Los conectores

Likewise, the more symmetrical complexes Pe(GHS)g(HgO)^ and Pe(ONS)^(HpO)g may well exhibit greater stability than the asymmetrical complexes Fe(CNS) ( H g O a n d

Fe(ONS)g(HgO )®“. This at least might be suggested by the data in Table II.

It would appear, therefore, that Method III is not truly applicable, though the error involved may appear greater than it actually is.

Fig.I and II show curves log.Fe^yFe^ vs log.(CNS”). Fig.I shows the experimental curve for series A and given with it are plots calculated from the values of nk^ (All)

in conjunction with equation (1). Similarly, Fig.II represents experimental and computed data for series B

(BII and BIII^^). The agreement between calculated and

experimental curves is excellent in both cases where calcul-

f.

'■V- >-■*'' ■' ■'■ ■ . ■■ ■ ', .■.-■■.■ -■■ • -■■■■•‘- ■ .'.A , • ; - -■■ . , -.■■■ ■ » . " ■ ■ -

S Ui

u V

96

curve computed from the Bjerrum data (Billa) is lower than the experimental curve, indicating that less of the neutral molecule is extracted. This follows naturally from the

above discussion. By assuming that Bjerrum^s theory is applicable, the neutral complex is assumed to have greater stability than, in fact, it has. Hence the calculated partition ratio [Pe(GH8)g]^ ♦ [Pe(0N8) = nk^ is greater, and,consequently, the calculated values of

log.Pe^JFe^ are less than the experimental values. It has been noted earlier that the values of nk^ = n calculated by Mebhods II and III are not compatible.

The calculated values of nk^ have been used to

construct nomograms from which may be read off the proport­ ions of the different ferric thiocyanate complexes in

solutions containing a wide range of thiocyanate concentrât' ions. These nomograms are shown in Pig.Ill and IV. The former is calculated from the series A constants, and applies, therefore, to ferric thiocyanate solutions in which the ionic strength is maintained at 1.8 with KNO .₃ The nomogram in Pig.IV is calculated from the series B data (symmetry method II) and, consequently applies to solutions of ionic strength 1.8 and containing NaOlO^. These nomograms apply strictly in cases where the iron

97

concentrations, though it is possible, with their aid, to to estimate the complex concentrations in any solution containing quite a range of iron and thiocyanate

concentrations by a method of approximations. So, also, it should be possible with their aid to prepare solutions containing predetermined complex concentrations*

100

APPLICATION of SPEOTROOHEMICAL ANALYSIS to the STUDY of the FERRIC THIOCYANATE COMPLEXES.

No study of ferric thiocyanate would he complete without an attempt "being made to measure, on a

quantitative "basis, the extent to v/hich the markedly

coloured solutions of ferric thiocyanate complexes ahsorh light in the visihle region of the spectrum. It is

realised, however, that such an attempt must "be

associated with considera"ble difficulty, since all "but the simplest of these solutions contain a number of coloured species, the capacity of which to a"bsor"b light will he expected to vary from one complex to another. However, with a knowledge of the various equilihrium constants

interrelating the individual complexes, the concentrations of the different ionic species in any particular solution may he calculated. In turn, it should he possible, by mathematical analyses, to separate, from the absorption

spectra of such solutions, the individual absorption spectra characteristic of the constituent ions.

Moreover, since the colour of solutions in which the thiocyanate concentration is small is due entirely to the simplest complex Fe(ONS) , any measure of the light

101

extent to which thiocyanate ion is united with ferric ion, and such a measure may be used to advantage in obtaining a value of the stability constant of the simplest complex Fe(ONS)^’^. And in turn, such a

method may be utilised in determining the effect which added ions may have in diminishing the colour of ferric thiocyanate solutions, either by the formation of

complexes, or by their capacity to alter ionic strength. The following pages are devoted to an account of:- (a) the construction, and calibration of a photoelectric

spectrophotometer, employed in this work, and the theory of spectrochemical analysis.

(b) the determination of the stability constant of the simplest complex Fe(ONS)^*** (absorption spectra of ferric thiocyanate solutions containing a small concentration of thiocyanate and varying

concentrations of iron).

(c) the absorption spectra of ferric solutions containing fixed amounts of ferric ion, and varying amoimts of thiocyanate ion.

(d) the absorption spectra of ferric thiocyanate solutions in various organic solvents.

(e) the absorption measurements on ferric thiocyanate solutions of different ionic strengths (activity coefficients).

108

TEBORY OF 8PB0TR00HBMI0AL ANAbYSIS

The colour of a solution arises from the capacity of the solute to absorb light of specific wavelengths;

the intensity of colour is a measure of the extent to which the solution is capable of absorbing light of these

specific wavelengths. When a beam of monochromatic light enters an absorbing medium, the intensity of light decreases exponentially as it passes through the medium, the intensity of the emergent beam being given by

I = lo<z

where I and lo are the intensities of the transmitted