CH. I.] 



INDICATORS. 



23 



Preparation of solutions * 



Alcoholic solutions are prepared by dissolving the solid in 

 alcohol and diluting with water. . 



(i) o-oi to 0-05 per cent, in water. (2) 0-04 

 per cent, in water by diluting 10 cc. of the stock 

 (1-2 per cent.) solution with 290 cc. water. 



(3) 0-02 per cent, in 50 per cent, alcohol. 

 (4) Hke (2). 



(5) o-oi per cent, in water. (6) 0-02 per cent. 

 in water. 



(7) 0-02 per cent, in 60 per cent, alcohol. 

 (8) as (2). 



(9) Strong aqueous solution, dialysed against 

 distilled water. 



(10) As (2). (n) o-oi per cent, in 50 per 

 cent, alcohol. 



(12) 0-02 per cent, in water, by diluting 

 10 cc. of the stock (0-6) per cent, solution with 

 290 cc. water. (13) as (12). 



(14) Same as solution (2). (15) 0-05 per cent. 

 in 50 per cent, alcohol. (16) 0-04 per cent, in 50 

 per cent, alcohol. 



Volume required. In most cases ten drops 

 to 10 cc. of the solution are about right. But 

 the amount varies with the range, colour of solution, 



etc. Thus 12 drops of no. (12) maybe required Fi f;5- Bottleand 



J ^ Dreyer s Drop- 



at P H = 6-9, and only 5 drops at P H = 8-0. It ping Pipette 



is essential that exactly the same amount be 



added to the measured volume of the fluid and to 



the same measured volumes of the standard solutions. The most 



convenient and accurate method of adding the drops is to have the 



bottle of indicators fitted with rubber corks pierced with Dreyer's 



dropping pipettes (fig. 5). 



Colour filters for dichroic indicators. Brom-phenol blue and 

 brom-cresol purple are dichroic. To get reliable results, especially 



* Concentrated standardised solutions can be obtained from the Cooper 

 Laboratory for Economic Research, Watford, Herts. 



