A POLAROGRAPHIC STUDY OF SOME WURSTER SALTS

By J. A. FRIEND* and N. K. ROBERTS*

[Manuscript receioed November 11, 19571

Four related Wurster salts are subjected to a polarographic investigation. In the case of Wurster's blue, results from the dropping mercury electrode, stationary platinum electrode, and rotated platinum electrode are compared.

The Wurster salt of p-phenylenediamine is unstable in aqueous solution but is fairly stable in a mixture of methanol, acetic acid, and water and the decrease of diffusion current with time indicates a disproportionation.

Wurster's red is also unstable in aqueous solution. In the solvent methanol, acetic acid, and water, a wave is observed with the stationary platinum electrode whose Et compares favourably with the potentiometric Eb.

Evidence from the three types of electrodes mentioned previously indicates two one-electron waves for Wurster's blue. The semiquinone formation constant quali- tatively appears much greater than that reported from potentiometric work. Decrease of diffusion current with time is perhaps due to a disproportionation (the very unstable di-imine has been shown to revert to the radical in aqueous solution).

Polarographic waves given by the Wurster salt of diaminodurene suggest that the radical does not exist in aqueous solution. Waves corresponding to the original amine and duroquinone (formed by hydrolysis of the di-imina) are obtained.

I. INTRODUCTIOE The Wurster salts, or univalent oxidation products of p-phenylenediamine

and its N-substituted derivatives, have been the subject of detailed physico- chemical studies along certain lines. Michaelis and his collaborators (Nichaelis, Schubert, and Granick 1939 ; Michaelis and Granick 1943) showed by potentio- metric and magnetochemical studies that the salts exist in solution as free radicals and in the solid state either as polymers or (in a few cases) as the monomeric free radical.

The free radicals are analogous to the semiquinone ions which exist in the free state in alkaline solutions of some quinones.

The Wurster salt cations are formed in, acid solution, and owing to increased resonance are more stable than the uncharged forms. The pH of optimum stability is approximately 4.6. The ions formed have formulae analogous to

"Department of Chemistry, University of Tasmania, Hobart.

POLAROGRAPHIC STUDY OF SOME WURSTER SALTS

A polarographic study of the oxidation of some p-phenylenediamine derivatives was made some years ago by Julian and Ruby (1950). The stationary platinum microelectrode was used. In the present work, as far as possible, the dropping mercury electrode was used, and attention was directed especially to four Wurster salts, the stability of which Michaelis had shown to vary between wide limits.

11. EXPERLMENTAL (a) Materials

The univalent oxidation products of four amines were studied. Oxidation was carried out with bromine water following Michaelis and Granick (1943) and the Wurster salts were usually isolated as the perchlorate by adding excess NaC10, to a cold solution. The perchlorate ion is polarographically inert.

(i) p-Pheny1enediamine.-A few preliminary experiments were carried out with this substance. A technical grade (B.D.H.) was used. The Wurster salt mas used in the form of the bromide, but as reported below, was too unstable to be of much value in the present work.

(ii) NN-Dimethyl-p-pheny1enediarnine.-This was prepared from dimethyl- aniline according to the method described by Gattermann (1948). The amine was purified by redistillation and boiled over a range of 1 "0. The Wurster saIt was prepared as the perchlorate from the amine dihydrochloride.

(iii) NNNfN'-Tetramethyl-p-phenylenediamine-This material was obtained in the form of the dihydrochloride by recrystallization of a sample of Eastman- Kodak product. The molar extinction coefficient of the Wurster salt at 5650 A in water was 12,400 which agreed closely with the value (12,470&60) found by Albrecht and Simpson (1955). The sulphate of tetramethyl-p-phenylenediamine was prepared by treating an ebhereal solution of the amine with an ethereal solution of H,SO,. The product had SO,, 53.4per cent. Calc. for (CH,) ,NH.C,H,.NH(CH,),(HSO,) : SO,, 53.3 per cent.

