223 



With starch conversion products it is possible to secure concordant readings between 

 different ohr-erver- only when (i per cent bichromate is used in a 3 cm cell. With 

 substances df higher <lispersii.ii than dextrin it would seem advisable tn use only 

 -odium light for polari/.ation. With all carbohydrate materials it would seem thai 

 the dispersion disturbances of white li-jht may In- eliminated by means of bichromate 

 solution. The results show, however, that the direct ions for operating saccharimeters 

 should specify the exact >ttvn<:th of bichromate solution to he used. 



A second and very discordant element in the unification of saccharimetric observa- 

 tions is in the use of clarifying agents. The several errors resulting from the use of 

 lead salts in clarifying sugar solutions have long been recognized. There is, first, the 

 volume of precipitate error; second, the precipitation of levulose error; third, the forma- 

 tion of -oluble lead levulosate of lower specific rotation than levoluse; and, fourth, 

 \\hen dry defecation is used, the error of dilution or change in volume. 



In .-fudyiiiir these various questions m \ attention was directed first of all to the 

 vrreat difference in composition of the , .minercial preparations of lead subacetale and 

 abo "l the solutions of this salt as ordinarily prepared for laboratory use. IVepara- 

 -f the anhydrous subacetate of lead sold by reliable chemical firms, and all 

 iruaranteed as to purity according to the food and drugs act. were found to var\ in 

 their c'-ntent of ba-ic lead ox id : - In lion- ..f lead subacetate 



prepared by di <_:<.- tini: litharge with the normal acetate of lead, according to the metho<l 

 of the association or other directions, will also vary greatly in composition, accord ini' 

 to the time and temperature ..f digestion. Solutions of the same specific ^raviiy thus 

 trad were found to \ary in the ratio of combined to basic PbOof from 5:2 to 1 : 1. 

 These \ariaiions in composition are not surprising when it is remembered that three 

 well-defined -ubacotatcfl have been prepared by the diu'i-.-tion of lithaire with normal 

 lead acetate. These are SPbvtf- I'M ), t he stihacetate ordinarily prescribed for clarifi- 

 cation; I'b.l'TbO, the monob.t .and I'b. lr_Tb< >, the diabasic acetate 



The ottinal dil r preparing basic lead acetate are explicit .- to i|,,. -p.-ejfie 



V'ra\ ity of lea<l solutions to be used, but are nil. .. point -t' -reaie-t importance, 



the content of ba.-ic lead. The differences which may result in saccharimetric work 

 from the u.-e of lead -olution- of \ar ;t\ may be seen from the follow M:L; polar- 



n- made upon a sirup and a sugar u.-ini: three different .-...luii-'iis of lead subace- 

 tate and "ne solution of the normal acetate all "i I _' \ specific LT:I\ ity. 



li/rrrnt solution* of lend subacetate. 



The solutions of greatest basicity have the greatest clarifying power and give the 

 highest polarizations owing to the greater precipitation and lowering of polarization 

 of the levulose and consequent increase in dextro-rotation. The 3:2 subacetate is 

 the one usually prescribed, and since this compound can be obtained of satisfactory 

 purity from one chemical house at least it might be well for chemists desiring uniform- 

 ity to prepare these solutions directly from this salt. The important point, however, 

 i- that in \\hate\er way prepared the solutions of basic lead used in saccharimetry 

 should have not only a constant specific gravity, but a uniform content of basic lead. 



The errors due to the volume of lead precipitate, a most serious one in the polariza- 

 tion of low-grade saccharine products, have been very largely eliminated by the ingen- 



