:236 J. M. Lewis: 



• concentration in the two solutions is identical. The most suit- 

 able concentration of thymolsulphonephthalein will give the solu- 

 tions a pale yellow colour. The titration is continued until the 

 first trace of blue in the alkali range is reached. At this point 



.all the benzoic acid has been neutralised. In carrying out such 

 a measurement it is most important that equal amounts of indi- 

 cator be added to both solutions, that the final volumes are the 

 same, and the colour matching be carried out in daylight of uni- 

 form quality. (3) 



The colorimetric method is however restricted in its scope. Gen- 

 erally speaking it cannot be used with coloured solutions, and fur- 

 thermore, the personal element must always enter largely into 

 any scheme which involves a nice discrimination between slightly 

 varying shades of colour. 



(2) The Hydrolysis of Esters. 



This depends upon the fact that the hydrolysis of methyl and 

 ethyl acetic esters is, in strong concentrations of hydrogen ions, 

 proportional to that concentration. 



For example, when methyl ester is acted upon by water in 

 -considerable dilution, the conversion to alcohol and acid is prac- 

 tically complete, thus : — 



CH...COOCH, + H 2 = CH 3 .COOH + CH :i OH. 



The technique of the method consists in the titration of the 

 acid-water-ester mixture at intervals against a N/20 baryta solu- 

 tion in order to ascertain the end point of the reaction. The flask 

 containing the mixture must be immersed in a thermostat. 



A water-ester mixture under identical conditions is kept under 

 observation as a control, and the calculation of the result is based 

 on Guldberg and Waage's formula for the velocity of chemical 

 reaction. (4) The method must be regarded as more or less cum- 

 bersome, and since the final result cannot be ascertained until 

 the reacting solutions have remained in the thermostat for 48 

 hours, it may be set aside as not fulfilling the necessary require- 

 ments for routine use in the laboratory. 



(8) The Electrometric method. 



This method of estimating the actual hydrogen ion concentra- 

 tion of a given solution is that which, in spite of certain difficulties 

 in the way of a rigid technique, must be regarded as the final court 

 <of appeal in any investigation which has for its object the estima- 