(iv) Diaminodurene (2,3,5,6-Tetramethyl-p-phenylenediamine).-This was pre- pared as the dihydrochloride from durene (Light and Co.) by the method of

106 J. A. FRIEND AKD X. K. ROBERTS

Nef (Nef 1887). The amine dihydrochloride was recrystallized and the Wurster salt prepared as the perchlorate.

All buffer salts and other chemicals used were of analytical reagent grade. The following buffer solutions were used :

pH 3.9-5.7 111 CH,COOH-1x CH,COONa. pH6.0-8 .0 0.51\1:Na,HPOpO.5~~KH2PO,. p H 9 ~ 3 5 0.311 Na2B,O,.1OH,O.

Half-wave potentials in acetate and phosphate buffer of the same pH were identical.

( b ) Apparatus Two instruments were used for the polarographic studies. One was a

Tinsley automatic ink-recording polarograpb, which was used primarily to obtain rapidly polarograms in the region of negative potentials. For much of the work a manual instrument of conventional design was used. This enabled mixed anodic-cathodic waves to be more easily recorded, and could be used readily a t positive potentials. I t also enabled half-wave potentials to be accurately determined. A Pye galvanometer of sensitivity 2 a02 x pA/mm was used to measure the current in this circuit.

A saturated calomel electrode was used as the reference electrode, in con- junction with a potassium nitrate salt bridge prepared according to the directions of Muller (1951), as the chloride ion is oxidized a t the positive potentials used in much of the present work. All potentials are quoted relative to the S.C.E. The cell resistance, as measured with an A.C. bridge, was 750 ohms.

The stationary platinum microelectrode used for potentials more positive than $0.2 V (v. S.C.E.) consisted of a piece of wire 0 .5 mm in diameter and 5 mm in length, sealed into a Pyrex tube bent a t right angles and supported horizontally. The potential was increased a t a rate of 20 mV/min.

(i) Rotated Platinum $Iicroelectrode.-This electrode was similar to electrode " A " described by Laitinen and Kolthoff (1941). A Gallenkanip laboratory stirrer was used to drive the electrode. The rate of rotation was increased until the current passing remained almost constant. I n contrast to the polarograms obtained by Laitinen and Kolthoff those of Wurster's blue were found to be reproducible from day to day.

All measurements were carried out in a thermostat controlled a t 25 .O&O.l "C, and oxygen was removed from the solutions by bubbling oxygen- free nitrogen through them. The pH of buffer solutions mas measured by means of a Leeds and Northrup glass electrode and potentiometer unit. Measurements of absorption spectra were carried out on a Unicam S P 500 spectrophotometer.

Conductance measurements were made using a Cambridge conductivity bridge and deionized water (specific conductance 1 x ~ O - ~ ohm cm-l) prepared by treating distilled water with " Bio-deminrolit ".

POLAROGRAPHIC STUDY OF SOME WURSTER SALTS 107

111. RESULTS

(a) Polarography of the Wurster Bait of p-Phenylenediamine

This salt was not stable in aqueous solutions, but was more stable in a mixture of water, methanol, and glacial acetic acid in the volume ratio of 5 : 4 : 1. The half-wave potential was too positive to be measured. Oxidation of the bromide ion distorted the shape of the wave.

oI_I--l 20 40 60 80 100

TIME (MINI

Fig. 1.-Wurster salt from p-phenylenediamine. Variation of diffusion ourrent with time, apparent pH 2.56. X Log id v. t (min)

left-hand ordinate. 0 l/id v. t (min) right-hand ordinate.

By studying the rate of decay of the radical, measured by the rate of decrease of t,he cathodic diffusion current, it was concluded that the decomposition was second-order as the plot of lli, against time was linear (Fig. 1). This is perhaps due to a disproportionation

two molecules of the salt giving one molecule each of the diamine and the di-imine, which is very unstable and decomposes immediately.

108 5. A. FRIEND AND N. K. ROBERTS

( b ) Wurster Salt of NN-Dimethyl-p-phenylenediamine (Wurster's Red) This salt is fairly stable in the water-methanol-acetic acid solvent, apparent

pH=2.56 (v. glass electrode). Decomposition was accompanied by a colour change from red to blue.

