ON THE VOLUllETRIC DETERIllNATlON OF LEAD. BS ALLERTON S . C U S H M A N
AND
J . HAYESCAMPBELL.
Received September 16. 1895.
F
R E S E N I U S in the last edition of his Quantitative Analysis commenting on the volumetric determination of lead, says : ‘ I Although there is no lack of proposed methods for the volumetricestimation of lead, we are still without a really good method for practical purposes, that is, a method which can be generally employed, and which is simple and exact.” Among the methods which have been proposed, one which has been so much used is that of Schwartz,’ briefly outlined. This depends upon precipitating the lead as chromate in a sodium acetate solution with a standard solution of potassium bichromate, the end point being determined by an outside indicator consisting of drops of a neutral solution of silver nitrate on a porcelain plate. It is exceedingly difficult to decide when the first red tinge makes its appearance in the indicator owing to the yellow color of the precipitated lead chromate. T h e end point is therefore frequently overrun. W. Diehlel modified the method by titrating the excess of bichromate in acid solution with sodium thiosulphate, the end point being indicated by the disappearance of the yellow color of the bichromate. In our experience this modification does not lessen the difficulty of determining accurately the end point, owing to the fact that the yellow color gradually shades off into a green in case a fair excess of bichromate has been added. These coilsiderations led us to endeavor to modify the original method in such a manner as to secure a simple and accurate means of determining the excess of bichroinate present. This we accomplish by titrating the solution after filtering off the precipitated lead chromate, with a standardized solution of ammonio ferrous sulphate, using potassium ferricyanide as an outside indicator under exactly the same conditions observed in standardizing bichromate solutions. T h e bichromate solution is made up of convenient empirical strength, and standardized against a weighed amount of pure dried amnionio ferrous sulphate. Slightly more than the equivalent weight of the latter salt is then 1 2
Dingl. poly. Jour.,1%. 284, Zlschr. ana[. Chem., 1880,306.
902
ilLLERTON S. C U S I I M A S .\SDJ . ll.hTES-ChJII'I~:-:I.I..
weighed out aiid dissolved in a liter of water bvitli tile acldition of a few drops of sulpliuric acid. T h e solution is transferred to a stock bottle into which is iiiiiiiediate!). poured :i suficit-nt qumtit,- of some light paraffin oil to form %; layel- o \ y r :lie solutioIi, thus protecting it from oxidation. '1'11~: stock 1)ottle i- fitted with ;t siplioii tulle :tiid piiiciicock so t h a t the d i i t i o i i C U I 1 ) ~ tlra\vii out irlieii needed. li7itli t?iis :irraiig~iiieiit c1i;iiige i n strength of the ;tiiiiiiotiio ferrous sulpiinte soliitic,ii t a k c s place very slowly, w!iiIc :I+ a i e ~ viiioiiii'iits c~iii!. a r e ityniretl to titrate it against the standard biclrrt~iii:itc,it, cs:tc:t >ti.cllgtli I . * < L I I i n . easily cieteriiiiiied from da\. to tl:i\ 111 order to test this iiioclification, we tiecitleci tic try it agaiiist other technical niethotls receiitll- proposed. a s \veil 2s ;igaiiist :t statitlard graviiiietric aiial!,si-. L': well iiiisetl stiiiple oi ;I crystallizetl galeiia coiitainiiig ( J l l i > - 3 little silicri 3' iiiiptirit!, 1\35 first aiia1yxt.d by the iiiethod of Rose :I: give11 i i i I:resetiius, Quantitative .iiial!.si.s. T h e 1e:iti is precipitated 3 s the s d p h i d r , with the proper precautions. the sulphitle i b then drieti. ignited geiitiy in a current of li)-clrogeii. a i i d \weig!ied. Tlie follon.iiig results ivere olitniiietl I
Z.o(XX,
3
I
.gS;o 1.9856
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,9856
99..;5 99.28 99.3j
86.08 86,t)o S6.C1 6
T h e next series of results ~ r e r cobtained 1))- the nietliocl oi Albert H. Lo\\..' Briefly, the iiietliotl consists i i i decccniposiirig the ore with nitric and sulphuric acids, adding fiirther an excess of sulphuric acid, dissolving tlie lead sulphate i n :I saturated solutiori of amtnoiiiuiri chloride and precipitating iiietallic lead 1)). means of strips of aluminuiii. T h e precipitated lead sponge is scraped off, pressed into a button, dried aiid irciglied. R!. this method the following results were obtained NO.
Wright taken Grams.
i,
