200 HYDltOGKX lt)X COXCKXTUATIOX IX S.A. SOILS. 



Gamble" as follows: — " If we add one drop of hydrochloric acid 

 to pure distilled water we probably change its pH by several 

 integers, say from 7 to 8. If we add the same amount of acid 

 to water containing a little sodium phosphate the change of pH 

 will be ver}' slight indeed." Salts of weak polybasic acids such 

 as phosphates and borates exhibit strong buffer action, while 

 salts of strong acids and bases show very slight buffer action. 

 Buffer action is of the utmost importance in Nature and is 

 exhibited to a marked degree by all physiological fluids. Its 

 im|)ortance from our present standpoint, as wall be seen when the 

 method of determining pH is described, is that it enables us to 

 j)rc'pare standard stock solutions of known pH, which are not 

 affected by the traces of alkali in the glass vessels, of ammonia 

 and of carbon dioxide m the atmosphere. 



The effect of diluting a solution of a strong highly dissociated 

 acid like hydrochloric acid is to cause a considerable change in 

 the hydrion concentration. Dilution of weak, partially dissociated 

 acids, however, causes very little change in pH. This will be 

 more clearly understood by reference to the example already 

 given of hydrochloric and formic acids. The former is 75 per 

 cent, ionised even in a normal solution, consequently there is 

 only 25 per cent, of reserve, undissociated electrolyte. Further 

 dilution soon produces almost complete ionisation, and, after this, 

 increasing dilution decreases the concentration of hydrogen ions 

 in a given volume of solution, that is, it increases the pH. Formic 

 acid, on the ocher hand, is only 1 per cent, ionised in a normal 

 solution, which therefore contains 99 per cent, of reserve, undis- 

 sociated electrolyte. Dilution of such a solution increases the 

 dissociation, and consequently causes the production of more 

 hydrogen ions, thus compensating for the increased volume of 

 solution. This etfect is seen in all buffer solutions. For example, 

 a normal sokition of asparagine has pH2'95, a tenth normal 

 solution has pH2"97 and a hundredth normal solution pH3'll. 



The Soil Solutiox. 



Plants obtain food substances from the soil in solution in 

 water, and the acid or alkaline reaction of the soil solution may, 

 therefore, be expected to produce marked effects upon the vegeta- 

 tion growing upon any particular soil. The possibility of the 

 reverse process — the vegetation itself producing changes in the 

 soil reaction — must also be kept in mind. Soil acidity has in fact 

 long been recognised as a factor of considerable importance to 

 plant life, but the older methods of measuring it are all eminently 

 unsatisfactory. The commonest method is by the use of litmus 

 paper, which is a very defective indicator of acidity for several 

 reasons, and often yields variable and contradictory results. 

 Other titration methods measure the total quantity of acid 

 substances present in the soil. Now the soil solution does not 

 inherently differ from any other solution, and as we have seen 

 the acidity of a solution depends, not on the total quantity of 

 acid present, but upon the hydrogen ion concentration of the 

 solution. The only satisfactory method, therefore, of measuring 