The variations of limiting current with (height of mercury)& indicated a diffusion-controlled wave. The half-wave potential Et determined with the stationary platinum microelectrode was approximately +O .39 V. The sulphate of the parent amine gave a similar wave a t the same potential, which may be compared with the potentiometric E,, value found by Michaelis and Hill (1933) of +0.36 V (pH=2.56).

(6) Wurster Salt of NNNIN'-Tetramethyl-p-phenylenediae This salt remained unchanged in the solid state. It was studied mainly in

acetate b d e r at ionic strength of 1.0.

(i) Dropping Mercury Electrode.-The polarogram of a fresh solution (prepared by adding a small quantity of salt to buffer solution from which the dissolved oxygen had been removed) consisted of an almost completely cathodic wave. As the solution aged the anodic portion of the wave increased, the total height remaining constant (Fig. 2).

0 HR 3 HR 40 HR

- 2 -

u

- - 2

E.M.F. (VOLTS u. S.C.E.1

Fig. 2.-Wurster's blue. Change of wave, with time (dropping mercury electrode) ; p H 5.3.

A solution of the sulphate of the parent amine (the chloride ion is oxidized st the dropping mercury electrode) gave an anodic wave of the same shape and with the same half-wave potential. Equal molar concentrations of radical and amine gave the same wave height. The wave of the amine gradually developed a cathodic portion on standing owing to atmospheric oxidation of the base to the free radical. When a steady state was reached the ratio of the height of the cathodic portion to that of the anodic portion was the same as for the free radical. The half-wave potential of the system remained constant.

POLAROGRAPHIC STUDY OF SOME WURSTER SALTS 109

The wave obtained was diffusion controlled for the following reasons :

(1) Limiting current was proportional to (height of mercury)*. (2) Limiting current was proportional to the concentration (over the range

0 . 5 3 m ~ ) . (3) Temperature coefficient of the diffusion current was about 2 per cent.

near 20 "0.

A graph of log i/(id-i) (i,=diffusion current, i=current a t potential E) as a function of E for the pH range 3.9-9 ~ 3 5 was linear and varied in slope from 60-70 mV/log unit

PH Slope (mV) PH Slope (mV) 3.9 60 6.5 63 4.6 60 7.2 7 0

e 5.3 69 7.5 68 5.7 62 8.0 64 6.0 60 9.35 60

The system is apparently reversible since the half-wave potential of the wave remained constant as a solution of the radical or amine aged. The slope a t pH 5.3, 7.2, and 7.5 is admittedly high for a fully reversible process but normal at the other pH values. An adsorption process may be involved. although no direct evidence of differential adsorption could be detected.

Application of the Ilkovi6 equation (Ilkovii: 1934) confirmed the result obtained from the logarithmic analysis of the wave.

Over the pH range studied the height of the wave remained constant. The main problem in applying the IlkoviG equation was the evaluation of the diffusion coefficient (D) of the Wurster cation. An approximate value of 7 x 10W cm2 sec-l was indicated by analogy with hydroquinone (D=7.2 x 10-%m2 sec-l). Con- ductance measurements at concentrations of 0 a25 and 0.5 x ~ O - ~ M yielded a value of the equivalent conductance A =108.5 mhos. Putting this value approximately equal to A, the value a t infinite dilution, and subtracting the ionic conductance of the perchlorate ion (68 mhos a t 25 " C ) , the ionic conductance of the cation is found to be 40.5 mhos. Applying the Nernst relationship for an ion of valency x (see Kolthoff and Lingane 1941)

(The superscripts indicate that the formula is valid a t infinite dilution.)

By a similar technique the diffusion coefficient of the amine dihydrochloride was found to be 9.8 X ~ O - ~ cm2 see-l. I n making these measurements, excess of free amine was added to reduce the hydrolysis of the hydrochloride. The value is only approximate as the free amine was rapidly oxidized by the air.

A value of 10.0 X ~ O - ~ em2 see-l was adopted as an approximate figure for D. However, the true value is probably considerably smaller since the concentration of the radical was usually 1 0 - 3 ~ and the base solution had a high ionic strength (1.0).

