524 ILLUSTRATIVE EXPERIMENTS 



mercury covered with HgCl in the presence of a definite concentration of KCl. 

 The other half-cell is a hydrogen electrode, i.e. ])latinuni black laden with 

 hydrogen and immersed in a solution containing H-ions. The difference of 

 E.M.F. between electrode and solution depends on the concentration of 

 H-ions in the latter. 



The difference of potential between the calomel and normal hydrogen 

 electrode can be ascertained. This value is subtracted from the total E.M.F. 

 of the cell to give a value from which the pJi may be calculated. 



24. Alterations of the Surface Tensions of Oil-Water Interface by 

 Alterations in Hydrogen Ion Concentration (Hartridge and Peters, Jour. 

 Fhystol. LIV., Proc. XLL). 



Oil free from fatty acids or soaps is essential. Pure olive oil, castor oil, 

 or cod-liver oil may be freed from these bodies by boiling for a number of 

 hours with frequent changes of a considerable excess of tap water. A series 

 of te.st tubes 6 X | in. each receives 5-10 c.c. of the treated oil. Into each 

 is put a capillary tube 4 in. long, open at both ends. The test tube is 

 almost filled with the fluid to be tested. Try solutions having a /jH of 9, 8, 

 7, 6. Measure the height to which the oil rises in the capillary. 



25. Buffer Solutions. 



(A) 8orensen's Phosphate Buft'ers. Two solutions are required, (a) M/15 

 solution of KHoPO, (9-078 gm. per litre). The salt should be chemically 

 pure, and its solution should be water clear and free from even traces of 

 chloride and sulphate. 



(6) M/15 solution of Na.,HP04 . 2H2O (11-876 gm. per litre). The salt 

 is prepared by exposing to the air the recrystallised Na2HP04 . I2H2O. 

 After two weeks' exposure on a porous plate, dissolve the salt in water and 

 test for chloride and sulphate : 8 c.c. of (a) KH.^PO^ + 2 c.c. of (6) Na2HP04 

 2Ho0 gives a solution of /)H 7-381. 



The water used for making these solutions must be free from CO^. This is 

 obtained by boiling the glass-distilled water in a hard glass flask for at least 

 5 minutes, and then fitting the flask with a bored rubber stopper carrying a 

 soda-lime tube so that the water may cool in an atmosphere free from COo. 

 The solutions may be kept in a hard glass double-necked bottle, of which one 

 neck is connectecl to a soda-lime tower fitted with a blow-ball, and the other 

 neck to the side tube of a burette. The burette is provided with a soda lime 

 tube at the upper end. It is as well to coat the inner surface of the bottle 

 with paraffin wax (see p. 557). 



(B) Clark and Lubs. Five solutions are necessary to provide a complete 

 ^H range. 



(i.) 0-2 M acid potassium phthalate. Recrystallise the salt from distilled 

 water and dry at 110° C. for some hours. Dissolve 40-828 gm. in distilled 

 water and make up to a litre. 



(ii.) 0-2 M acid potassium phosphate. Recrystallise the salt from distilled 

 water and dry as above. Dissolve 27-231 gm. in distilled water and make 

 up to a litre. 



(iii.) 0-2 M boric acid in 0-2 M KCl. Dry the boric acid in air on a porous 

 plate. Use pure ignited KCl. Dissolve 12-4048 gm. of HBO., and 14-912 gm. 

 of KCl in distilled water and make up to a litre. 



(iv.) 0-2 N sodium, hydroxide. Standardise this solution against weighed 

 amounts of pure potassium phthalate, using phenolphthalein as indicator. 



(v.) 0-2 N hydrochloric acid. Prepare this from a freshly distilled 20 per 

 cent, solution and standardise it against the standard N/5 soda, using methyl 

 red as indicator. 



