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HYDIKXiKX lOX COXCEXTRATIOX IX S.A. SOILS. 



When these solutions are mixed in certain definite proportions, 

 mixtures of known pH vahies between 4*49 and 9' 18 are obtained, 

 as will be seen from the accompanying table taken from 

 Prideaux.' 



Prom this table a graph can be constructed by plotting the 

 pH values against the number of c.c. of N/15 Na.HP04 in 10 c.c. 

 of a mixture. From this graph it is possible to read off the 

 composition of any mixture of given pH, and hence to prepare 

 solutions of any desired pH. 



A set of six indicators is used, each indicator showing a 

 gradual coloiir change at different pH values. Thus bromthymol 

 blue is yellow for values of pH below 6"5, between 6"5 and 7'5 

 it changes through various shades of green, until for higher values 

 still it becomes blue. It can, therefore, be used to determine 

 pH values 6"5 — 7'5. For more acid solutions three other indi- 

 cators are used: — Bromcresol purple for values between 6'0 and 

 6'5, methyl red 4"5 — 6"0, and Bromphenol blue 3'5 — 4'5. For 

 more alkaline solutions Phenol red indicates values between 

 T'O and 8"5, and Cresolphthalein those between S'o and lO'O. 



The soil extract is prepared by thoroughly mixing 10 gms. 

 of soil with 50 c.c. of water. Tap water may be used provided 

 its ])H is not far removed from 7; in our experiments the pH of 

 our tap water was determined as 7*2. The mixture is allowed 

 to stand for about twelve hours, and the supernatant liquid is 

 then poured off. In the actual determination of pH, 10 c.c. of 

 this soil extract is taken and 10 drops of the indicator added. 

 In the event of the extract being strongly coloured, it may be 

 considerably diluted as already explained without appreciably 

 altering the pH value. The extract is first tested with 

 Bromthymol bkie. Three possibilities then arise: 



