~Reaction of Meat Extract.~--Meat extract thus prepared is acid in its reaction, owing to the presence of acid phosphates of potassium and sodium, weak acids of the glycolic series, and organic compounds in which the acid character predominates. Owing to the nature of the substances from which it derives its reaction, the total acidity of meat extract can only be estimated accurately when the solution is at the boiling-point. Moreover, it has been observed that prolonged boiling (such as is involved in the preparation of nutrient media) causes it to undergo hydrolytic changes which increase its acidity, and ~the meat extract only becomes stable 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° 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°C.--_not_ 15°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.]