CH. V.] 



METHOD OF AMOS PETERS. 



135 



means of a solution of sodium thiosulphate. From the 

 amount of copper reduced the amount of glucose in the 

 volume of solution taken can be determined. 



Solutions required. 



1. Copper sulphate. 69-278 grams, of the purest crystalline salt CuSO 4 , 

 5H 2 O, is dissolved in water and the volume made up to i litre. 



2. Alkaline tartrate. 346 grams, of Rochelle salt and 250 grams, of pure 

 potassium hydroxide are dissolved in water and the volume made up to i litre. 



3. Sodium thiosulphate. 99-2 grams, of the purest thiosulphate are 

 dissolved in boiled out distilled water and the volume made up to i litre with 

 boiled out distilled water. It should be prepared at least a week before it is 

 standardised. 



4. Potassium iodide. Saturated solution. 100 grams, of the solid are 

 treated with 70 cc. of hot distilled water and the solution allowed 'to cool. 



5. Soluble starch. Shake i gram, of soluble starch (see p. 391) with about 

 10 cc. of distilled water and pour the suspension into 90 cc. of boiling water. 



Standardisation of the thiosulphate. Measure 20 cc. of 

 the copper sulphate into a 200 cc. Erlenmeyer flask. Add 40 cc. 

 of distilled water and 20 cc. of strong (33 per cent.) acetic acid. 

 Insert a thermometer and cool or warm to 20 C. Run in about 

 6-5 cc. of the saturated potassium iodide, the thermometer being 

 withdrawn and its stem washed with this solution. The iodine 

 liberated is titrated at once with the thiosulphate. When approach- 

 ing the end point add about i cc. of the soluble starch. The colour 

 changes to a chocolate brown when very near the end point. This is 

 best determined by the " spot test " method. Allow a drop of the 

 thiosulphate to fall on the quiet surface of the liquid. If the end 

 point has not been reached, a very perceptible white area is seen 

 around the drop. This is very readily distinguished from the 

 diminution of the slightly yellowish colour of the suspended cuprous 

 iodide. The volume of the drop delivered by the burette must be 

 deducted from the total volume added. 



The copper value of the thiosulphate is calculated as shewn in 

 the following example : 



20 cc. of the copper sulphate = 352-93 mg. Cu. 

 This required 27-6 cc. of thiosulphate. 



So i cc. of thiosulphate = 



27-6 



= 1278 mg. Cu. 



