638
T H E J O U R N A L O F I N D U S T R I A L A N D EA’GIhlEERIA’G C H E M I S T R Y .
tion, 30-50 cc. conc. HC1, and about two grams of tartaric acid. Never omit the tartaric acid, which serves t o hold the antimony in solution. Insert a rubber stopper having a small swan-neck thistle tube in it to condense and retain any iodine vapors t h a t might be evolved in heating. Heat the flask almost to boiling for a few minutes, whereupon the sulphides are changed t o iodides: SnS 41 = SnI, S SnS, 41 = SnI, zS SbS, 31 = SbI, 3s Sb,S, 61 = 2Sb1, 5s As will be seen, the tin is completely oxidized, whereas the antimony goes only t o the triiodide form. COO1 the flask, wash out the bulb tube and dilute somewhat. Titrate the excess of iodine with a tenth-normal solution of sodium thiosulphate, using a starch indicator. Now wash, Or filter (if a s h a v e r end Point is desired), ‘into a beaker, neutralize with Na,CO, and add a n excess of powdered sodium bicarbonate. NOWtitrate with a tenth-normal solution of iodine (Mohr’s method). I n this alkaline solution the antimony triiodide is oxidized t o the quinquivalent condition: SbI, I, = SbI,. From this last titration the amount of antimony Present can be calculated. MultiPlY the volume of iodine solution used in this titration by 3/2 and subtract this from the volume of iodine consumed in the first titration. This will give the volume of iodine consumed by the tin alone. I cc. 0 . I IY iodine = 0.006 g. Sb. SbI, I, = SbI,. SnS, I, = SnI, 25. I cc. 0.1 N iodine = 0 . 0 0 2 9 8 g. Sn. This method. of getting the tin and antimony b y the titration of the sulphides is, as far as I know, original and for it I can claim several advantages. One is t h a t the antimony is p u t in just the stage of oxidation t h a t is required for Mohr’s titration. Another is t h a t i t does not matter in what condition of valency the sulphides are, even free sulphur does not interfere. This fact was ascertained by taking two samples of tin of 0.1 gram each, dissolving one in cone. HC1, forming SnCl,, and the other in HC1 + ”o,, forming SnCI,, Analyzing as above, gave almost identical results. The separation of the insoluble sulphides from the absolute b y tin and antimony can be made dissolving these sulphides of cu, Pb, pe and zn in of NaOH and hot HC1 and reprecipitating by this manner any traces of Sb or Sn that N~,s. may have been retained by the first precipitate are removed and added t o the main filtrate. T o test the above method of titrating the sulphides of iodine, I have weighed out samples of by pure tin and run them through with this method and have obtained the following close results, using the theoretical tin factor for the iodine:
+ + + +
+ + + +
+
+
+
Tin taken. Gram. 0.0664 0.0702 0.0923 0.1132
Tin obtained. Gram. 0.0669
0.0705 0.0922 0.1135
+
Sept., 1911
To further test the method, known amounts of pure copper, lead, tin and antimony were dissolved together and tested in the above manner with the following result: Taken. Gram.
Obtained. Gram.
Copper. . . . . . . . . . . 0.2995 Lead. . . . . . . . 0.2990 Tin.. . . . . . . . . . . . . . 0 , 0 9 4 6 -4ntimony. . . . . . . . 0.0942
______
0.2996 0.2997 0,0953 0.0936
THE VOLUMETRIC CHROMATE DETERMINATION O F LEAD. By
JOHN
WADDELL.
Received July
IO.
1911
In the Transactions of the American Institute of Mining Engineers, Vol. 35, page 359 (I905), Mr. H. A. Guess gives a method for determining lead in ores. Though i t can be used for nearly all kinds of ores, i t is most satisfactory in just those cases for which the results obtained by the molybdate method are least reliable, namely, with ores poor in lead and containing a large amount of calcium carbonate. Guess claims that the method is easily carried out and 1 understand that it is employed at the ill Smelter in British Columbia. It does not seem, however, t o have become a t all general. I have not found it in any of the books on quantitative analysis nor have I seen i t described in any of the chemical periodicals, ~ u ~ describes s s the method as f o ~ ~ o m s : “ T o the ore charge of from I to grams in a 2 5 0 cc. flask add from 3 t o 5 cc. of strong nitric acid and 15 cc. of strong hydrochloric acid; digest until everything is in solution and the excess of acid has been reduced to about 8 CC. The whole operation on the hot Plate requires b u t 1 5 minutes. The flask is then removed, and slightly diluted ammonia is added SlOWlY in slight excess, the neutralizing action being SUfiCiently vigorous to render the contents of the flask quite hot. Acetic acid of 80 per cent. strength is then added slowly, the flask being shaken vigorously until the smell indicates a decided excess of acetic acid. Then 5 CC. of strong ammonium acet a t e are added t o insure the solution of any lead compounds remaining undissolved by the ammonium acetate already formed in the flask. If the ore contains no antimony or separated gelatinous silica and if the siliceous residue in the bottom of the flask is only in slight amount (as is usual with heavy lime Ores or with concentrates) add t o the hot, undiluted and unfiltered solution an excess-about 1 0 . cc.-of a ten per cent. potassium chromate solution. Under these conditions.the bulk of the contents Of the flask will not exceed 5 0 CC.7 and after shaking and letting the precipitated lead chromate settle for about five minutes the contents are filtered Of any rapid and close through an I I cm. paper. If these directions are carried out the lead chromate will be quite granular and will show no tendency t o run through. The precipitate in the flask and on t h e filter is washed several times with hot water containing about 0.5 per cent. of acetic acid until free from soluble chromates. The funnel
Sept., 1 9 1 I
T H E JOUR,VAL OF I.VDUSTRI.4L
A N D EHGINEERING C H E A T I S T R Y .
with the filter is then set over the original flask and hot, dilute hydroch1o;ic acid ( I : I ) poured through the filter, dissolving the lead chromate. Further additions of hydrochloric acid are made if necessary and all lead chromate is dissolved from the filter; then i t is washed with warm water until free from chromate. The original flask now contains nothing b u t the hydrochloric acid solution of the lead chromate and the washings which, after adding a small crystal from 0 . .j t o 2 grams in weight of potassium iodide, is titrated direct n-kh standard “ h y p o ” solution whose value is known i n terms of lead, the m’sst suitable strength being t h a t in which I cc. is equal t o 5 mg. of lead. I n this operation, b y using only a small quantity of potassium iodide and having the solution fairly strong with hydrochloric acid (about 50 cc. of hydrochloric acid I : I in a total cmf zoo cc. of solution) and somewhat warm, a n y tendency of the lead t o form yellow scales of lead iodidz and thus somewhat obscure the end reaction with starch is completely checked and the end reaction is sharp. The sharpness of the end point with starch, as opposed t o the indistinctness of the end point in the molybdate method, is given as one of the special features. Students working in my own and other laboratories have found difficulties with this method. Sometimes the blue color of the starch disappeared on addition of thiosulphate, only t o come back in a minute or t w o ; in other cases the precipitate of lead chromate seemed not to be washed free from soluble chromate in the course of an hour or more ; somctirnes even when no error was visible the results were not
