Nessler jars, if the amount of zinc is below 0 . 5 mg. With larger amounts aliquot portions are taken for comparison. Birckner states that the method gives reliable results, and if properly carried out the experimental error should not exceed ~ 0 . 0 5mg. Owing to the sensitiveness of the turbidimetric comparison the method was used in this laboratory in an investigahionof .thezinc content of marine organisms,6 and our experience as to its applicability confirms Birckner’s findings. MODIFICATIONS OF BRBYER-BIRCKNER METHOD However, it was found necessary to modify the preliminary procedure. The material for analysis was prepared, ashed, and the copper removed as previously described by Rose and Bodansky.s The filtrate obtained from the copper sulfide precipitation was boiled to remove the hydrogen sulfide and made alkaline with ammonia. This solution was acidified with citric acid and the zinc was precipitated as the sulfide. The tendency of zinc sulfide to form colloidal suspensions when precipitated from acid and even from alkaline solutions and the difficulties involved in filtering such precipitates are well known. Where zinc occurs in traces this tendency is especially marked, and the introduction of a considerable error is almost unavoidable. By using citric acid in our analyses of marine organisms there was frequently obtained, during the heating preliminary to the introduction of the hydrogen sulfide, a precipitate of calcium citrate, the calcium being derived from the chitinous protective covering of some of the animals or from the skeletal structures of the marine vertebrates. It was noted that in the presence of calcium citrate the filtrates were clear and free of zinc. The calcium citrate is not an interfering substance and may be dissolved along with the zinc sulfide, and the zinc determined turbidimetrically as in Birckner’s procedure. The more complete recovery of minute quantities of zinc in this way may be due to the adsorption of the colloidal zinc sulfide particles by the calcium citrate. Indeed, Bancroft7 has recently pointed out that there is some evidence to show that when a colloidal solution is precipitated the finer particles attach themselves to the coarser ones. As the turbidimetric method afforded a means for the estimation of minute quantities of zinc, a brief study of the behavior of colloidal zinc sulfide in the presence of calcium citrate was possible. Furthermore, in anticipation of future work on the occurrence and distribution of zinc in various 1 Received March 14, 192 1. 2 W W Scott, “Standard Methods of Chemical Analysis,” New York, 2nd Edition, 1917, 487. a Chem. News, 116 (1917), 295, 308. 4 J . B i d Chem., 38 (1919), 191. 6 Bodansky, Ibid , 44 (1920), 399. 6 Ibrd., 44 (19201, 99. 7 THIS JOURNAL, 13 (1921), 153.
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TEXAS,GALVESTON, TEXAS
tissues, it became necessary to study the precipitation of zinc sulfide in the presence of calcium citrate and to determine the applicability of such a method. PRECIPITATION OF ZINC SULFIDE I N PRESEXCE OF CALCIUM CITRATE ZINC SULFIDE SUSPENSION-For preliminary experiments, a zinc sulfide suspension was prepared according to the method of Mul1er.I One gram of zinc was treated with sufficient sulfuric acid to dissolve it, and diluted with 30 cc. of glycerol and 100 cc. of distilled water. Fivehundred cc. of ammonium sulfide solution were then added, and the whole diluted to 1 liter and allowed to stand for several days. One hundred %c.of Muller’s suspension and 50 cc. of glycerol were gradually diluted to 1 liter with hydrogen sulfide water, shaking vigorously after each dilution. The suspension of finely divided zinc sulfide prepared in this way was not permanent, but when diluted with water, preparations were obtained which resembled very closely the zinc suifide suspensions that are frequently formed during a precipitation. To determine qualitatively the degree of subdivision of the