The boedeker reaction—I

J. lnorg. Nucl. Chem. 1965,Vol. 27. pp. 831 to 840. PergamonPress Ltd. Printedin Northernlreland

THE BOEDEKER REACTION--I T H E I N T E R A C T I O N OF A L K A L I M E T A L SULPHITES A N D N I T R O P R U S S I D E S IN AQUEOUS SOLUTION*

W. MOSER, R. A. CHALMERS and A. G. FOGG~ Chemistry Department, The University, Old Aberdeen, Scotland (Received 25 May 1964; in revisedform 9 October 1964) Abstract--The association of nitroprusside and sulphite ions in aqueous solution has been sho~n colorimetrically to be greatly affected by alkali metal ions. An increase in alkali metal ion concentration intensifies the red colour characteristic of the sulphitonitroprusside ion, Fe(CN)sNOSO~ ~ . Further, for equivalent concentrations of alkali metal ion, the extent of intensification increases with the size of the alkali metal ion. It is suggested that the sulphitonitroprusside ion exists in solution

mainly as the ion pair, Fe(CN)sNOSOaM3-, where M is an alkali metal. The stability constants for this system are considered. The values obtained indicate that the degree of association is much lower than has previously been reported. WHEN solutions of sodium nitroprusside and sodium sulphite are mixed, the resulting solution is coloured red. The addition of a concentrated solution of zinc sulphate considerably intensifies the colour of the solution and a red precipitate usually forms. I1) This precipitate has been characterised as zinc sulphitonitroprusside, Zn2Fe(CN)~NOSO3. ~) The colour intensification produced by the addition of zinc ion is considered to be due to the formation of insoluble zinc sulphitonitroprusside.13,4~ The bonding of the nitroprusside and sulphite groups in the sulphitonitroprusside ion is not fully understood. When extremely concentrated solutions of sodium nitroprusside and sodium sulphite are mixed, the yellow sulphitopentacyanoferrate (II) ion, Fe(CN)sSQ ~-, is formed351 As the sulphitonitroprusside ion must be an intermediate in the formation of this ion, SCAGLIAR1N~considers it probable that the sulphite group is bonded to the nitrosyl group. C6) Further, by analogy with the sulphidonitroprusside ion, Fe(CN)sNOS 4-, and the nitropentacyanoferrate (II) ion, Fe(CN)sNO24 , he considers it likely that the sulphite group is bonded directly to the nitrogen atom of the nitrosyl group. ~) DWORZAK et al. have studied colorimetrically the interaction of solutions of sodium sulphite and sodium nitroprusside, c4) They confirm that the main coloured ion present in these solutions is the 1:1 (nitroprusside:sulphite) ion, but maintain * This paper represents part of the work submitted by A. G. FOGGto the University of Aberdeen for the degree of Doctor of Philosophy. t Present address: Department of Applied Science, College of Advanced Technology, Loughborough, Leicestershire, England. l~l C. BOEDEKER,Ann., 117, 193 (1861). f~ T. PAVOLINI,Boll. chim.farm. 69, 713, 719 (1930). ~3}j. FAGES, C.R. Acad. Sci., Paris 134, 1143 (1902). t~ R. DWORZAK, K.H. BECHT, L. REITTER and E. RUF, Z. Anorg. Chem. 285, 143 (1956). ~5~ K. A. HOFFMANN,Ann., 312, 1 (1900). ~"~ G. SCAGLJARINX,Atti (IV) congr, naz. chim. pura applicata, 1933, 597 (1932). 831

