176 



acetate. This would naturally be expected, as the Herles' solution forms a precipi- 

 tate in itself, hence causing concentration, and the excess of wet lead subacetate 

 causes an increase and also a change of precipitate, thereby changing the concentra- 

 tion, while the normal acetate produces no more precipitate with an excess, and hence 

 no change of concentration. 



As regards the danger of adding an excess, this is the least in case of the neutral 

 acetate, as an excess is indicated when no more precipitate continues to form. When 

 using wet subacetate a better clarification is reached before the point where more ace- 

 tate will produce a further precipitation. With dry lead it is difficult to determine 

 when enough has been added. To add by weight takes much time, but where many 

 determinations are to be made varying measures or cups could be used, the weight 

 of the contents having been previously determined. It has the fault of precipitating 

 reducing sugars in as large quantities as the wet subacetate, as noted in last year's 

 report; besides, an excess of this reagent increases the volume of the solution, thereby 

 lowering polarization. This effect is shown in the following experiment: Six hundred 

 cubic centimeters of a solution of pure sucrose were made up and five 100 cc flasks were 

 filled and the following quantities of the dry subacetate were added, shaken, and then 

 polarized, care being taken that the polarization was made at 20 C. 



Polarization of pure sucrose solution with varying amounts of dry lead. 



It is noted from the table that the polarization has been lowered 0.35 by the addition 

 of 2 grams of the dry lead. 



The greater part of the lead subacetate went into solution even up to the 2 gram 

 quantity, and only a cloud was noted. The meniscus of the liquid in the flasks 

 containing the added lead subacetate was above the 100 mark in each case, showing 

 an increase in volume. 



Experiments were tried to determine this increase in volume. Five accurately 

 graduated flasks with glass stoppers were used, and into these were weighed the varying 

 quantities of dry subacetate, as in the previous experiment. A 100 cc pipette was 

 used and an equal amount of solution of sucrose was added to each flask, the flasks 

 being shaken during the addition of the solution. \Vhen added, the flasks were 

 corked up and allowed to stand over night. The height of the liquid being marked 

 on the neck of the flasks, the contents were poured out and the flasks cleaned and 

 dried. By means of a Morse-Blalock pipette, capable of reading to 0.005 cc, the flasks 

 were filled to the mark. The results are tabulated below: 



Volumes of solution of pure sucrose when adding various amounts of dry subacetate. 



