456 PRACTICAL PHYSIOLOGY 



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. Schiff's reagent (see later) and 

 water to bring the volume to 100 c.c. The flask is stoppered, inverted a ftw 

 times to mix its contents, placed in a glass vessel containing water at 15 C., 

 and left for 30 minutes in diffuse daylight. The Schiff's reagent reacts with 

 the aldehyde present, giving a red colour, which reaches a maximum in 30 

 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 30 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. 2 cm. 1*48 cm. 



10 -r readings, 4 '13 5 6 "85 



A ,| O 



Then a is equivalent to 3 c.c. +^ 



' 



O'oO 4*lo 



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

 The amount of lactic acid in the liquid originally employed = 

 3-32x3-435xw 



-01 --- mg ' 

 whero 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 prepared 

 containing 0'5 c.c. Schiff's reagent and 1'5 c.c., 2 c.c., 3 c.c., 5 c.c. respectively 

 of dilute standard formaldehyde solution, made up to 100 c.c. with water. 

 These are placed in a dark cupboard till required. The colour develops very 

 slowly, and is fairly permanent, so that the standards may be used any time 

 within three days after the first twelve hours. 



The dilute standard Formaldehyde Solution. 10 c.c. commercial formalin 

 (40 per cent, formaldehyde) are diluted to 100 c.c. This solution will keep 

 practically indefinitely. To make the dilute standard solution 5 c.c. of this 

 solution are diluted to 500 c.c. This dilute solution will keep practically unaltered 

 for a week if well stoppered. It is standardised, unless made from an already 

 standardised formaldehyde solution, by the following method : 40 c.c. are 



measured into a stoppered bottle, 25 c.c. j^ iodine solution are added, and 



then 10 per cent, caustic soda, till the liquid assumes a light yellow colour. 

 The mixture after standing for 10 minutes is acidified with dilute hydrochloric 



N 

 acid and titrated with j^ sodium thiosulphate solution, until the colour of the 



iodine just disappears. The volume in c.c. of thiosulphate solution required is 

 subtracted from 25 c.c. Let the remainder = 6 c.c. Then the formaldehyde in 



1 -49 x b 

 mg. present in 1 c.c. of the solution = n = -- 



The value of n should be nearly 0'4 mg. 