eine sulfide particles, 10-cc. portions of the suspension, equivalent to 1 mg. of zinc, were filtered cold through four grades of filter papers, 9 cm. in diameter; the residues were carefully washed with small portions of 2 per cent ammonium thiocyanate2 and the amounts of zinc in the residues as well as in the filtrates were determined turbidimetrically. The results given in Table I show that only from 30 to 60 per cent of the zinc sulfide particles in the suspension are retained by filter papers of good make, such as are frequently used in qualitative and quantitative analysis. The limitations of the turbidimetric comparison do not permit of a closer approximation than that given in the table. TABLE I-RBTRNTION OF ZINC
SULFIDE SUSPENSION BY STANDARD FILTER PAPERS (10cc. zinc sulfide suspension I1 mg. Zn filtered cold) Zinc Recovered Filter Mg. Residue Filtrate Total Paper 1 0.39 0.60 0.99 1.03 0.60 0.43 2 0.57 1.02 3 0.45 4 0.31 0.75 1.06 ADDITION OF CALCIUM CITRATE-BY using varying dilutions
of the zinc sulfide suspension, with and without the addition of calcium citrate, and going through the usual analytical procedure, comparative data have been obtained on the adsorptive (?) effect of calcium citrate. Four series of determinations were performed on definite volumes of the suspension, varying in zinc content from 0 . 1 to 10 .O mg. In the experiments of Series I, measured portions of the suspension were diluted to 125 cc. After boiling these solutions for 15 min., a rapid stream of hydrogen sulfide was passed through the liquids until they had cooled to about room temperature. After allowing the precipitates to settle for 2 hrs., the flasks were heated on the water bath for 30 min. The precipitates were collected on small filter^,^ washed with 2 per cent ammonium thiocyanate, dissolved in hot hydrochloric acid, and the zinc was estimated turbidimetrically in the total volume or in an aliquot portion. In Series 11, the suspensions were diluted to 115 cc. and treated with 10 cc. of 50 per cent citric acid. In Series 111, 10 cc. of citric acid solution and 4 g. of calcium citrate, prepared from Merck’s calcium carbonate and citric acid, were added in each experiment. In Series IV, the dilutions were made 1 2
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Chem.-Zlg., 28 (19041, 35. Compare Jamieson, THISJOURNAL, 11 (1919), 323. Whatman No. 40.
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T H E JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
to 109 cc., and there were added 16 cc. of citric acid solution and 2 . 1 g. of calcium carbonate. I n these experiments the calcium citrate precipitated as the solutions were boiled, the amounts separating approximating the undissolved portion& of calcium citrate obtained in the experiments of Series 111. The same procedure as detailed for Series I was closely adhered to in all the experiments. An examination of the results in Table I1 shows that the presence of calcium citrate facilitates the complete recovery of zinc as the sulfide, and that this is especially marked in the case of quantities less than 2.5 mg. Where zinc is present in higher concentrations complete recovery may be obtained in the absence of calcium citrate, as is indicated by the results obtained with 10-mg. quantities. This is probably due to the formation of a number of coarse zinc sulfide particles sufficient to adsorb the finer ones. It should also be noted that the recovery of the zinc sulfide is somewhat more complete when the calcium citrate is precipitated in the solution than when added pre-formed. TABLE 11-RECOVERY OF FINELY DIVIDEDZINC SULFIDE IN THE ABSENCE AND IN THE PRESENCE OF CALCIUM CITRATE Zinc Present Mg 0.10 0.20 0.30 0.40 0.50 I .oo 2.50 10.00
Series I
....
0.06 0.12 0 . I6 0.16 0.75 2.20 8.96
Zinc Found Mg. Series I1 Series I11 Series IV 0.08 0.11 0.15 0.06 0.18 0.26 0.28 0.09 0.36 0.21 0.39 0.48 0.20 0.47 0.99 0.88 1.00 2.20 2.40 2.28 10.20 10.40 10.32
.....
