ADVANCED CHEMICAL PHYSIOLOGY 295 



pure sulphuric acid are rapidly added from a dropping funnel, the flask 

 being shaken and cooled under the tap. The flask is then fitted with 

 a rubber cork carrying an inlet tube for steam and a thermometer, 

 so arranged that both dip well below the surface of the liquid. It is 

 then placed in a slanting position on wire gauze on a retort stand and 

 attached to a good vertical condenser. (For this purpose the exit 

 tube of the flask must be bent at a suitable angle.) A flask of about 

 300 c.c. capacity, immersed in cold water, is placed as the receiver 

 of the condenser with its mouth just touching the jacket of the con- 

 denser, so as to prevent loss of aldehyde by evaporation. A gentle 

 current of steam from an ordinary steam generator is then passed into 

 the distillation flask, which is vigorously heated with a Bunsen burner. 

 Distillation will generally begin at about 140 C., but the heating is 

 continued till the temperature reaches 155 C., when the current of 

 steam is increased, and the heat applied to the flask adjusted so that 

 the temperature is kept between 153 and 157 C. When about 100 c.c. 

 have collected in the receiver, or the distillation has lasted nearly 

 thirty minutes, the decomposition is complete. The contents of the 

 receiver are rendered just permanently alkaline by the addition of 

 2 per cent, caustic soda solution and a little litmus solution, diluted 

 to about 150 c.c., and redistilled into a flask with a 100 c.c. mark in 

 the neck, using the same precautions to prevent loss as before, until 

 about 50 c.c. have been collected. (When the amount of lactic acid 

 is excessively small, as is the case in normal urine, a 50 c.c. flask may 

 be employed, the quantities given in what follows being halved.) 



To the second distillate are added 0-5 c.c. SchifPs reagent (see later) 

 and water to bring the volume to 100 c.c. The flask is stoppered, 

 inverted a few times to mix its contents, placed in a glass vessel con- 

 taining water at 15 C., and left for thirty minutes in diffuse daylight. 

 The Schiff's reagent reacts with the aldehyde present, giving a red 

 colour, which reaches a maximum in thirty minutes and then slowly 

 fades. This reaction may be used qualitatively as a test for lactic 

 acid. For quantitative estimation the coloured liquid (a) is transferred 

 at the end of thirty minutes to one tube of a colorimeter. A convenient 

 depth of liquid is selected. The two formaldehyde standards (see 

 later) are selected which are nearest to a in colour, and the depth of 

 each determined which gives the same intensity of colour as the 

 selected depth of a. 



The calculation is best described by an example. 



Formaldehyde 4 c.c. a Formaldehyde 3 c.c. 



Readings of equal depth of colour, 2-42 cm. 2cm. 1-46 cm. 

 10 -T- readings, 4-13 5 6-85 



5 - 4-13 



Then a is equivalent to 3 c.c. -f- ^-5^ ^-5 



b-85 4-13 



= 3-32 c.c. standard formaldehyde solution. 

 The amount of lactic acid in the liquid originally employed = 



3-32 x 3435 x n 



-04 mg ' 



where n is the standard value of the formaldehyde. 



If the colour of a is much greater than that of any of the standards, 

 another determination must be made, using a more dilute solution of 

 lactic acid. 



The Formaldehyde Standards. A series of four stoppered flasks is 



