5i8 CHAPTER XXV 



cedure, 13 grms. sucrose and 5 c.c. of 38 per cent, acid in 100 c.c. was found by 

 him as i • 4266. Other determinations with this acid concentration or with 38 • 8 

 per cent, acid and the same sucrose concentration are 1-4278 (Walker), 1-4288 

 (Tolman), 1-4305 (Steuerwald). With the greater proportion of acid {v. sup.) 

 Steuerwald found 1-4554. These values are in some doubt since the use or 

 othervvise of bichromate-filtered light is not stated. The ver^^ careful and 

 exact determinations of Jackson andGillis'"^ give a value of 1-4325. referred 

 to bichromate-filtered light, 13 grams of sucrose per 100 c.c. and 5 c.c. of 38 -8 

 . per cent, acid ; and this value should be accepted as the most probable. 

 The influence of concentration is noteworthy and the values of the constant 

 given above refer only to that one particular concentration. The rotation 

 of invert sugar falls with dilution and hence also the value of the Clerget 

 constant. 



Concentration of the Sugar^r^A different constant obtains with each 

 different concentration of sugar. In the Herzfeld routine the value of the 



constant is given by the expression 141.84 where i is the direct reading 



in the 20 cm. tube after the inversion, which at a concentration of 13 grams 

 sugar per 100 c.c. gives the value 142.66. In the scheme given below due 

 to Jackson and Gillis, and representative of the latest work, the appropriate 

 value of the constant for each concentration and temperature is given in 

 tabular form. 



Constancy of the Value of y. — The presence of basic lead salts diminishes 

 the rotation of the fructose originally present, which is afterwards restored 

 in the process of inversion, thus giving a variable value of y {v. sup.). Pellet''^ 

 was the first to correct for this, and he acidified the material used for the direct 

 reading with sulphurous acid. This process reads as follows: — Two hundred c.c. 

 of normal weight solution of material are placed in a 220 c.c. flask, clarified 

 with basic lead acetate, completed to 220 c.c. and filtered. One hundred 

 c.c. of the filtrate are treated with 30 c.c. sulphurous acid in saturated 

 solution, made up to 200 c.c, filtered if necessary, and observed. A 

 second portion of the original filtrate from the lead clarification is used to 

 obtain the invert reading. The rotation due to the sucrose is then obtained 

 after making the necessary allowance for dilution and selecting the ap- 

 propriate constant. The method generally used in Java, due to Ter- 

 vooren,^*' makes a similar correction in principle, acidifying the filtrate from 

 the lead clarification with acetic acid. 



These last two methods eliminate a very substantial source cf error, 

 but still do not fulfil the postulate that there be no change in the value of y 

 {v. sup.), since the media in which the direct and inverted readings are made 

 are not the same. An attempt to eliminate this error is due to Andrlik, 

 who proposed to take the direct reading in the presence of urea and hydro- 

 chloric acid, the former body inhibiting inversion long enough to allow an ob- 

 servation to be made ; this method has not found general acceptance. That 

 method which most nearly meets all the conditions necessary for accuracy 

 is the double neutral polarization method first proposed by Saillard.^i In 

 his method a quantity of sodium chloride, equivalent to the hydrochloric 

 acid used in inversion, is added to the material used for direct polarization, 

 and, after inversion, the hydrochloric acid present is exactly neutralized 

 with caustic soda. There thus result two systems similar except for the 



