346 METHODS OF ANALYSIS [Chap. 



23 CALCroM.-TENTATIVE. 



Heat the combined filtrate and washings, obtained in 22, to 50°C. and add an 

 excess of ammonium oxalate solution. Allow to stand in a warm place until the 

 precipitate has settled, filter, wash the precipitate with hot water, dry, ignite over 

 a Bunsen burner and finally over a blast lamp. Cool in a desiccator and weigh as 

 calcium oxid. 



24 POTASSIUM AND SODIUM.— TENTATIVE. 



Evaporate an aliquot of the solution, prepared as directed under 21, nearly 

 to dryness to remove the excess of hydrochloric acid, dilute and heat to boiling. 

 While still boiling add barium chlorid solution so long as a precipitate forms and 

 then enough barium hydroxid solution to make the liquid strongly alkaline. As 

 soon as the precipitate has settled, filter and wash with hot water, heat the filtrate 

 to boiling, add sufficient ammonium carbonate solution (1 part of ammonium car- 

 bonate in 5 of 2% ammonium hydroxid solution) to precipitate all the barium, 

 filter and wash with hot water. Evaporate the filtrate to dryness and ignite the 

 residue below redness to remove ammonium salts. Add to the residue a little 

 water and a few drops of ammonium carbonate solution. Filter into a tared plat- 

 inum dish, evaporate, ignite below redness and weigh the mixed potassium and 

 sodium chlorids. 



Determine potassium in the mixed chlorids as directed in I, 45, beginning with 

 "Digest the residue with hot water, filter through a small filter". Calculate the 

 potassium so found to its equivalent of potassium chlorid and subtract this from 

 the weight of the mixed chlorids to obtain the weight of sodium chlorid. 



25 PHOSPHORIC ACID.-OFFICIAL. 



Mix 5 grams of the sample with a little magnesium nitrate solution, dry, ignite, 

 dissolve in dilute hydrochloric acid and dilute the solution to a definite volume. 

 In an aliquot of the solution determine phosphoric acid as directed under I, 6 or 9. 



26 SULPHURIC ACID^i.-TENTATIVE. 



Boil 5 grams of the sample gently for 1^ hours with a mixture of 300 cc. of water 

 and 15 cc. of concentrated hydrochloric acid. Dilute to 500 cc, draw off a 100 cc. 

 aliquot, dilute considerably, precipitate with 10% barium chlorid solution, filter 

 the precipitated barium sulphate on a Gooch, wash with hot water, dry, ignite and 

 weigh. 



27 AMMONIA.-TENTATIVE. 



Introduce 2 grams of the sample into a distillation flask, add 300-400 cc. of water 

 and an excess of sodium hydroxid solution, connect with a condenser and distil into 

 a measured amount of standard acid. Titrate the excess of acid in the distillate 

 with standard alkali, using methyl red or cochineal as an indicator. 



Ammonia alum is often an ingredient of cream of tartar substitutes and baking 

 powders, and ammonium carbonate is- occasionally present in baking powders. 



LEAD. 



Method I. Colorimetric Method^^. — Tentative. 



(Applicable in the absence of alum and phosphates. Approximate method for 



preliminary work.) 



28 REAGENTS. 



(a) Sodium bisulphite solution. — Dissolve 10 grams of anhydrous sodium car- 

 bonate in sufficient water to make 100 cc. and pass sulphur dioxid into the solution 



