PRESIDBNT's address — SECTION B. 76 



After a review of all the work done on this subject, Sorensen 

 (1909) gives the number 10~"" at 28° C. as being the most probable 

 value for the dissociation constant of water, or 10~'^" as the concen- 

 tration of hydrogen ion with respect to a normal solution. 



With 'the fixing of a definite value for hydrogen ion concentration 

 in water the Theory of Indicators for acids and alkalies has been 

 established on a firm basis ; so that one can now use a few drops of a 

 suitable indicator to determine in many cases by observation of the 

 colour the concentration of hydrogen ion in an acid or alkaline 

 solution. I have found it convenient to have in the laboratory a set 

 of the following indicators, giving a complete range from concentration 

 of hydrogen iou^ 10~' to 10~'^ Beside each indicator is given the 

 concentration of hydrogen ion present at the point of changing colour : — 



Methyl Violet 



Methyl Orange 



Sodium Alizarin Sulphonate 



Methyl Red 



Rosolic Acid 



Phenolphthalein 



Thymolphthalein 



Resorcin Yellow (Tropaolin 0) 



It is not possible that the indicator method wiU supersede titrations 

 generally, but for the determination of acid or alkali in cases of hydro- 

 lysis, it seems likely that this method will be found to be as satisfactory 

 as any other, and it has the advantage of being very easily carried out. 



There are certain of the artesian waters in New South Wales which 

 are specially corrosive to iron and steel casings. All these waters have 

 been analysed, but as the H * ion concentration is not widely different 

 from that in pure water, acidity or alkalinity determinations are not 

 included in the original Government analyses. 



Of late, however, the reaction of many bore waters to various 

 indicators has been determined, and I was informed that the chemist 

 in charge only found noticeable corrosion in the bores with the more 

 acid reactions. 



The corrosion of iron has occupied attention for many years, and 

 is still being largely investigated. In support of the so-called electro- 

 lytic theory, the author has pointed out (1911) that corrosion of iron 

 takes place in aqueous solutions of sodium carbonate. Supporters of 

 the so-called acid theory maintain that water and oxygen alone are not 

 sufficient to start corrosion, and that the usual cause is a small quantity 

 of carbonic acid. It is unnecessary to make sure in the case of sodium 

 carbonate solution that all carbon dioxide has been boiled out of the 

 solution, as this would aU be bound by the carbonate to form a small 

 quantity of bicarbonate. The concentration of hydrogen ion in a, 