832

W. MOSER, R. A. CHALMERSand A. G. FOGG

that small quantities of a 1:2 (nitroprusside : sulphite) ion are also present. They have o b t a i n e d the following dissociation c o n s t a n t s : Fe(CN)sNO(SO3)26- ~- Fe(CN)sNOSO3 a- + SO32- (k = 0.4) F e ( C N ) s N O S O z 4- ~ F e ( C N ) s N O 2- + SO32- (k = 0.067) T h e Boedeker reaction has been reexamined by the present authors. This first paper is concerned with the interactions of alkali metal sulphites a n d nitroprussides in a q u e o u s solution. GENERAL EXPERIMENTAL Solid reagents. All solids used were of reagent grade. The potassium nitroprusside was obtained especially from May and Baker Ltd. Reagent solutions. Standard nitroprusside solutions were prepared daily using a weighed amount of solid reagent. Sulphite solutions were stabilized against air oxidation by the addition of small quantities of a dialkali metal salt of ethylenediaminetetraacetic acid (EDTA). The maximum EDTA concentration permitted in the measured solutions was 0.001 M. At this concentration the EDTA had no observed effect on the equilibria studied. In Table 1 the rates of deterioration of a stabilised and an unstabilised 0.01 M sulphite solution are compared. TABLE 1.--STABILITY OF

0'01 M

SODIUM SULPHITE SOLUTIONS TO AIR OXIDATION.

THE SOLUTIONS WERE PREPARED FROM PREVIOUSLY BOILED AND COOLED, DEIONIZED WATER~ AND WERE CONTAINED IN 500 m l VOLUMETRIC FLASKS

Time (days) Unstabilized solution

0

3

7

10

100

24"5

0'7

<0'4

100

97"5

96.5

94"5

Percentage of original sulphite ion remaining Stabilized solution (0-0003 M EDTA)

Standard alkali metal salt solutions were prepared accurately by weighing the solid, but the concentrations were checked flame-photometrically. Standardization ofsulphite solutions. Sulphite solutions were standardised iodometrically. The iodine solution was prepared as required by acidifying an aliquot of an N/10 or an N/100 potassium iodate/potassium iodide solution. Aliquots of the sulphite solution were added to the iodine solution and the excess iodine titrated with a standard sodium thiosulphate solution. When in use the sulphite solutions were standardised every three hours. The p H limits in these investigations. Sulphite ion is the only species of sulphurous acid which associates with the nitroprusside ion. ta~ From the dissociation constants of sulphurous acid one can deduce that in solutions of pH greater than 8.5 over 97 per cent. of the sulphurous acid is present as sulphite ion. Solutions of sodium sulphite of concentration 0"01 M and above fulfil this pH requirement. If sulphite solutions of concentration less than 0.01 M are examined, a suitable buffer must be used. The pH of the solutions studied must not be raised much above pH 9, however, as the nitroprusside ion is converted ultimately into the nitropentacyanoferrate (II) ion,TM according to the following equation: Fe(CN)sNO ~- + 2OH- ~ Fe(CN)sNO~ 4- + H20 Theoretically, small quantities of the nitropentacyanoferrate(II) ion should be evident at pH 9, but, fortunately, its rate of formation at this pH is slow. tT~ When the equilibrium is reached at pH 10, equivalent quantities of the two ions are present; at pH 11 conversion to the base form is complete. tT~I. M. KOLTHOFFand P. E. TOREN,J. Amer Chem. Soc. 75, 1197 (1953).

The Boedeker reaction--I

833

Instrumentation. Absorption spectra were obtained using a Unicam SP 500 spectrophotometer. A Spekker Absorptiometer, fitted with a mercury vapour lamp, was used to obtain measurements at a fixed wavelength, It was found that for all additives used in the work described in this paper, the single visible absorption maximum of the coloured species always occurred at 475 rap. An Ilford 602 gelatin filter was used with the absorptiometer. All optical densities and molar extinction coefficients quoted in this paper refer to the wavelength range of the Ilford 602 filter unless stated otherwise. Apparatus. Graduated pipettes were used to measure out volumes of solution up to 10 ml. It was found convenient to prepare solutions for colorimetry in 25 ml or 50 ml volumetric flasks. EXPERIMENTAL Initially, the method of continuous variations as used by JOB~ was employed to investigate the constitution and stability of the coloured species formed in these solutions. It was found that the coloured species was extremely unstable, and, therefore, the method of BETTS and MICHELS~9~ was used to obtain a value for the stability constant. It is convenient at this point to note that the experimental curves illustrated in this paper are drawn as broken lines. Calculated curves are drawn as full lines. The optical density due to the new coloured species--the sulphitonitroprusside ion and its ion pairs--has been derived by deducting the optical density due to the total added nitroprusside ion from the experimentally-obtained optical density. This is justified by the low stability of the new coloured species. The colour intensification produced by alkali metal ions. A comparison of the magnitude of the intensification produced by different alkali metal ions was made by adding the same molar proportion of salts of each alkali metal to identical solutions containing sodium nitroprusside and sodium sulphite. The results are shown in Table 2. TABLE 2.--INTENSIFYING ACTION OF ALKALI METAL IONS.

