34 LA BORA TOR Y ME THODS OF UNI TED S TA TES ARMY 



caused by the excess of chlorine introduced into the solution. 

 If the final solution fails to give a momentary flash of color 

 with alcoholic solution of phenolphthalein it should be 

 rejected. If the solution shows color with powdered phenol- 

 phthalein it must be titrated with boric acid as described 

 above, for preparation from Bleaching Powder, until this 

 defect is corrected, or it must be discarded. The solution 

 should be titrated for hypochlorite concentration every 

 twenty-four to forty-eight hours and discarded when it falls 

 below the desired lower limit (usually 0.45 per cent.). 



If a chlorine meter is not available, chlorine gas may be run 

 into the 1.5 per cent, carbonate solution through any impro- 

 vised diffuser until the hypochlorite concentration has reached 

 0.5 per cent. The amount of chlorine required to give a hypo- 

 chlorite concentration of 0.5 per cent, is approximately twice 

 the amount required to cause decolorization of powdered 

 phenolphthalein. It is therefore convenient to add powdered 

 phenolphthalein and note the amount of chlorine required to 

 cause its decolorization. When almost twice that amount of 

 chlorine has been introduced, frequent titrations of the hypo- 

 chlorite content must be commenced to determine the proper 

 point at which to stop the introduction of the chlorine. 



TlTRATION OF DAKIN's SOLUTION. To IO C.C. of the 



Dakin solution, add approximately 5 c.c. of a 10 per cent, 

 solution of potassium iodid and 3 c.c. of glacial acetic acid, 

 lodin is liberated and dissolves in the excess of iodid present. 

 Dilute with water to about 50 c.c. A standard r ^ thiosulphate 

 solution is then added from a burette until the solution is 

 just rendered colorless. The number of cubic centimeters 

 required to effect this result multiplied by the factor 0.0372 

 gives the percentage of sodium hypochlorite present in the 

 Dakin solution. 



PREPARATION OF STANDARD -^ THIOSULPHATE SOLUTION. 

 Dissolve exactly 24.82 grams of pure crystalline sodium thio- 

 sulphate in water and make up to exactly i liter. One c.c. of 



