296 PRACTICAL PHYSIOLOGY 



prepared containing 0-5 c.c. Schiffs 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 Standards 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 formalde- 

 hyde solution, by the following method : 40 c.c. are measured into a 



N 

 stoppered bottle, 25 c.c. ,-^ iodine solution are added, and then 10 



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

 The mixture after standing for ten minutes is acidified with dilute 



N 

 hydrochloric acid and titrated with y^ 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 mg. present in 1 c.c. 



1-49 X b 

 of the solution = n = ^ . 



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



Schiffs Reagent. 1 gm. finely powdered rosaniline hydrochloride 

 and 100 c.c. water are placed in a small bottle with a closely fitting 

 stopper. Sulphur dioxide is passed in from a syphon, till the dye just 

 dissolves to a yellow solution, when the liquid is very nearly saturated 

 with the gas. The reagent loses sulphur dioxide rather readily, so 

 that it must be kept closely stoppered, and must be resaturated occa- 

 sionally with sulphur dioxide. The formaldehyde standard with -5 c.c. 

 of the reagent and 5 c.c. dilute formaldehyde solution (2 mg. formalde- 

 hyde) made up to 100 c.c. with water should be of such a depth of colour, 

 that by the colorimeter 1-3-1-7 cm. is equivalent in colour to 0-7 cm. 



N 

 YQQ potassium permanganate. 



The method can be applied to urine either directly, or after rendering 

 alkaline with sodium carbonate and evaporating on the waterbath, 

 but not more than 40 c.c. of urine of specific gravity 1,020 should be 

 employed for one distillation in either case, as with more urine frothing 

 is liable to occur. 



Glycuronic acid forms a source of error, but may be removed by 

 means of basic lead acetate. For this purpose 25200 c.c. urine are 

 measured into a 500 c.c. graduated flask. Slight excess of basic lead 

 acetate solution, 10 c.c. strong ammonia and water to make 500 c.c. 

 are added. The contents of the flask are well mixed, allowed to stand 

 for a short time and filtered through a dry filter into a dry flask. A 

 measured volume of the filtrate (350 c.c. or less) is evaporated in a dish 

 on the waterbath, sodium carbonate solution being added to keep 

 the liquid alkaline. The residue in the dish is then washed into the 

 distillation flask with 40 c.c. water and 45 c.c. sulphuric acid and treated 

 as above. This treatment causes a small loss of lactic acid, so that 

 only about 50 per cent, of minute quantities of lactic acid added to urine 