[Fe(CN)sNO 2-1 = 0.004 M, [SO3~-] = 0.13 M,[Na +] -:= 0.27 M, and further[M +] =- 0.31 M Further alkali metal ion (M +) Optical density

None added 0"118

Li +

Na +

K+

Rb +

Cs

0.130

0-178

0'342

0.436

0.604

It was found subsequently that the effect of ammonium ion on the colour intensity of this solution was of similar magnitude to that of sodium ion. Magnesium ion had no noticeable effect on the colour intensity.

The relative dependence of the colour intensity on the alkali metal ion concentration and on ionic strength. The colour intensities were obtained for a series of solutions having the same added sulphite and added nitroprusside ion concentrations, but with an increasing sodium ion concentration. This procedure was repeated using sodium salts of various mono-, di-, and tri-basic acids to increase the total sodium ion concentration of the solutions. A similar set of results was obtained using solutions containing potassium nitroprusside, potassium sulphite, and potassium salts of mono-, di-, and tri-basic acids. In all cases curves were plotted of optical density against sodium or potassium ion concentration (Figs. 1 and 2), and of optical density against ionic strength. The optical density is proportional to the sodium or potassium ion concentration and is almost independent of "inert" anions such as sulphate and nitrate. The dependence of the colour intensity on the sulphite and nitroprusside ion concentrations. A curve was obtained of optical density against added sulphite ion concentration at constant added nitroprusside and added sodium ion concentrations (added nitroprusside ion concentration -- 00126 M, sodium ion concentration = 0.525 M). A standard sodium sulphate solution was used to maintain the sodium ion concentration constant. A similar curve was obtained using potassium nitroprusside, potassium sulphite and potassium chloride (added nitroprusside ion concentration = 0,012 M, potassium ion concentration = 0.424 M). ~ P. JoB, Ann. chim. 9, 113 (1928). ~9~ R. H. B~'rTS and R. K. MICnELS, J. Chem Soc. (Suppl.), S 286 (1949).

834

W. MOSER, R. A. CHALMERS and A. G. FOGG

A 06i E

>,

8

04

0.2

0'

l

I

0.25

I

0.50

I

0.75

I00

Sodium ion concentration, M F]O. ].---Effect of sodium ion. A. chloride, B. nitrate, C. tartrate, D. sulphate and [SO3 ~-] : 0"104 M

E. citrate [Fe(CN)sNO 2-] = 0'0126 M

0,4-

x

S

B

~ 0.2 0

0

1

0 25

I

O50 Potassium

1

0.75

I

1.00

ion concentration,

M

FIG. 2.--Effect o f potassium ion. A. chloride, B. nitrate, C. tartrate, D. sulphate and E. citrate. [SOa 2-] = 0.0437 M [Fe(CN)sNO 2-] = 0.0088 M

Curves were obtained also o f optical density against a d d e d nitroprusside ion concentration at constant a d d e d sulphite and added sodium or potassium ion concentrations (added sulphite ion concentration = 0.0588 M, sodium ion concentration = 0.329 M ; a d d e d sulphite ion concentration = 0.0261 M, potassium ion concentration = 0.290 M). All these curves are linear up to and beyond the equivalence point for a 1 : 1 (nitroprusside: sulphite) species.

