NE UTRALISA TION OF MEDIA 33 



test by titration will show that the medium is still acid to phenol- 

 phthalein, to the extent sometimes of from 05 to i per cent This 

 discrepancy is perhaps due to minor reactions which are not under- 

 stood ; the reaction of the medium, however, must be brought to 

 the desired point by the further addition of sodium hydrate, and 

 the titrations and additions of alkali must be repeated until the 

 medium has the desired reaction.^ 



NeutPalisation in bulk {Procedure recommended) — 

 Place the medium in a wide-mouthed enamelled saucepan, 

 and neutralise to litmus with caustic soda solution. Accurate 

 standardisation is not necessary at this stage, the only object 

 being to save time in the subsequent neutralisation to phenol- 

 phthalein. Bring slowly to the boiling-point over the Bunsen 

 flame. All risk of burning can be avoided by the employment 

 of a water-jacketed pan, or the interposition of an asbestos 

 mat between the pan and the flame. Boil for five minutes. 

 Remove and place on one side in a beaker about 100 cc. 

 of the medium, to be subsequently returned as explained 

 below. Hold ready to hand a beaker of boiling water, half a 

 dozen small white porcelain evaporating dishes, and two ordinary 

 dessert spoons marked on their inner surface with a line show- 

 ing the level of 5 cc. of liquid. By means of one of these, 

 place quickly 5 cc. of the boiling medium in one of the evaporating 

 dishes, and add with the other an equal quantity of the boiling 

 water. Drop in two drops of a i per cent solution of pheholph- 

 thalein in 50 per cent alcohol.'- If the mixture remains colourless 

 as seen against the white porcelain after stirring with a glass rod, 

 add a sufficient quantity of normal soda solution to the bulk, and 

 repeat until a pinkish tinge is seen. A slight excess of alkali does 

 not matter at this stage as the addition of the 100 cc withdrawn 



^ The method of expressing the degree of reaction is as follows :— Acid 

 media are designated by the plus sign, and alkaline media by the minus sign, 

 and the degree of acidity or alkalinit>' is noted in parts per hundred ; thus, a 

 medium marked +1-5 would indicate that the medium was acid, and that 

 1-5 per cent, of normal sodium hydrate solution is required to make it neutral 

 to phenol phthalein, while - 1-5 would indicate that the medium was alkaline, 

 and that 1-5 per cent of normal acid must be added to make it neutral to the 

 same indicator. 



2 In view of the slightly acid properties of this indicator this solution should, 

 in course of preparation, be rendered neutral by first cautiously adding a few 

 drops of centi-normal alkali until the appearance of a very faint redness, and 

 then adding one drop of centi-normal acid. 



C 



