easure; rinse out the measure with a very small quantity of boiling
distilled water from the wash-bottle, and then add this rinse water to
the meat extract already in the beaker.

4. Run in about 0.5 c.c. of the phenolphthalein solution and immerse the
beaker in the water-bath, and raise to the boil.

5. To the medium in the beaker run in n/10 NaOH cautiously from the
burette until the end-point is reached, as indicated by the development
of a pinkish tinge, shown in figure 98 (b). Note the amount of
decinormal soda solution used in the process.

NOTE.--Just before the end-point is reached, a very slight
opalescence may be noted in the fluid, due to the
precipitation of dibasic phosphates. After the true
end-point is reached, the further addition of about 0.5 c.c.
of the decinormal soda solution will produce a deep magenta
colour (Fig. 98, c), which is the so-called "end-point" of
the American Committee of Bacteriologists.

[Illustration: FIG. 98.--a, Sample of filtered meat extract or
nutrient gelatine to which phenolphthalein has been added. The medium is
acid, as evidenced by the unaltered colour of the sample. b, The same
neutralised by the addition of n/10 NaOH. The production of this faint
rose-pink colour indicates that the "end-point," or neutral point to
phenolphthalein, has been reached. If such a sample is cooled down to
say 30 deg. or 20 deg. C., the colour will be found to become more distinct
and decidedly deeper and brighter, resembling that shown in c. c, Also
if, after the end-point is reached, a further 0.5 c.c. or 1.0 c.c. n/10
NaOH be added to the sample, the marked alkalinity is evidenced by the
deep colour here shown.]

(B) Perform a "control" titration (occasionally two controls may be
necessary), as follows:

1. Measure 25 c.c. of the meat extract into one of the beakers, wash out
the measure with boiling water, and add the phenolphthalein as in the
first estimation.

2. Run in n/1 NaOH from the pipette, just short of the equivalent of the
amount of _deci_-normal soda solution required to neutralise the 25 c.c.
of medium. (For example, if in the first estimation 5 c.c. of n/10 NaOH
were required to render 25 c.c. of medium neutral to phenolphthalein,
only add 0.48 c.c. of n/1 NaOH.) Immerse the beaker in the water-bath.

3. Complete the titration by the aid of the n/10 NaOH.

4. Note the amount of n/10 NaOH solution required to complete the
titration,