The Boedeker reaction--I

835

Method of continuous variations. Curves were obtained of optical density against solution composition for a number of series of solutions. These are shown in Figs. 3-5. The sodium ion concentration was controlled by additions of sodium acetate.

//

J l

/

08 ! !

/

d

d

!

4 c

06

-g o o. 0

04

02

J

0 0 024

002 0020

0!04 0 016

006 008 0.012 0,008

O'.Io 0 004

Nitroprusside ion concentration, Sulphile ion concentration ,

012 0 M M

FIG. 3.--Method of continuous variations. [Na +] ::: 0.93 M

Method of Betts and Michels. A number of solutions were prepared with high sulphite and potassium ion concentrations (approx. M and 2'5 M respectively), but with very low added nitroprusside ion concentrations (approx. 3 × 10 4 M). The optical densities of these solutions were used to apply the method of BETTS and MICHELS. The colour of these latter solutions faded quite rapidly, probably due to the formation of sulphitopentacyanoferrate (II) ion at such high sulphite ion concentrations. It was necessary to determine the colour intensities of the solutions within two minutes of preparation. The colour hTtensification produced by non-electrolytes. It was found that the addition of urea or glycine to a solution of sodium nitroprusside and sodium sulphite increased the colour intensity of the solution. Curves of the optical density of a solution of sodium nitroprusside and sodium sulphite against the concentration of non-electrolyte in the solution were obtained for urea, glycine, hexamine, urethane, sucrose and pyridine. These are shown in Fig. 6. The relative intensification by urea and sodium ion is shown in this figure. RESULTS

AND

DISCUSSION

The nitroprusside:sulphite ratio in the coloured species. DWORZAK gives the m o l a r e x t i n c t i o n coefficients o f the t w o c o l o u r e d species w h i c h he c l a i m s to be present in these solutions as 37-6 (1 : 1) and 48 (1:2) at 546 m#. This would indicate a molar e x t i n c t i o n coefficient for the 1 : 1 s p e c i e s - - t h e m a i n species p r e s e n t - - o f

a b o u t 70 at

W. MOSER, R. A. CHALMERS and A. G. FOGG

836

/

\

/J~

I0

\\

/

,

11 //

/

0-8

!

!

~g

O6

o

Q.

0

04

I

,/ 0.2

0 008

J 0.01 007

I

l

0'02 0 03 0"06 005

I

0 04 004

r

005 003

~

0 06 002

i

007 0 Of

Nitroprusside ion concentrotion, Sulphite ion concentration,

008 0

M M

FIG. 4.--Method of continuous variations [Na +] = 0'93 M

06 ~4)"4~ "t~'" ~'

1

0.4

0-2

0-

040

0.004 0.008 O.OJ2 0-30 0 ZO O.tO Nitroprusside ion concentration, M Sulphite ion concentration, M FIo. 5.--Method of continuous variations. [Na +] = 0'88 M

0.016 0

The Boedeker reaction--I

837

the wavelength of its maximum absorption (475 m/z). The present investigations, however, indicate a single coloured species with a molar extinction coefficient of about 3300 at the wavelength range of the Ilford 602 filter, corresponding to a molar extinction coefficient of 3800 at 475 mlt. Thus the present authors consider the association between nitroprusside and sulphite ions to be much smaller than indicated by DWORZAK. The high molar extinction coefficient of the coloured species is readily demonstrable. A solution with an added nitroprusside ion concentration of 0.00010 M and 06

05

A

x

04 >, c

._t2 ~.

