r, the "optimum" usually approximates
fairly closely to +10; and as experiment has shown that this reaction is
the most generally useful for routine laboratory work, it is the one
which may be adopted as the standard for all nutrient media derived from
meat extract.

Briefly, the method of standardising a litre of media to +10 consists in
subtracting 10 from the initial _titre_ of the medium mass; the
remainder indicates the number of cubic centimetres of normal soda
solution that must be added to the medium, per litre, to render the
reaction +10.

~Standardising Nutrient Bouillon.~--For example, 1000 c.c. bouillon are
prepared; at the first titration it is found

1. 25 c.c. require the addition of 5.50 c.c. n/10 NaOH to neutralise.

Two controls give the following results:

2. 25 c.c. require the addition of 5.70 c.c. n/10 NaOH to neutralise.

3. 25 c.c. require the addition of 5.60 c.c. n/10 NaOH to neutralise.

Averaging these two controls, 25 c.c. require the addition of 5.65 c.c.
n/10 NaOH to neutralise, and therefore 1000 c.c. require the addition of
226 c.c. n/10 NaOH, or 22.60 c.c. n/1 NaOH, or 2.26 c.c. n/10 NaOH.

Initial _titre_ of the bouillon = +22.6, and as such requires the
addition of (22.6 c.c. - 10 c.c.) = 12.6 c.c. of n/1 NaOH per litre to
leave its finished reaction +10.

But the three titrations, each on 25 c.c. of medium, have reduced the
original bulk of bouillon to (1000 - 75 c.c.) = 925 c.c. The amount of
n/1 NaOH required to render the reaction of this quantity of medium +10
may be deduced thus:

1000 c.c.:925 c.c.::12.6 c.c.:x.

Then x = 11.65 c.c. n/1 NaOH.

Whenever possible, however, the required reaction is produced by the
addition of dekanormal soda solution, on account of the minute increase
it causes in the bulk, and the consequent insignificant disturbance of
the percentage composition of the medium. By means of a pipette
graduated to 0.01 c.c. it is possible to deliver very small quantities;
but if the calculated amount runs into thousandth parts of a cubic
centimetre, these are replaced by corresponding quantities of normal or
even decinormal soda.

In the above example it is necessary to add 11.65 c.c. normal NaOH or
its equivalent, 1.165 c.c. dekanormal NaOH. The first being too bulky a
quantity, and the second inconveniently small for exact measurement, the
total weight of soda is obtained by substituting 1.16 c.c. dekanormal
soda solution, and either 0.05 c.c.