30 TESTING MISCELLANEOUS SUPPLIES. 



110 C., cool, and weigh as lead chlorid. Add 0.0085 gram to the weight of the 

 lead chlorid to correct for its solubility in alcohol. Multiply the result thus 

 obtained by 0.74478 to obtain the amount of lead present. 



(4) CHROMIUM. 



Evaporate the filtrate from the lead chlorid determination to dryness on a 

 steam bath to expel the alcohol, add about 5 cc of hydrochloric acid, then 

 about 50 cc of water, and warm till the solution is complete. Cool, transfer 

 to a volumetric flask, and make up to a definite volume. To an aliquot add 

 sodium peroxid in sufficient amount to render the solution alkaline and to 

 oxidize the chromium to chromate, boil until all of the hydrogen peroxid is 

 driven off, cool, make acid with sulphuric acid, add a measured excess of 

 standard ferrous sulphate and titrate the excess of iron with standard potas- 

 sium dichromate. 



(5) SULPHATE. 



In another aliquot of the filtrate from the lead chlorid determination, deter- 

 mine the sulphate in the ordinary way by precipitation and weigh as barium 

 sulphate. 



(6) TREATMENT IN THE PRESENCE OF SOLUBLE IMPURITIES. 



If the sample contains no soluble impurities, the preceding method will give 

 a complete analysis. When the qualitative test shows calcium it may be neces- 

 sary to make the analysis in a somewhat different manner, for gypsum would 

 by this treatment be precipitated with the lead chlorid and render the deter- 

 mination worthless. In such a case dissolve and separate the insoluble as 

 above. Dilute the filtrate from the insoluble to 400 cc, cool, pass in a rapid 

 stream of hydrogen sulphid until all of the lead is precipitated as lead sulphid, 

 filter, wash with water containing some hydrogen sulphid, dissolve the lead 

 sulphid in dilute nitric- acid (hot), add an excess of sulphuric acid, heat to 

 fumes and complete the determination of lead as sulphate in the usual way. 

 In the presence of chromium the solution must be very dilute and the hydrogen 

 HI 1 1 'hid passed in rapidly to get a good separation of lead as sulphid. Trans- 

 f-r the filtrate from the lead sulphid to a 500 cc flask, add ammonium hydroxid 

 until it is slightly alkaline, pass in hydrogen sulphid until all of the zinc present 

 is precipitated (in case no zinc is present the addition of hydrogen sulphid is 

 nnnet-essary) ; make up to volume, mix, allow to settle, draw off aliquots of 

 the clear liquid, and determine the calcium (and, if necessary, the magnesium) 

 in tin- usual way. All the zinc and chromium will still be in the 500 cc flask; 

 dissolve in hydrochloric acid, make up to the mark, mix, take out an aliquot, 

 and d. -tcrminr the /inc volumetrically by the ferrocyanid method as described 

 under lead /.in- whites. In another aliquot determine chromium as already 

 dMcribed. 



Sulphate ran he determined by dissolving another portion in hydrochloric 

 tilteriim from it the insoluble, and precipitating with barium chlorid from 

 tile dilute hot filtrate; if the solution is kept dilute and hot, the barium sul- 

 phate pn-< -ipitate will not he contaminated by lead. 



<; S( " hroiurs contain harium phosphate instead of lead sulphate, in which 



case test for phosphoric arid, or barium sulphate, calcium sulphate, etc., may 

 replace lead sulphate. Lead citrate is also sometimes present instead of lead 

 sulphate. 



