rV] WATERS 47 



Add some solid ammonium nitrate and heat in the water bath at a temperature of 

 45*-50°C. Add the molybdate solution and keep at the above temperature for 30 

 minutes. The yellow precipitate formed at this point appears generally only in 

 traces; if more than traces are present, filter and wash with cold water until entirely 

 free from nitric and molybdic acids. Transfer the precipitate and filter to a beaker, 

 add a little water, and beat the paper and contents to a pulp. Dissolve the yellow 

 precipitate in a small amount of the standard potassium hydroxid ; add phenolphtha!- 

 ein and titrate with the standard acid. From the data so obtained calculate the 

 phosphoric acid ions in the water to milligrams per liter. 



52 Manganese. Iodin, Bromin, Arsenic and Boric Acid. 

 Evaporate large quantities of water to dryness, after the addition of small amounts 



of solid sodium carbonate. Boil the residue thus obtained with water, transfer to 

 a filter, and wash thoroughly with hot water. Make the alkaline filtrate up to a 

 definite volume. 



MANGANESE.— OFFICIAL. 



53 REAGENTS. 



(a) Dilute nitric acid {1 to 1). 



(b) 0.2% silver nitrate solution. 

 (C) Ammonium persulphate. 



(d) Standard manganous sulphate solution. — Dissolve 0.2877 gram of pure potas- 

 sium permanganate in a small amount of water, add an excess of sulphuric acid, 

 reduce carefully with oxalic acid and make up to 1 liter. One cc. of this solution is 

 equivalent to 0.1 mg. of manganese. 



54 DETERMINATION. 



Dissolve the insoluble residue under 52 in an excess of the dilute nitric acid, evap- 

 orate to dryness, treat with water, add about 1 cc. of strong nitric acid and a little 

 of the silver nitrate. If a precipitate of silver chlorid appears, add more of the 

 silver nitrate until all the chlorin is precipitated. Add an excess of about 10 cc. of 

 the silver nitrate for each mg. of manganese present in the sample. Filter, add 

 1 gram of ammonium persulphate to the filtrate, and place the beaker or flask con- 

 taining the solution on the steam bath until a pink color develops (usually about 20 

 minutes). Compare the color developed with standards similarly prepared by treat- 

 ing solutions containing known amounts of the standard manganous sulphate with 

 nitric acid, silver nitrate, and ammonium persulphate. 



IODIN AND BROMIN.— TENTATIVE. 



55 REAGENTS. 



(a) 10% sodium hydroxid solution. 



(b) Sulphuric acid {1 to 5). 



(C) ;?% potassium, or sodium nitrite solution. 



(d) Carbon disulphid.— Freshly purified by distillation. 



(e) Chlorin water. — Saturated and freshly prepared. 



56 DETERMINATION. 



Evaporate to dryness an aliquot of the alkaline filtrate under 52, add 2-3 cc. of 

 water to dissolve the residue and enough 95% alcohol to make the percentage of 

 alcohol about 90. This precipitates the chlorids. Heat to boiling, filter and repeat 

 the preceding solution and precipitation once or twice. Add 2 or 3 drops of the 