110 J. A. FRIEKD AND K. K. ROBERTS

The IlkoviE equation gives the number of electrons involved in the reduction process independently of any assumption of reversibility, and so provides necessary information for the interpretation of the logarithmic plot. It was used in the form

i -605nDll~cm2/3t1 16 d-

where n electrons are involved in the reduction of an ion of diffusion coefficient D at concentration e m ~ / 1 using a capillary of drop-time t , and a flow- rate of mmgHglsec. In the present instance, id=2.3 PA, c = l m ~ , m2'%llR=1 '444 mg213 see-l'" D=lO-5 om2 see-l. This gives n=O -83.

Fig. 3.-Wurster's blue. Half-wave potential against pH (dropping mercury electrode).

A graph of the E, against pH was linear over the pH range 3-9-6.0, with a slope of 56 mV/pH unit. From pH 6.0-7.5 the slope mas 22 mV/pH unit and from pK 7.5-9-36 zero slope (Fig. 3). Acetate buffer was employed from pH 3.9-5.7, phosphate buffer from pH 6.0-8.0, and borate buffer for pH 9.35. This E&-pH relationship is consistent with a one-electron system." Interpolation gives pH 6.6 as the dissociation constant of the system. Because the slope becomes flatter a t this point, the dissociation must refer to the reduced form.

(ii) Stationary Platinum Electrode.-Because only one wave was obtained with the dropping mercury electrode, which was shown to be a one-electron step, a platinum electrode was constructed to investigate the positive potential range inaccessible to the dropping mercury electrode for the presence of an expected anodic one-electron wave.

At the stationary platinum microelectrode, a wave as in (a) was observed, but a second wave a t a positive potential was found as well (Figs. 4 and 5). The ratio of the heights of the two waves was approximately 1 : 2.5 a t all pH's studied. mTaves obtained with this electrode were not as reproducible as those

* The range 6.0-7.5 is regarded as a transition range with an average slope of 22 mV/pH unit. It is unlikely that two dissociation constants woidd be so close.

POLAROGRAPHIC STUDY OF SOME WURSTER SALTS

I E.M.F. (\iOLTS U. S.C.E.1

Fig. 4.-Parent amine of Wurster's blue. Wave from stationery platinum electrode ; pH 5.3.

I E.M.F. (VOLTS 0. S.C.E?

Fig. 5.-Wurster's blue. Wave from stationary platinum electrode ; pH 5.3.

112 J. A. FRIEND AND N. K. ROBERTS

found a t the dropping mercury electrode, A comparison of the half-wave potentials and slopes of the wavcs obtained a t the two electrodes follows :

(1) Small Wave pH .. . . . . .. 3.9 4.6 5.3 E* (Pt) . . . . . . 0.17 0.13 0 .09 E$ (Hg) - . . . . . 0.175 0 el36 0.098 Slope from log plot (Pt) 60 60 69 Slope from log plot (Hg) 60 60 69

(2) Large Wave E4 (Pt) . . . . . . 0.44 0.42 0.39 Slope from log plot . . -60 -60 -60

The large wave had the slope of a one-electron step ; however, it is not clear why it was so large. Changing the base electrolyte to a mixture of water, methanol, and glacial acetic acid in the volume ratio 5 : 4 : 1 did not alter the relative heights of the t,wo waves.

E.M.F. [VOLTS o. S.C.E.1

Fig. 6.-Wurster's blue. Wave from rotated platinum electrode. pH 4.7.

Variation of E+ with pH was linear over the pH range 2 .5-5.3 for both the small and large wave. For the small wave a value of 55 mV/pH unit and for the large wave 30 mV/pH unit were obtained. The shift of 30 mV/pH unit is low for a one-electron system.

(iii) Rotated Platinum Microelectrode.-Recourse was had to a rotated platinum microelectrode to ascertain whether the stationary electrode was responsible for the anomalous results.

