538 CHAPTER XXVI 



compound occurs. Neutral lead acetate is frequently specified to be used 

 as a clarificant followed by the removal of the excess of lead by sodium 

 carbonate, oxalate or sulphate. The use of the first-named salt is irrational 

 since a basic lead acetate will be formed resiilting in the precipitation of 

 reducing sugars. In addition, Meade and Harris^^ have shown that the 

 results of the analysis are affected by the quantity of neutral lead acetate 

 used, and also by the de-leading agent employed. They recommend, instead, 

 that kieselguhr should be employed as the clarificant in quantity sufficient 

 to give a clear filtrate. To this recommendation the writer would add that 

 alumina cream or intra-precipitation of alumina is equally efficacious. 



Standardization of Solution. — The quantity of copper reduced depends 

 on the exact composition of the solution, particularly on the amount of alkaU 

 present. It also depends on the time of boiling and even on the surface 

 area of the beakers in which the reaction takes place. Every fresh prepara- 

 tion of copper and alkaline tartrate should therefore be standardized under 

 the precise routine of the analyst against a pure preparation of glucose 

 or invert sugar. From the results of the standardization a correction may 

 be applied to the quantity of reducing sugar, as found from the correspondence 

 in the table employed. For example : — The analyst has found that with his 

 preparations and routine (all intended to conform with those of Munson and 

 Walker) 0-1140 gram invert sugar corresponds to 0-2203 gram copper, 

 the value found by Munson and Walker being 0-2176 gram copper; he 

 should therefore in subsequent analyses with this stock material multiply 

 the weight of copper found by 0-988 before using the correspondence in 

 Munson and Walker's tables. 



Direct Volumetric Methods. — The original process for the determination 

 of reducing sugars was a volumetric one, and as such it is described in the 

 older textbooks. It was also accepted that no correction was necessary for 

 variation in the concentration of the reducing sugar solution, or for the pres- 

 ence of cane sugar. Neglect of these points tended to discredit volumetric 

 processes. Ling,^*' however, has always supported the use of such, and in 

 conjunction with Rendle and with Jones has arranged tables for the correc- 

 tion of the errors so introduced. His treatment of the question is followed 

 below. 



Solutions Required. — {a) 69-3 grams CUSO45H2O in 1,000 c.c. {b) 142 

 grams caustic soda and 346 grams Rochelle salt in 1,000 c.c. 



For the analysis 5 c.c. each of the above are mixed immediately before use. 

 A solution of i-o gram of ferrous ammonium sulphate and 1-5 gram of 

 ammonium sulphocyanide in 10 c.c. water and 25 c.c. hydrochloric acid is used 

 as indicator. This solution is decolorized if necessary with zinc dust, and is 

 preserved out of contact with air. The treatment with zinc dust may be 

 repeated if necessar}^ but eventually it will be necessary to make up a fresh 

 stock. In the presence of cupric salts an intense red coloration is produced. 

 The analysis is performed by adding gradually the reducing sugar solution 

 from a burette to the boiling Fehling solution. The reducing sugar solution 

 should contain not less than o-i or more than 0-25 gram reducing sugar. 

 The approach of the complete reduction of the cupric salt is indicated by 

 the waning blue colour of the solution. When this is no longer distinctly 

 blue, a drop of the unfiltered liquid is withdrawn by a glass rod, placed on a 



