STANDARDISING THE REACTION OF MEDIA 33 



Standardisation of Reaction of Media. While the above 

 procedure of dealing with the reaction of a medium is sufficient 

 for ordinary work, it has been thought advisable to have a more 

 exact method for making media to be used in growing organisms, 

 the growth characteristics of which are to be described for 

 systematic purposes. Such a method should also be used in 

 studying the changes in reaction produced in a medium by the 

 growth of bacteria. It, however, involves considerable difficulty, 

 and should not be undertaken by the beginner. It entails the 

 preparation of solutions of acid and alkali which may be used 

 for determining the original reaction of the medium, and for 

 accurately making it of a definite degree of alkalinity. Normal l 

 and decinormal solutions of sodium hydrate and hydrochloric 

 acid are used. 



Preparation of Standard Solutions. The first requisites here are 

 normal solutions of acid and alkali. The latter is prepared as follows : 

 85 grammes of pure sodium bicarbonate are heated to dull redness 

 for ten minutes in a platinum vessel and allowed to cool in an exsiccator ; 

 just over 54 grammes of sodium carbonate should now be present. Any 

 excess is quickly removed, and the rest being dissolved in one litre of 

 distilled water, a normal solution is obtained. A measured quantity is 

 placed in a porcelain dish, and a few drops of a '5 per cent solution of 

 phenol-phthaleine in neutral methylated spirit is added to act as indicator. 

 The alkali produces in the latter a brilliant rose pink which, however, 

 disappears on the least excess of acid being present. The mixture is 

 boiled and a solution of hydrochloric acid of unknown strength is run 

 into the dish from a burette till the colour goes and does not return 

 after very thorough stirring. The strength of the acid can then be 

 calculated, and a normal solution can be obtained. From these two 

 solutions any strength of acid or alkali (such as the decinormal solution 

 of NaOH mentioned below) may be derived. 



As Eyre has suggested, the reaction of a medium may be 

 conveniently expressed by the sign + or - to indicate acid or 

 alkaline respectively, and a number to indicate the number of 

 cubic centimetres of normal acid or alkaline solution necessary 

 to make a litre of the medium neutral to phenol-phthaleine. 

 Thus, for example, "reaction = - 15," will mean that the medium 

 is alkaline, and requires 15 c.c. of normal HC1 to make a litre 



1 A "normal" solution of any salt is prepared by dissolving an "equivalent" 

 weight in grammes of that salt in a litre of distilled water. If the metal of 

 the salt be monovalent, i.e. if it be replaceable in a compound by one atom 

 of hydrogen (e.g. sodium), an equivalent is the molecular weight in 

 grammes. In the case of N'aCl, it would be 58'5 grammes (atomic weight of 

 Na = 23, of Cl = 35'5). If the metal be bivalent, i.e. requiring two atoms of H 

 for its replacement in a compound (e.g. calcium), an equivalent is the 

 molecular weight in grammes divided by two. Thus in the case of CaCl 2 an 

 equivalent would be 55'5 grammes (atomic weight of Ca = 40, of C1 2 -71). 



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