As befoqe, two waves were obtained but the anodic wave was the same height as the cathodic wave (Fig. 6). At pH's greater than 6 - 0 the anodic wave increased in height, until from pH 7 - 2 to 8.0 the ratio of the height of the anodic to the cathodic wave was 2 : 1. Half-wave potentials followed closely

POLAROGRAPHIO STUDY O F SOME WURSTER SALTS 113

those obtained with the dropping mercury electrode. A comparison of some slopes and half-wave potentials with those obtained with the latter electrode follows :

Cathodic Wave PH . . . . 3 . 9 4 . 3 4 . 9 5 - 4 6 . 0 E* (Pt) . . . . 0.166 0 a140 0 .I15 0 .078 0.052 E* (Hg) * . . . 0.175 0 el50 0 -120 0.090 0.060 Slope (Pt) (mV) . . 66 68 66 72 65 slope (Hg) (my) 60 60 ' 63 69 60

Reproducibility of polarograms was much better than that obtained with the stationary platinum electrode.

The slope of the plot of E v. log i/(i,--i) was linear at all pH values.

I I I 2 3 4 5 6 7 8 9

pH

Fig. 7.-Wurster's blue. Half-wave potential against pH ; cathodic wave from rotated platinum electrode.

In the table below the E+ and the slope of the plot of E against log i/(i,-i) for the anodic wave at pH 3 -9, 4.6, and 5 -3 is compared with the values given by the stationary platinum electrode.

pH . . . . . . . . 3 - 9 4 - 6 5 . 3 E, (stat. Pt ) . . . . 0.44 0-42 0 -39 E* (rot. P t ) . . . . 0.465 0.453 0 -441 Slope (stat. Pt ) (mV) . . 60 60 60 Slope (rot. P t ) (mV) . . 59 64 55

As limiting currents were very much larger than those obtained with the dropping mercury electrode and with the stationary platinum electrode, the potential drop due to the resistance of the electrolyte was considerable. Both the half-wave potentials and the slopes of the log plots have been corrected for this effect (" iR drop ").

The of cathodic wave given by the radical varied with pH (Fig. 7). &'*/pH relationship followed closely that observed with the dropping mercury electrode.

114 J. A. FRIEND BED N. K. ROBERTS

From pH 3.9-6-0 the slope was 5ti mV/pH unit. Between pH 6.0 and 7 - 5 the average slope was 23 mV/pH unit. Above 7.5 there was no significant change of E+. As before, the range pH 6.0-7.5 must be regarded as a transition range.

Again the Ek/pH relationship for the anodic wave gave a ~ a l u e too low for a one-electron system (Fig. 8). The cathodic and anodic waves were approximately of equal height from pH 3.9-6.0 and the Et/pH slope dependence was linear with a slope of 18 mV/pH unit. From pH 6.8 to 8.0 no change in E+ occurred, however, the anodic wave increased in height. A break occurs in the Ek/pH graph at pH 6 -7. If this break could be interpreted as due to a dissociation of the reduced form, it mould agree quite well with the value obtained potentio- metrically by Michaelis and Hill (1933) of pH 6.5.

Fig. 8.-Wurster's blue. Half-wave potential against pH ; anodic wave from platinum electrode.

There does not appear to be any obvious explanation for this increase in wave height and the low Et/pH dependence of the wave.

(iv) Amperomet~ic Titration.-Amperometric titration of the amine dihydro- chloride with bromine water showed that the addition of two equivalents of bromine produced the maximum (steady) cathodic current, which Tvas in fact the same as that obtained from the same niolar concentration of the Wurster salt a t the same potential. Addition of bromine water to the Wurster salt, as expected, did not increase the cathodic current, As theoretically only one equivalent of bromine is necessary to produce the radical, the above observation must have its explanation in the extreme lability of the di-imine with which the intermediate radical is in equilibrium.

The radical from NNN'N'-tetramethyl-p-phenylenediamine is known to be quite stable in acid solution (Michaelis, Schubert, and Granick 1939). Hence

POLAROGRAPHTC STUDY O F SOME WURSTER SALTS 115

one would expect to obtain a polarogram having equal anodic and cathodic parts. With the mercury electrode a single cathodic wave involving one electron is observed. Both the stationary and rotated platinum electrodes reveal an expected anodic wave. However, only the rotated platinum electrode produces an anodic wave equal in height to the cathodic wave. Logarithmic analysis of the anodic wave obtained with the rotated platinum electrode indicates a one-electron step, but the E,/pH dependence is too low for a one-electron system (18 mV/pH unit). For a one-electron system the E&/pH dependence can only be 0, 60 mV/pH unit, or some multiple of the latter figure.

