5i6 CHAPTER XXV 



The Determination of Sucrose as opposed to Polarization. — It only 

 occasionally happens that sucrose is the sole optically-active body present 

 in a material presented for analysis. Should other active bodies be present 

 they will be returned as sucrose with an influence either positive or negative. 

 The influence of such adventitious bodies may be eliminated by the following 

 procedure developed by Clerget''^ at the instigation of Biot. Let x be the 

 rotation due to sucrose and let y be that due to other active bodies. Then, 

 if d, be the direct polarization, d =x-\-y. Let an operation be made on 

 X changing the value of a; to a x, the value of y remaining unchanged. If i 

 be the reading now observed in the polarimeter, i = a x -\-y. Subtracting 

 this second equation from the first, d — i = x-\-y — ax — y = x {i-a), 



d — i 



whence x = , so that if a be known, x, or the rotation due to the sucrose 



I — a 



alone, can be calculated. The quantity i — a, or generally (i — a) x lOO is 

 known as the Clerget constant. The operation by means of which this 

 determination is made is the inversion or hydrolysis of sucrose under the 

 influence of a catalyst into equal parts of glucose and fructose. The 

 catalyst usually employed in analysis is hydrochloric acid, and in the immedi- 

 ately succeeding pages reference is made solely to this means. Sucrose 

 after inversion into glucose and fructose (invert sugar) possesses a left- 

 handed rotation, so that the value of a in the equation above is negative 

 and I — a is greater than unit}^ The reading after the operation, or the 

 inverted reading, i, will also be negative unless the value of y is suffici- 

 ently great to counterbalance the negative rotation of the invert sugar 

 formed. 



In order that this analysis may be justified, the following postulates are 

 necessary. 



I. — The operation of inversion must be conducted in such a way that the 

 same value can always be found for a. 



2. — The influence of temperature and concentration must be accurately 

 known. 



3. — ^The value of y must remain unchanged. 



Of these influences that due to temperature has alwa5^s been recognised 

 and allowed for ; it is only recently that the other factors have been taken 

 into consideration and the great majority of textbooks ignore them. 



Temperature. The rotation of invert sugar decreases with rise of tem- 

 perature and is such that 



d — i 



t 



I —a 



200 



constant 



where a is the observed value at 0° C and i is the temperature of observation. 

 This correction for temperature was given by Clerget and has been uniformly 

 confirmed by all subsequent observers. For example, Clerget found that 

 under his routine a sugar solution polarizing 100°, after inversion polarized 

 — 44° at 0° C, — 39° at 10° C, — 34° at 20° C, etc. The Clerget constant, 

 then, becomes i — (—0-44) = 1-44, or, as generally expressed, 144, in which 



case 0*5 tis used as the temperature correction in place of , 



