Estimation of Glucose or Cane Sugar

Estimation of Glucose or Cane Sugar in Solution by means of Fehling s Solution 

FeJdmg s Solution consists of two parts. The first, a solution of 69-28 gms. of pure crystalline copper sulphate dissolved in water with the addition of 1 c.c. of pure sulphuric acid, and the whole made up to 1 litre the second, a solution of 350 gms. of Rochelle salt (sodium potassium tartrate) and 120 gms. of sodium hydroxide (purified from alcohol) dissolved in water and made up to 1 litre. Equal volumes of these two parts are mixed just before use. Each c.c. of the resulting solution is equivalent to 0-005 gm. glucose or to 0-00475 gm. cane sugar. The solution deteriorates after a time, and should be standardised frequently against pure glucose or pure cane sugar. 

Standardisation.—Some pure glucose is dried for 12 hours in a vacuum desiccator over sulphuric acid and a solution of known concentration (0-5—1%) made. 

of each part of the Fehling s solution are measured out into a porcelain dish, diluted with 40 c.cs. of dilute caustic soda solution, and gently boiled. Glucose solution, about 1 c.c. at a time, is then run from a burette, and the mixture boiled after each addition until the blue colour is finally discharged. 

The titration is then repeated, all but 2 c.cs. of the volume of glucose solution used in the first determination being run in at once, and the remainder in drops until the blue colour just vanishes. The end point is more easily observed when the dish is slightly tilted. Several determinations are made until concordant results are obtained. If the end point is indistinct, a dilute acetic acid solution of potassium ferrocyanide spotted on a white plate may be used as external indicator. A brown coloration is observed so long as copper is present in solution. 

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