8 The Estimation of Carbohydrates 



lead precipitate, thus causing considerable error in the analysis. Pellet 

 has stated that in some cases as much as 23 % of the laevulose is preci 

 pitated at the ordinary temperature, whilst at 50° the whole of the 

 laevulose is removed. The view that laevulose is actually 'precifilated 

 has been universally adopted in the standard treatises on sugar 

 analysis^. 



It is true that Watts and Tempany, like Gill himself, showed that 

 if to a solution of pure invert sugar basic lead acetate is added, although 

 the rotation is greatly changed in the positive direction no laevulose is 

 actually precipitated and on adding just sufficient acetic acid to combine 

 with the lead oxide of the basic lead acetate present the original rotatory 

 power is restored. Prinsen Geerligs, however, maintained that whilst 

 this is true of pure solutions of invert sugar or laevulose it is not so 

 when other substances such as sodium chloride or sulphate are present 

 or in fact any compound capable of forming a precipitate Avith the 

 lead. In such cases Geerligs considered that the laevulose was actually 

 precipitated and lost; whether it was regarded as thrown out jper se, 

 in an insoluble form or co-precipitated by a process of adsorption is 

 not made clear. In adopting this view the early statement of Gill 

 in 1871, that when the lead is precipitated by sulphur dioxide the 

 original rotation of the invert sugar is restored, was entirely ignored. 



The object of the present paper is to show that whilst the experi- 

 mental facts recorded by the writers cited above are perfectly correct 

 they have been misinterpreted. The results given below show that, 

 at least in dilute solutions, laevulose is never jjrecipitated by basic 

 lead acetate even in presence of salts such as chlorides, sulphates or 

 carbonates. No loss of laevulose occurs, indeed, unless the excess of 

 basic lead acetate is alloioed to act for some length of time upon the sugar 

 before the lead is precipitated. If to the solution of pure laevulose basic 

 lead acetate is added in excess (5 c.c. or 10 c.c. to 50 c.c. of solution) 

 and the lead is immediately precipitated by sodium carbonate or sodium 

 sulphate, practically 100 % of the laevulose is recovered in the solution. 

 But if, on the other hand, the basic lead acetate is left with the sugar 

 solution for different lengths of time, for example, 15 minutes, 1 hour, 

 24 hours, and the lead is then precipitated by the same reagents, 

 increasing proportions of laevulose are found to have disappeared. 

 The amount of laevulose lost depends solely on the length of time the 



^ For example by von Lippmann in Lunge's Chem. Techn. Untersuch. Methoden, 

 3, 485 (5th Ed.); by Tiing (Thorpe's Diet. App. Chem. article "Sugar," p. 244). Compare 

 also Allen's Commercial Organic Analysis, 4th Ed., 1, 311. 



