QUALITATIVE AND QUANTITATIVE ANALYSIS 73 



transferred to a large beaker containing about 3 grams KI and 



10 cubic centimeters strong HCl in aqueous solution. The 



contents are diluted to 500 or 600 cubic centimeters and titrated 



with the thiosulphate. Starch paste is added toward the end 



of the reaction, and at the end point the solution turns from a 



blue to a light green. If the thiosulphate used is pure, the 



volume will be a little less than 25 cubic centimeters, and the 



solution can be readily adjusted to O.IN by dilution with water. 



Thus if the titration is 24.7 cubic centimeters thiosulphate, every 



24.7 cubic centimeters of thiosulphate solution should have added 



to it 0.3 cubic centimeters water, or to 914 cubic centimeters of the 



914 

 remaining solution add ^-^ X 0.3 or 11.1 cubic centimeters of 



water. The resulting solution should be O.liV and can be 

 readily checked against the O.IA^ dichromate. The O.IA^ thio- 

 sulphate solution thus prepared will keep its strength for more 

 than a year. To make 0.005iV thiosulphate, 25 cubic centimeters 

 of the O.liV solution are diluted to 500 cubic centimeters in a 

 volumetric flask and mixed. This solution keeps for only a few 

 days and is best prepared anew for each set of determinations. 



(d) Basic lead acetate, Home reagent — A 33 per cent solution 

 is used. 



(e) Phosphate solution — For removing excess lead, a 10 per 

 cent solution of Na2HP04'12H20 is used. Three cubic centi- 

 meters are required for every cubic centimeter of the lead acetate 

 solution used. Add phenolphthalein and if alkaline or acid 

 neutralize. 



Place 10 or 25 cubic centimeters of culture, depending on the 

 percentage of sugar, in a 50-cubic centimeter volumetric flask, 

 add a few drops of phenolphthalein and neutralize with sodium 

 hydroxide. Add 1 cubic centimeter of lead acetate solution, 

 shake, and then add 3 cubic centimeters of phosphate solution. 

 If alkaline or acid, neutralize, dilute to exactly 50 cubic centi- 

 meters and mix thoroughly by inverting. Let stand for 3 

 minutes and then remove an aliquot for analysis by means of a 

 pipette. If chlorides or other compounds precipitable by lead 

 are present, a little more of the lead acetate solution can be used. 

 An excess of lead acetate is to be avoided for some of the sugar 

 will be carried down with the precipitate. 



