24 THE PROPERTIES OF SOLUTIONS. [CH. I. 



with turbid fluids, it is necessary to compare the solutions by 

 using a source of light from which the blue rays have been 

 screened off. This can be done by painting a piece of trans- 

 parent tracing paper with a strong acid solution of phenol red, 

 prepared by mixing one part of the stock 0-6 per cent, solution 

 with one part of the standard 0-2 M acid potassium phosphate. 

 The paper, while still wet, is pinned across the front of a box 

 containing one or two powerful carbon filament lamps. The ex- 

 amination should be conducted in a dark room, or the external 

 light should be cut off by using a dark cloth. 



Another method is to take an unexposed photographic plate, 

 fix it in hypo in a dark room, wash for some hours in running water, 

 stain by immersion in the dye, drying and mounting a piece on the 

 comparator on the side towards the light, in place of the ground 

 glass screen. 



Standard solutions of definite P H . 



The most convenient sets of solutions that have been 

 worked out are those of CJark and Lubs. A constant 

 volume (5occ.) of a standard solution of acid potassium 

 phthalate, acid potassium phosphate or of boric acid is 

 measured into a 200 cc. measuring flask. A given amount 

 (x cc.) of standard NaOH or HC1 is then added, and the 

 volume brought to the mark with distilled water. The 

 PH obtained with the different solutions are given in the 

 tables below. If intermediate points are desired, they can 

 be obtained from curves drawn from the points given. 



Preparation of solutions. 



0-2 M acid potassium phthalate. Dissolve 40-828 gm. in dis- 

 tilled water and make up to I litre. The salt should be recrystallised 

 from distilled water and dried at 110 C. for some hours. 



0-2 M acid potassium phosphate. Dissolve 27-231 gm..in dis- 

 tilled water and make up to i litre. The salt should be recrystallised 

 from distilled water and dried at 110 C. for some hours. 



0-2 M Boric Acid in 0-2 KCl. Dissolve 12-4048 gm. of air 

 dried boric acid and 14-912 gm. pure ignited KG in distilled water 

 and make up to i litre. 



