able in this respect after it has been maintained at the
boiling-point for forty-five minutes~.

Although meat extract always reacts acid to phenolphthalein, it
occasionally reacts neutral or even alkaline to litmus; and again, meat
extract that has been rendered exactly neutral to litmus still reacts
acid to phenolphthalein. This peculiar behaviour depends upon two
factors:

1. Litmus is insensitive to many weak organic acids the presence of
which is readily indicated by phenolphthalein.

2. Dibasic sodium phosphate which is formed during the process of
neutralisation is a salt which reacts alkaline to litmus, but neutral to
phenolphthalein. In order, therefore, to obtain an accurate estimation
of the reaction of any given sample of meat extract, it is essential
that--

1. The meat extract be previously exposed to a temperature of 100 deg. C.
for forty-five minutes.

2. The estimation be performed at the boiling-point.

3. Phenolphthalein be used as the indicator.

The estimation is carried out by means of titration experiments against
standard solutions of caustic soda, in the following manner:

_Method of Estimating the Reaction._--

_Apparatus Required_: _Solutions Required_:

1. 25 c.c. burette graduated 1. 10N NaOH, accurately
in tenths of a centimetre. standardised.

2. 1 c.c. pipette graduated in 2. n/1 NaOH, accurately
hundredths, and provided standardised
with rubber tube, pinch-cock,
and delivery nozzle.

3. 25 c.c. measure (cylinder or 3. n/10 NaOH, accurately
pipette, calibrated for standardised.
98 deg. C.--_not_ 15 deg. C).

4. Several 60 c.c. conical 4. 0.5 per cent. solution of
beakers or Erlenmeyer phenolphthalein in 50 per
flasks. cent. alcohol.

5. White porcelain evaporating basin, filled with boiling water and
arranged over a gas flame as a water-bath.

6. Bohemian glass flask, fitted as a wash-bottle, and filled
with distilled water, which is kept boiling on a tripod stand.

METHOD.--Arrange the apparatus as indicated in figure 97.

(A) 1. Fill the burette with n/10 NaOH.

2. Fill the pipette with n/1 NaOH.

[Illustration: FIG. 97.--Arrangement of apparatus for titrating media.]

3. Measure 25 c.c. of the meat extract (previously heated in the steamer
at 100 deg. C. for forty-five minutes) into one of the beakers by means of
the m