32G PHYSIOLOGY OF NUTRITION. 



some conditions it is also advisable to lixiviate the crashed masses 

 with a little water, and filter the fluid obtained. If it is desired 

 to obtain absolute values for the acidity of the plant structures, 

 they are first weighed, then the pulp obtained by pounding them 

 is mixed Avith water and warmed for half an hour in thick- 

 walled glass vessels, in the water-bath, at a temperature of from 

 80 C. to at most 90 C., to drive off Carbon dioxide, then trans- 

 ferred to a filter and washed with as little hot water as possible 

 (see 60). In investigating the behaviour of free organic acids in 

 the Crassulaceae (see 131), the best method of procedure is to 

 weigh the leaves which we select for examination, crush them in 

 a mortar, then for half an hour heat the pulp, together with the 

 water employed for rinsing (which should be as little as possible), 

 on a water-bath in thick- walled glasses to a temperature of from 

 80 C. to at most 90 C., then again transfer to a mortar, rinsing 

 out the vessels with water and pouring the rinse-water into the 

 mortar, and finally after cooling at once titrate. 



For titrating saps, extracts or pulp, we use dilute potash or 

 soda lye. We dissolve 1 gr. of caustic potash or caustic soda in 

 1,000 c.c. of water, add slight excess of baryta water, and then 

 Sodium sulphate in order to precipitate the excess of baryta. The 

 clear solution, which is now free from Carbonic acid, should give 

 no precipitate with Sulphuric acid. To fill the burette we employ 

 an arrangement such as that represented in Fig. 98. In titrating 

 we run the potash or soda solution from the burette into the acid- 

 containing sap, extracts, or pulp, and if we are working with 

 very clear saps, add from 3 to 5 drops of a dilute alcoholic solu- 

 tion of phenolphthalein to serve as an indicator. In other cases, 

 especially in the titration of pulp, turmeric paper must be used. 

 In comparative work it is not necessary to determine the titration 

 equivalent of the soda or potash solution. If, however, this is 

 required, we prepare a normal solution of an acid, i.e. a solution 

 which contains 1 equivalent weight in grams of a monobasic acid 

 to every 1,000 c.c. In using Oxalic acid (C 2 H 2 O 4 + 2H 2 0-126) 

 63 gr. of pure acid must be dissolved in 1,000 c.c. of water. 

 From this solution the equivalent titre of the soda or potash solu- 

 tion is easily determined. 2 



1 See G. Kraus, Abhandlungcn der Naturforschcnden Gescllschaft zu Halle, 

 Bd. 16. 



2 On methods of titrating [see Mohr, Lehrbuch <l. analytisch-chem. Titrir- 

 incthode. 



