5o6 CHAPTER XXV 



The Errors Inherent to the Use of Lead Salts as Clarificants. — The use of 



lead salts in sugar analysis introduces two small, but distinct, sources of 

 error. These are recognized by analysts, but in general, in technical work, 

 are neglected. These inherent errors are discussed below : — 



In the majority of the schemes for clarification detailed below, an insoluble 

 precipitate is formed, which occupies an appreciable volume, so that if, 

 after clarification, the solution be made up to loo c.c. the actual volume is 

 100 c.c. less the volume occupied by the precipitate ; prima facie, an error 

 is thus introduced, though that this is the case is denied by certain chemists. 

 H. Pellet^* in particular claims that the precipitate formed by the addition 

 of basic acetate of lead entrains sugar, and that this entrainment compensates 

 for the volume occupied by the lead precipitate. In his experiments he shows 

 that a weight of sugar material dissolved in water and made up to lOO c.c. 

 in the presence of its precipitate gives a reading of, say, 50; the same weight 

 of sugar material made up to 200 c.c. in the presence of its precipitate will 

 give a reading exactly half the first, in this case, 25 ; if the lead precipitate 

 exercised an influence proportional to its volume, the first solution would be 

 more than twice as concentrated as the second, and hence the first reading 

 should be more than twice as large as the second. This phenomenon he 

 attributes to the entrainment of sugar by the lead precipitate, and claims 

 that it is unnecessary to apply a correction for its volume. 



The writer in investigating the same subject found also that a fixed 

 weight of sugar material made up of different volumes in the presence of 

 the precipitate tends to give identical polarizations independent of the dilu- 

 tion, and explains the apparent non-influence of the lead precipitate by an 

 increase in the specific rotation of cane products with dilution. 



Home's very detailed experiments also point to the conclusion that the 

 lead precipitate introduces a positive error and that sugar is not entrained. 



Correction for the volume of the lead precipitate is made by the following 

 methods : — 



1. Scheibler's Method.^^ — The material under analysis is first made up 

 to a volume of 100 c.c. in the presence of its precipitate, and the reading 

 taken ; a second reading is taken under identical conditions, except that the 

 volume is now made up to 200 c.c. 



Let X be the volume of the precipitate ; let a be the reading in 100 apparent 

 c.c, and h the reading in 200 apparent c.c. 

 Then (100 — x) a = (200 — x) h. 

 Solving this equation x is found. 



2. Deerr's Method.^^ — The material under analysis is first made up to 

 100 c.c. in the presence of its precipitate, filtered, and 50 c.c. of the filtrate 

 diluted to 100 c.c, and the reading observed ; let it be a. The same weight 

 of material is made up to 200 c.c. in the presence of its precipitate, and the 

 reading taken ; let it be 6 ; let the volume of the precipitate be x ; then 

 2a (100 ~ x) = (200 — x) b. 



Solving this equation x is found. 



The object of this procedure is to obtain both readings in the same 

 concentration and at the same part of the scale, thus eliminating errors 

 due to Siny change in opticity with dilution, errors in the zero point and errors 

 in scale graduation. 



3. Method of Sachs.^'' — The precipitate obtained is collected on a filter 

 and washed until free from sugar ; it is then transferred to a graduated flask, 