EFFECT OF IRONSALTS As is known, zinc sulfide, p.recipitated in the usual way, may retain traces of iron, and it was to be expected that the retention of this metal would be even greater in the presence of calcium citrate. The presence of iron in the solution to be used for the turbidimetric estimation should be avoided, because in the presence of ferrous salts there is produced potassium ferri-ferrocyanide which imparts a blue color to the solution. I n our analyses of animal tissues known to contain appreciable quantities of iron, the interference due to this metal was not always experienced, and when i t did occur one reprecipitation usually sufficed to make the final filtrate iron-free. To test the method further and to study the removal of iron salts, the experiments recorded in Table 111 were performed. I n the experiments of Column A the zinc was precipitated as the sulfide from a citric acid solution. I n those of Column B the precipitations were performed in the presence of 5 mg. of ferric chloride and sufficient calcium carbonate to give on boiling about 1 g. of calcium citrate. In Expts. lB, 3B, and 5B, two precipitations, and in 2B and 4B, three precipitations were required to remove the iron. As will be observed, even under these conditions the zinc was almost entirely recovered. TABLE111-COMPARISON BETWEEN EXPT. N O
1 2 3 4 5
THE O R ~ D I N AMETHOD L MODIFIED PROCEDURE Zinc Recovered Mg. Zinc Added A B Mg. Original Modified 0.10 0.03 0.1 0.17 0.09 0.2 0.12 0.28 0.3 0.47 0.33 0.5 0.65 0.9s 1.0
AND
THE
SUMMARY OF MODIFIEDMETHOD The weighed material, if organic in nature, is oxidized with sulfuric acid, then with nitric acid, ashed, and the ash is repeatedly extracted with hot dilute hydrochloric acid. After filtration the combined extracts are evaporated to dryness. The residue is dissolved in 2 cc. concentrated hydrochloric acid and 50 cc. of distilled water. The copper is precipitated as the sulfide, and removed by filtration. The filtrate, containing the zinc, is boiled to expel the hydrogen sulfide, cooled, neutralized with ammonium hydroxide
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and treated with 10 cc. of 50 per cent citric acid. The resulting solution is again heated to boiling and, if no calcium citrate separates, small quantities of calcium carbonate are added until a precipitate of about 1 g. of calcium citrate is formed. A rapid stream of hydrogen sulfide is then passed through the liquid until the latter has cooled. The flask is allowed to stand for several hours, part of the time on a water bath, until the supernatant liquid is clear. The precipitate is collected on a filter, washed with 2 per cent ammonium thiocyanate, and dissolved on the filter paper with hot dilute hydrochloric acid, the filtrate being collected in the flask in which the precipitation occurred. The a p pearance of a reddish color due to ferric thiocyanate indicates the presence of iron, in which case the zinc should be reprecipitated. A turbidity due to colloidal sulfur may be dispelled by boiling the solution. If clear and colorless, the solution is ready for the turbidimetric comparison. For this purpose the solution or a n aliquot portion is diluted with water to 45 cc. in a 50-cc. Nessler jar. Other Nessler jars are prepared, containing definite volumes of standard zinc solution ( 1 cc. =z= 0. $ mg. Zn), 3 cc. of concentrated. hydrochloric acid, and distilled water to make 45 cc. It is important that the Nessler jar containing the unknown, should have an equivalent volume of concentrated acid, Five cc. of potassium ferrocyanide solution (34.8 g. per liter) are then pipetted into each jar. The liquids are! quickly mixed, and the turbidities are compared by viehiving longitudinally when the jars are held over a sheet of fine print. More of the standard zinc solution may be added from a buret to the jar having a turbidity which approximates the turbidity of the unknown most closely, until the turbidities in the two jars are identical.'
CONCLUSION I n our hands the modified method has been found more satisfactory than any other method for the estimation of less than 5 mg. of zinc. Even with larger quantities, 10 to 20 mg., this procedure compares favorably with other standard methods. ACKNOWLEDGMENT Acknowledgment is due to Professor W. C. Rose of this laboratory for his helpful suggestions and advice.
Platinum Theft The Sierra Magnesite Co., of Porterville, Cal., reports a platinum theft from its laboratories some time during the night of June 14, 1921. Three platinum crucibles, weighing a s follows, were taken. No.
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1 2 3
Grams 12.100 12.328 14.307
In addition, one Palau crucible weighing 14.491 grams was taken. Exports of cottonseed oil from the United States in May 1921 amounted to 18,947,796lbs. valued a t $1,332,165, as compared with 11,655,272 lbs. valued a t $2,595,235 in May 1920. This is an increase of 62 per cent in quantity, but a decrease of 48 per cent in value. The total cottonseed oil shipments during the 11-month period ended May 1921 were 269,105,939 lbs. valued a t $30,287,371, as compared with 148,127,351lbs. valued a t $33,652,238 for the same period in 1920. The consumption of cottonseed oil during May 1921 was 307,000 bbls., the largest for any month this season. 1
Zinc borosilicate glassware should not be used in zinc determinations.
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