PROCEDURE!

The optical cells were kept scrupulously clean. Immediately prior to use, they were thoroughly cleaned with concentrated nitric acid and rinsed succe^vely with water and alcohol* Ho difficulty was experienced

in drying and polishing them with a clean white cloth. One cell was strictly reserved for use with solvent, and the other for solution. In most oases, the colour- ^less solution (’solvent*) was of similar concentration

to the coloured solution save that one of the ions required for the production of colour was omitted, usually the iron. Thereby, error arising from the

presence of that ion as impurity in the other stock solutions could he eliminated. The solutions were

filtered to free them from dust, and in certain instances thermostated before use.

Ho measurements were made until the light source had been illuminated for some minutes, in order that

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The coloured solutions, themselves, were protected from light in so far as that was possible. Once in the instrument, the solutions were completely isolated from any light other than that transmitted by the spectrometer. That decomposition was negligible was shown by repetition of earlier readings at the completion of a run. A

minimum of four extinction measurements were made at each wavelength and a mean value taken. The order in which

I and Iq are measured are not relevant, but I was measured

alternatively first and second in actual practice. This led to a slight economy of time and a complete spectral analysis of a single solution could be made in about one hour.

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THE STABILITY CONSTANT OF THE COMPLEX PeCNS®'' FeCNS®* 3 = Pe“* + CHS

[PeCNS^^] “ [Pe“][CNS"J

= Stability Constant

The complex PeONS®"^ differs from the others in that it is possible to obtain a solution in which it is formed almost exclusively. On the other hand any attempt to obtain a solution in which any of the other complexes is in a

state of virtual isolation must fail in as much as each of these complexes must at all times be in equilibrium with other complexes in such proportions that do not allow of isolation. It is generally accepted that the simple ion PeCNS®^ is the only complex formed to any measurable extent when the thiocyanate concentration is small. Provided

the thiocyanate concentration is maintained at a sufficiently low level, it may be assumed that the red colour of such

solutions is due, essentially, to the complex PeCHS^^ and, consequently,, that any estimation of the colour of such solutions serves as a measures of the concentration of the simple complex. If the total concentrations of iron and thiocyanate in the coloured solutions are known, it is

-.Vf: : V':v \

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(possible to derive a value for the stability constant of the complex. A variety of such values have been obtained by various workers e.g. Prank and Ostwalt (9 ), Edmonds

and Birnbaum ( 8 ), Bent andl French {4^ ), and Babko ( i ). In no case did these workers employ values of ionic

strength greater than unity and it is significant that none of them performed their experiments at constant

temperature, even though the colour of ferric thiocyanate solutions is believed to be sensitive to changes in

temperature. The object of the experiments about to be reported was to obtain, at 18^0, a value of solut­ ions in which the ionic strength was maintained at 1.78 with HH^OIO^ and with KNO3 . The values so obtained have been employed in the partition calculations previously described.

Theory

Although a number of methods have been devised for the determination of K^, all are substantially the same, being developed from the assumption that, under conditions

of dilution, no complex, other than PeOHS^*^ is formed in appreciable amounts. The method adopted was essentially that described by Prank and Ostwalt (loc.cit.).

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The optical densities of solutions containing a small fixed amount of thiocyanate and varying amounts of iron

are measured over a range of wavelengths, the acid content and ionic strength of all the solutions being the same. If the original concentrations of iron and thiocyanate are

’a* and *b*, and if the concentration of PeCNS^^is *x’, then

^ 1 (a-x) (b-xj

for any particular ionic strength, and assuming for the moment that no hydrolysis of the ferric ions occurs. It follows that

x ® - ( a ^ b + K ) . x + a b = ^ 0 K = 1 -î-K

1

Since ’a* and *b* are small, the second term on the right hand side may be ignored. Hence

a + b -f K

But, since it is as aimed that all the colour is due to PeCH8^+

log Iq/i - t .x.d

for any particular wavelength, log Iq/I being the extinctLoo,

S the molecular or ionic extinction coefficient for the same wavelength, and *d* the thickness of the optical cell, in this case unity. By combining the last two equations on# obtains

: . / ' ' ' '

Dx

whence

5% r(a+b) + 1 ...

Equation (1) represents a straight line, the slope of which is equal to the reciprocal of the extinction

coefficient for any particular wavelength and the Intercept of which is equal to the quotient I f&K*. If sufficient experimental data is available, the equation may be solved by th^e method of least squares. On the

other hand, should optical measurements be made on two specific solutions only in which the iron concentrations are a and a ’, £ the extinction coefficient for any

wavelength, is given by

6 = A a . . . (2 ) Knowing values of 6 , values of K, and hence K^# may readily be found. This latter method was chosen for obtaining values of K^. At the same time a series of extinction curves were obtained for ferric thiocyanate solutions in which the thiocyanate concentration was small and the iron concentration varied over the largest possible range set by the instrument.

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EXPERIMENTAL

The experiments are divided into two sections

A The effect upon the absorption spectra of ferric thiocyanate solutions of varying the iron

concentration,

and B The determination of

â The absorption spectra of ferric thiocyanate solutions containing a range of iron concentrations. The solutions analysed photometrically had the following compositions*

NH^CNS 3x10"^ M

Pe (010^ )g 5*786x10'“^ M x x X = 1-15

HCIO4 0.182 M

Ionic strength 1.78 with NaClO^ The thiocyanate concentration was small and sufficient only to permit of the formation of PeONS^^ . The various solutions from which the coloured solutions were prepared were preheated to 18+.1®C in a thermostat. Measurements of

extinction were made immediately onr mixing. A complete analysis from 3850Â to 7000A, with four readings at each wavelength, took about an hour. That no decomposition or

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