2G8 REPORTS ON THE STATE OF SCIENCE. 



The Sensitiveness of Indicators. By H. T. Tizard. 



[Ordered by the General Committee to be printed in extenso.] 



Before we could distinguish so accurately as we can at present between 

 degrees of acidity, an indicator was regarded as a substance with the 

 remarkable property of suddenly changing its colour as the solution in 

 which it was contained became either ' acid ' or ' alkaline,' pure water 

 being taken as the standard of neutrality. It was Ostwald who, as is 

 well known, brought forward a simple and convincing ionic theory of 

 indicators which was sufficient to explain all the facts known at that 

 time, and which even now it has only been found necessary to alter to a 

 relatively unimportant extent. According to him, the change of colour 

 was brought about directly by a change in the ionisation of the indicator ; 

 an indicator was to be looked upon as a weak acid or base, the ions of 

 which had a different colour in solution from that of the undissociated 

 molecule. Modern experience teaches us that simple ionisation of this 

 kind is not as a rule accompanied by any deep-seated change in colour ; 

 the colours of copper sulphate, and potassium permanganate, for example, 

 appear absolutely unaltered on dilution from moderately strong to very 

 dilute solutions, although these salts are completely ionised in the latter, 

 and largely in the undissociated state in the former. Sometimes we 

 observe in such solutions a slight alteration in the depth of colour on dilu- 

 tion, but it is now generally admitted that a considerable change in the 

 absorption is only brought about by a considerable change in the constitu- 

 tion of the molecule. We therefore regard indicators not as true, but as 

 pseudo-acids or bases, and assume that the undissociated molecule is 

 really a mixture of one or more tautomeric forms in equilibrium, only one 

 of which ionises to any considerable extent. Thus on the old theory, an 

 indicator was an acid, say XOH, which dissociated into ions XO' and H°, 

 XOH and XO' being differently coloured in solution. On the new theory 

 an indicator, in the undissociated form, is a mixture of two (or more) forms, 

 HXO and XOH, in equilibrium ; and under suitable conditions the form 

 XOH ionises, giving rise to the ions XO' and H°. The equilibrium 

 equation may then be written in the form : — 



[HXO ^ XOH] V- XO' + H°. 



Of these different molecular species, XOH and XO' must be similarly 

 coloured, or, to speak more strictly, must absorb light to a similar extent 

 in solution : HXO, being a differently constituted molecule, may exhibit 

 an entirely different colour in solution. Thus HXO may be deep red, 

 XOH and XO' pale yellow, in equivalent solutions. If the equilibrium 

 between HXO and XOH leans very much to the side of the former — if, for 

 example, there are always 1,000 molecules of HXO to every molecule of 

 XOH — then the colour of the solution which contains practically no XO' 

 ions will be entirely due to HXO. When ionisation takes place, the ions 

 XO' and H° are formed from XOH, some of the latter disappears, and 



hence the equilibrium HXO ^I'l XOH readjusts itself. Finally we may 



