THE DETERMINATION OF CANE SUGAR 503 



The German normal weight is due to Ventzke. 27 In his original publica- 

 tion he compared the polarization of various sugars in 25 per cent, solution, 

 the specific gravity of cane sugar at this point being I 1056, and defined this 

 as a normal solution. He also used a tube 23-4 cms. long. Later this 

 standard was changed to a sugar solution of density I I, with a tube length 

 of 20 cms. This again was referred to weight, the equivalent being 26 048 

 grams of sugar dissolved in 100 c.c. Originally metric c.c. were specified, 

 but in 1855 with the general adoption of Mohr's c.c. a change to this standard 

 was made with no change in the weight. The International Sugar Com- 

 mission sitting at Paris in 1900 recommended a change to metric c.c. and the 

 adoption of 20 C. as the temperature of observation. The normal weight 

 now became 26-0082 grams, for which 26 grams is substituted. A third 

 normal weight, namely 10 grams, is that used by Wild in the few instruments 

 of this type that have come into use. 



In 1896, Sidersky, at the International Congress of Applied Chemistry, 

 proposed the adoption of 20 grams as a normal weight, and at the Inter- 

 national Congress of 1906, this quantity was specifically adopted as the Inter- 

 national normal weight referred to metric c.c. and a temperature of 20 C.* 



Unfortunately, however, the exact rotation of cane sugar is uncertain. 

 By definition 26 grams of sugar in 100 metric c.c. observed at 20 C. in a 

 20 c.m. tube should read 100 on the Ventzke scale. Bates and Jackson 28 

 were the first to challenge this basic standard, and in a research of very 

 great care, conducted at the U.S. Bureau of Standards, they found a value 

 of 99-895. A very little later Walker 29 in Hawaii confirmed their work, 

 finding a value of 99 86. The result first quoted makes the normal weight 

 26 027 grams. This value is very appreciably lower than that due to 

 Schonrock on whose measurements of the equivalence of quartz and cane 

 sugar the graduation of all except the French instruments is based. On 

 the other hand, Herzfeld 30 has challenged the correctness of the work of 

 Bates and Jackson, so a state of confusion prevails. 



To remove an element of uncertainty the careful analyst should confirm 

 the graduations of his instrument, and determine at the average temperature 

 of his laboratory what is his proper 'normal weight. This is the procedure 

 followed by Harrison 31 in British Guiana : " Each 100 c.c. flask in use 

 for sugar polarization is verified by weighing into it 99 533 grams of recently 

 boiled distilled water at 20 C. The exact weight of chemically pure sugar, 

 which, when made up to a bulk of 100 c.c. in one of the corrected flasks at 

 28C., gives a polarization reading of 100, is ascertained by experiment for 

 each instrument, and this weight of sugar is invariably used instead of the 

 maker's weight for 17-5 C." 



It is at once evident that cane sugar can be determined bj' direct polari- 

 zation only in the absence of other bodies which also rotate the plane of 

 polarization. In very many routine analyses in the sugar-house the dis- 

 turbing effect of other optically active bodies is small, and the polarization 

 very closely measures the quantity of cane sugar present. For the purposes 

 of trade and for the imposition of customs duties the polarization of raw 

 sugars is accepted as the percentage of sucrose, from which, however, the 

 polarization is to be carefully distinguished. In the presence of other 

 optically active bodies the sucrose may be determined by the Clerget or 

 double polarization methods, described in detail elsewhere. 



* The 26-gram weight has also come to be referred to as the International normal weight. 



