ESTIMATION 139 



According to Boddener and Tollens,* a considerable saving 

 in time may be effected by precipitating the phloroglucide in 

 hot solution, i.e. between 80 and 85°. The reaction then 

 takes place according to the equation — • 



QH.O^ + QH0O3 = C„H,0. + sHp 



so that the precipitate actually weighs less than the one pro- 

 duced in the cold ; the precipitation is, however, complete in 

 from one and a half to two hours. The weight of furfural 

 corresponding to the precipitate so obtained may be calculated 

 by adding -ooi (to allow for the phloroglucide remaining in 

 solution) and multiplying the resulting figure by 0-571. The 

 number so obtained if multiplied by i -935 gives the correspond- 

 ing amount of pentose or if multiplied by 1703 gives the 

 amount of pentosane. The method is, however, not suitable 

 if it is desired to estimate the methyl-pentosans as distinct 

 from the pentosans, in which case Krober's method as modified 

 by Ellett f and Mayer J should be employed. 



Reducing Sugars Other than Pentose. 



Various gravimetric methods of estimating reducing sugars 

 have been suggested ; the outstanding feature of all these 

 methods is that they yield reliable results only if carried out 

 under strictly controlled conditions. One of the most reliable 

 methods is that of Brown, Morris, and Millar ; § the pre- 

 cipitated cuprous oxide is washed, dried, and weighed after 

 conversion into cupric oxide by ignition ; 1| from the weight 

 of cupric oxide obtained, the equivalent weight of either 

 dextrose, levulose, invert sugar, or maltose may be determined 

 by reference to tables which will be found in the original 

 paper. 



Compared with Brown, Morris, and Millar's method, that 

 of Allihn,^ once extensively used, is more cumbersome. The 



* Boddener and Tollens : " J. Landw.," 1910, 58, 232. 

 t Ellett : id., 1905, 53, 13. 

 X Mayer : id., 1907, 55, 261. 



§ Brown, Morris, and Millar : " J. Chem. Soc," 1897, 71, 94. 

 II Alternatively, the cuprous oxide may be reduced in a current of 

 hydrogen and weighed as copper. 



T[ Allihn : " J. prak. Chem.," 1880, [2], 22, 63. 