O2

o

--0----©

eG

I

I

I

I

I

0.5

I.O

~5

2.0

2.5

Concentration of

additive,

M

FIe. 6. -Effect of organic additives. A. Na ÷ (for comparison), B. glycine, C. urea,

D. sucrose, E. urethane, F. hexamine and G. pyridine. [SOa2-] = 0'076 M [Fe(CN)sNO2-] - 0.014 M [Naq = 0"18 M For curve A the abscissa represents the concentration of the additional sodium ion added. sulphite and potassium ion concentrations of 0.50 and 2-50 M respectively has an optical density of 0.220. This indicates that the coloured species has a molar extinction coefficient of at least 2200. Applying the method of BExTS and MICHELSto the optical densities of pairs of solutions similar to the above, the value obtained for the molar extinction coefficient of a 1 : 1 species is 3100 ± 400. The presence of a very unstable species is indicated also by the linear curves of optical density against added nitroprusside and added sulphite ion concentrations. The fact that the optical density is directly proportional to the added sulphite and added nitroprusside ion concentrations indicates the presence of either a very stable or a very unstable 1 : 1 species. In the case of a single, very stable species, however, the curves would flatten off beyond the equivalence point. The method of continuous variations provides further evidence of the composition of the coloured species and of its low stability. For a very unstable 1 : 1 species the maximum of the curve should be central over a wide range of values of b, where b is the ratio of the limiting concentrations of sulphite and nitroprusside ions for the particular curve.

838

W. MOSER, R. A. CHALMERSand A. G. FOGG

It is seen that, for curves in which the values of b are 0.2, 1 and 25 (Figs. 3-5 respectively), the maximum does not deviate appreciably from the central position. The slight dissymmetry of the curve shown in Fig. 5 can be attributed in part to the high value of b. It was the dissymmetry of his continuous variations curve (b = 1) that led DWORZAK to suggest that small amounts of the 1 : 2 species were present in these solutions. Part of this dissymmetry, however, can be attributed to the underestimation of the amount of free nitroprusside ion present. It seems probable that the dissymmetry in the present curves is due to changes in the "inert" anion composition of the solutions (see Fig. 1), and not to the presence of a 1 : 2 species. Finally, colorimetric studies of the "zinc ion intensified" system-described in the next paper of this series--again indicate that only a 1:1 (nitroprusside:sulphite) species is formed by the association of nitroprusside and sulphite iens and that its molar extinction coefficient is approximately 3300. The effect of alkali metal ion. The dependence of the colour intensity on the alkali metal ion concentration is extremely marked. Indeed, if the curves of optical density against alkali metal ion concentration (Figs. 1 and 2) are extrapolated to lower alkali metal ion concentrations, they pass very close to the origin. Therefore, very little, if any, association of nitroprusside and sulphite ions seems to occur in the absence of alkali metal ions. Further, the relative intensification produced by an alkali metal ion increases with its size. The intensification produced by potassium ion, for example, is about three times that produced by an equivalent amount of sodium ion. Thus the intensification by alkali metal ion is not simply an ionic strength effect. This was confirmed further by comparing the intensifying powers of various alkali metal salts of mono-, di- and tri-basic acids. The intensification may be correlated more readily with the concentration of a particular alkali metal ion than with the ionic strength. It would appear, therefore, that the alkali metal ion plays an active part in the association of nitroprusside and sulphite ions. It is suggested by the present authors that an ion pair of general formula Fe(CN)sNOSO3M a- is formed by the sulphitonitroprusside ion and an alkali metal ion, M +. In the case of the sodium ion system, the linearity of the curve of optical density against added sodium ion concentration at constant added nitroprusside and added sulphite ion concentrations indicates the participation of one sodium ion in the formation of the coloured species. The analogous curves for potassium ion deviate slightly from linearity. A mathematical treatment of the curves obtained to illustrate the effect of nitroprusside, sulphite and alkali metal ions on the colour intensity is of interest. If it is assumed that the'ion pair is the only coloured species formed in these solutions, then the equilibrium existing is as follows: Fe(CN)sNO 2- + SOa2- + M + ~- Fe(CN)~NOSOaM aThus, E = Ek[Fe(CN)sNO~-][SOa~-][M +] Differentiating with respect to the concentration of one particular ion gives an expression for the slope of the curve of optical density against the concentration of that particular ion.

