Aug., 1921
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. In our analyses of animal tissues known to contain appreciable quantities of iron, the interference due to this metal was not always experienced, and when it 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. In 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
697
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 an 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.
......................... ......................... .........................
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.
*
