76 METHODS OF ANALYSIS [Chap. 



by 2, gives the number of cc. of N/10 ammonium sulphocyanate corresponding to 

 the potassium cyanic! used by the formaldehyde. Calculate the per cent of for- 

 maldehyde present U cc. of N/10 ammonium sulphocyanate is equivalent to 3 mg. of 

 formaldehyde (HCHO)). 



LIME-SULPHUR SOLUTIONS.^i 



TOTAL SULPHTJR.-OFFICIAL. 



68 PREPARATION OF SOLUTION. 



Weigh 10 grams of the solution and dilute to the mark in a 250 cc. graduated 

 flask with recently boiled and cooled water. 



69 DETERMINATION. 



Transfer a 10 cc. aliquot to a 400 cc. beaker, add about 3 grams of sodium peroxid, 

 cover immediately with a watch glass and warm on the steam bath, with frequent 

 shaking, until all the sulphur is oxidized to sulphate, adding more sodium peroxid 

 if necessary. Dilute, acidify with hydrochloric acid, evaporate to dryness, treat 

 with water acidified with hydrochloric acid, boil, and filter to remove silica, if pres- 

 ent. Dilute the filtrate to 300 cc, add 50 cc. of concentrated hydrochloric acid," 

 heat to boiling, and precipitate with 10% barium chlorid solution slowly and stir- 

 ring constantly. (The rate is best regulated by attaching a suitable capillary tip 

 to the burette containing the barium chlorid solution.) Evaporate to dryness on 

 the steam bath, take up with hot water, filter through a quantitative filter paper, 

 wash until free from chlorin, ignite and heat to constant weight over a Bunsen 

 burner. Calculate the sulphur from the weight of barium sulphate. Previous to 

 use test the reagents for sulphur and, if present, make corrections accordingly. 



SULPHID SULPHTJR.-OFFICIAL. 



70 REAGENT. 



Ammoniacal zinc solution. — Dissolve 50 grams of pure zinc chlorid in water, add 

 ammonium hydroxid in sufficient quantity to redissolve the precipitate first formed, 

 then add 50 grams of ammonium chlorid^' and dilute to 1 liter. 



71 DETERMINATION. 



Dilute 10 cc. of the solution, prepared as directed under 68, to about 100 cc. and 

 add the ammoniacal zinc solution until the sulphid is all precipitated, indicated by 

 the addition of a drop of the clear solution to a few drops of nickel sulphate solution. 

 Filter immediately, wash the precipitate thoroughly with cold water and transfer 

 it and the filter paper to a beaker. Cover with water, disintegrate with a glass rod 

 and add about 3 grams of sodium peroxid, keeping the beaker well covered with a 

 watch glass. Warm on the steam bath with frequent shaking until all the sulphur 

 is oxidized to sulphate, adding more sodium peroxid if necessary. Make slightly 

 acid with hydrochloric acid, filter to remove shreds of filter paper, wash thoroughly 

 with hot water, and determine the -sulphur in the filtrate exactly as under 69. 



72 THIOSULPHATE SULPHTJR.-OFFICIAL. 



Dilute 50 cc. of the solution, prepared as under 68, to about 100 cc. in a 200 cc. 

 graduated flask. Add a slight excess of the ammoniacal zinc chlorid and dilute to 

 the mark. Shake thoroughly and filter through a dry filter. To 100 cc. of the fil- 

 trate add a few drops of methyl orange and exactly neutralize with N/10 hydrochloric 



