276 



URINE. 



[CH. XII. 



comparator (see p. 21). The concentrations of the indicator in the 

 urine and the two buffer solutions, and also the intensity of the 

 pigment in the three tubes containing urine are kept constant by 

 the addition of equivalent quantities of water and soda respectively. 



Solutions and Apparatus required. 



i . A comparator for holding the tubes (see fig. 37) . This should be fitted 

 with a ground glass screen, as shewn in the diagram. 



2. Tubes of clear resistance glass (7 x i in.) to fit the 

 comparator. These should have the same internal diameter. 

 They can be calibrated by measuring 25 cc. of water from a 

 pipette into a number and selecting those tubes in which the 

 fluid reaches the same level. A better method is to use a hard 



Fig. 37. Cole and Onslow's Comparator for large tubes. 



wood gauge, which is slightly conical and is marked with rings 

 corresponding to every 1-64 in. diameter. The gauge is pressed 

 into the tubes and those selected of the same internal diameter. 

 The method of using this is shewn in fig. 38. It is convenient to 

 choose sets of tubes and to mark each member^of a set with a 

 distinguishing letter by means of a diamond. 





Fig. 38- 

 Gauge. 



3. Buffer solution, P H = 7'4 2 - Prepared by treating 50 cc. of 0-2 M. 

 KH 2 PO 4 with the equivalent of 39-9 cc. of 0-2 N.NaOH and diluting to make 

 200 cc. with distilled water (see p. 27). 



4. Buffer solution, P H = 7-47. Prepared as above, but use the 

 equivalent of 40*2 cc. of 0*2 N.NaOH. 



5. Standard alkali, see p. 380. It is convenient to use o-i N. from an 

 ordinary burette or 0*2 N. from a microburette. 