The Boedeker reaction--I

839

e.g.

(d[Fe(cN)dEsNo2 _])

[so3~ ],tM+] = Ek[$032-][M+]

Thus, values for ek, the product of the molar extinction coefficient and the stability constant, can be obtained from these curves. They are given in Table 3. TABLE 3.--VALUES OF ek, e~k~ AND e2k,,.k~. V INDICATES THE CONSTITUENT OF VARYING CONCENTRATION. THE SODIUM ION AND POTASSIUM ION CS)NCENTRATIONS WERE ADJUSTED USING SODIUM SULPHATE AND POTASSIUM CHLORIDE RESPECTIVELY [Fe(CN)5 NO2 l

[SO32 ]

[Nail

ek

Flkl

e2k2k~

0.01 V 0.032 0.0126 V 0.0316

V 0-107 0.0806 V 0"0588 0.0435

0.32 0.32 V 0.525 0.329 V

417 440 320 401 447 321

-36.1 --45-1

328 304 320 315 309 321

[?e(CN)sNO 2 ]

[ S O 3 ~-]

[K ~]

ek

61ka

0"03 V 0'015 0'03 0"045 0'012 V 0"012

V 0"0412 0"0412 0"0412 0-0412 V 0"0261 0-0567

0"25 0'25 V V V 0"424 0'290 V

1320 1350 1530 1480 1500 1280 1220 1280

e~k2kl

-------

1170 1180 1530 1480 1500 1180 1090 1280

If, on the other hand, the sulphitonitroprusside ion exists also as the free ion, then the pertinent equilibria are: Fe(CN)sNO z- + SO32- ~

kl

Fe(CN)~NOSO34-

k2

Fe(CN)sNOSO34- + M + . - Fe(CN)sNOSO3M 3E -: elka[Fe(CN)~NOZ-][SO32-] + E2k~kl[Fe(CN)sNO2-][SO32-][M +] The intercept of the curve of optical density against sodium ion concentration at the ordinate axis gives a value for ~1kl. Using this value and the slope of the curves of optical density against added sulphite and added nitroprusside ion concentrations, values can be obtained for e~k2k 1. These values, which are given also in Table 3, show a slightly closer agreement than do those of Ek. The results in general, however, agree well and support the ion pair theory. Assuming that the molar extinction coefficients of the free sulphitonitroprusside ion and its ion pairs are all 3300, the values of k, kl and k 2 may be determined. The value obtained for k~ is 0-011. For the sodium ion system the calculated values of k and k s are 0.13 and 8.6 respectively. For the potassium ion system they are 0.39 and 36. The value of k obtained for the potassium ion system using the method of Betts and Michels is 1-4 ± 0.7. This slightly higher value may indicate that more than one

840

W. MOSER,R. A. CHALMERSand A. G. FOGG

potassium ion is associated with the sulphitonitroprusside ion at high potassium ion concentrations (see Fig. 2). The effect o f dielectric constant. The effect of the dielectric constant of the solution on the association of nitroprusside and sulphite ions is seen readily in Fig. 6. Urea and glycine, both of which increase the dielectric constant of water, ~1°) intensify the colour of a solution of sodium nitroprusside and sodium sulphite. Qualitatively it is seen that an increase of dielectric constant causes an increased association of nitroprusside and sulphite ions. This is to be expected as the force of repulsion between the two negative ions will decrease as the dielectric constant increases. The ease of ion pair formation by the sulphitonitroprusside ion and alkali metal ion, however, will be reduced. One can only assume that the increase in ease of combination of nitroprusside and sulphite ions more than compensates for the increased difficulty of ion pair formation. Acknowledgment One of us (A. G. F.) wishes to thank Shell Research Ltd. (London), part of whose grant to the University of Aberdeen went to maintain him during this work, and the University of Aberdeen as Trustees of the Courts Scholarship which he was pleased to receive.

~xo~p. S. ALLBRIGHT,J. Amer. Chem. Soc. 59, 2098 (1937).