TITRATION AND ADJUSTMENT OF CULTURE MEDIA 233 



taken as an index a sample^^ titrated at the boiling point with 

 phenolphthalein as the indicator. Boiling has also a marked 

 effect on the ionic concentration in media mixtures, hence the 

 boiling titration called for in the Standard Method and followed 

 more or less closely by many workers and recommended in the 

 various text books, is subject to greater error than titrations 

 performed at a temperature of 20° to 30° C. 



In the adjustment of culture media not only is the desired 

 end point more closely approximated by titration at 20° to 

 30° C. (see chart 6), but the misleading information of the boil- 

 ing titration is avoided (see chart 4). Although according to 

 Eyre (see page 211) certain acids are detected only at the boil- 

 ing point, the actual reaction of media at the temperatures at 

 which they are used is the object of vital importance. The 

 conditions prevailing at these temperatures (incubator, 37.5°C., 

 for some organisms and room temperature fui- others) arp morft 

 closely indicated by titrations conducted at room temperature. 



As shown in chart 4 the actual reaction of a medium 

 titrated by the boiling method is really from 0.5 to 1 per 

 cent lower than is indicated by the boiling titration figures. 



The tests made on the reaction towards phenolphthalein of 

 the various peptones on the market showed them to differ greatly 

 in acidity. The effect on peptones of prolonged heating in 

 tests similar to those on the meat infusions showed that hydroly- 

 sis occurred on the application of heat. The development of 

 acidity took place after successive adjustments of reaction with 

 sodium hydroxid, as with the meat infusions and the total 

 amount of hydrolysis approximated closely that of the uncor- 

 rected portions. Here, too, prolonged heating of fourteen hours 

 in the autoclave did not give complete hydrolysis. 



In the usual titration methods, no one indicator gives all the 



'6 That is, if the titration is performed after a preliminary boiling of the whole 

 batch of meat infusion. When a titration is made on meat juice pressed out in 

 the cold and containing added peptone dissolved at a low temperature, the boil- 

 ing of the sample in the casserole is necessary to approximate future conditions 

 after the boiling of the whole batch. 



