40 Estimations occurring in Agricultural Analysis [60 



Phenol -phthalein. Must be dissolved in alcohol. Colour- 

 less when acid or neutral red when alkaline. It should not be 

 used with ammonia, but is very useful in the titration of weak 

 organic acids. 



Lacmoid. Dissolves in water, and gives the same colour 

 effects as litmus, but is more sensitive. 



Methyl orange. Is a most convenient indicator and 

 strongly to be recommended, its colour being bright yellow with 

 alkalis and red with acids. It should be made up by dissolving 

 a gram in a litre of water. It is not affected by carbonic acid. 



In using indicators to determine the end of a reaction, the 

 solutions should always be used in the same way. For in- 

 stance, if methyl orange be used to determine when a certain 

 amount of H 2 S0 4 is neutralised by running in alkali from a 

 burette, a slightly different result will be obtained from that 

 which would be got by running the acid into the alkali. 



PREPARATION OF SEMINORMAL SULPHURIC 

 ACID 



60. Half fill a litre flask with distilled water. Measure out 

 35 c.c. of strong sulphuric acid in a graduated 100-c.c. cylinder, 

 and pour it, a little at a time, into the water in the 

 flask, shaking the liquid round after each addition. 

 The acid need not be measured very exactly, as 

 the solution has to be standardised afterwards. 

 When all the acid has been added, cool the flask, 

 either by allowing it to stand over night or by 

 placing in a current of cold water, as shown in fig. 

 24. The beaker placed over the mouth of the 



Fig. 24. ^ r 



flask prevents any water getting into the liquid. 

 Next fill up to the litre mark with water, and mix thoroughly. 



N.B. — When large quantities of standard acid have to be prepared, 

 this mixing becomes a matter of some difficulty. The best method is to 