Since the other evidence indicates that the anodic wave is a one-electron step, i t seems some other explanation must be found for the low E,/pH dependence.

The almost purely cathodic wave produced by the radical at the dropping mercury electrode develops a larger anodic portion on standing ; the overall height is constant. Also the initial purely anodic wave given by the amine changes to a wave possessing the same relative ratio of anodic to cathodic part as the wave from the radical. Atmospheric oxidation of the base produces the latter change. Both the polarographic wave and the absorption spectrum suggest the radical is being reduced to the amine. An equilibrium is apparently set up between the oxidation of the base and the reduction of the radical. The mechanism of the reduction is not clear, but it may proceed via the di-imine with which the radical is in equilibrium. Michaelis, Schubert, and Granick (1939) found that the extremely unstable di-imine reverted to the radical in acetic acid solution (the initially colourless di-imine returned to the deep blue of the radical), but no explanation was given.

(d) Wurster Ealt of 2,3,5,6-Tetramethyl-p-phenyZe'y1ediamine

, The Wurster salt was very stable in the solid state.

(i) Dropping Mercury Electrode.-At concentrations higher than 2 x 1 0 - 3 ~ the solution turned from yellow to green indicating polymerization (Michaelis and Granick 1943) and on standing for a short time yellow needle-like crystals of duroquinone were formed (LuValle, Glass, and Weissberger 1948). At lower concentrations the solution was yellow and no precipitation occurred.

A fresh solution of the Wurster salt produced an anodic and a cathodic wave. On aging, the cathodic wave increased and the anodic wave decreased in height until only the cathodic wave remained (Fig. 9). The total wave height remained the same throughout.

The polarogram obtained from a fresh solution of the amine consisted of an anodic wave of the same Eb and slope as that from the radical. Aging produced first an anodic and a cathodic wave entirely analogous to those produced by the radical and finally a solely cathodic wave (Fig. 10).

Both waves were diffusion controlled because

(i) Limiting current was proportional to the (height of mercury)*. (ii) Limiting current was proportional to concentration over the range

0.5-0 -125 ma1.

A stationary platinum electrode did not reveal any further waves. B

116 J. A. FRIEND AND N. K. ROBERTS

A graph of log (i/i,--i) v. E was linear at all pH's studied for both the anodic end cathodic waves. For the cathodic wave the slope was close to 37 mV/log unit from pH 3.9-6.4 and 31 mV/log unit for pH 7.1-9.35.

The anodic wave had a slope close to 45 mV from pH 3 ~9-5.9 which then slowly decreased from 40 mV at pH 6.4 to a steady value of 32 mV at pH 7.96.

A value of 30 mV/log unit applies to a two-electron reversible wave at 25 OC. In the case of both waves this value is approached a t high pH's. The rela]tively high value of 45 mV for the anodic wave in the range pH 3.9-5 -9 is most likely due to intermediate radical formation.

Fig. 9.-Wurster salt from diaminodurene. Change of wave with time ; p~ 4.6.

Application of the IlkoviE equation also indicated a two-electron step. As before, the main difficulty was the calculation of the diffusion coefficient of the Wurster cation. The diffusion coefficient was evaluated by conductance measurements on solutions of the Wurster salt and on the amine dihydrochloride in the presence of excess amine. Values for Do were 11 a2 x and 8.3 x cm2 see-l respectively. Since conductances increased rapidly in the case of the radical and were quite steady in the case of the amine dihydrochloride, the latter figure was accepted as correct.

Now, i,=2.60 pA m21St116=1 -619 mg2J3 sec-112, o = 0 . 5 m ~ D = 8 ~ 3 ~ 1 0 - ~ c r n ~ s e c - ~ ,

POLAROGRAPHIC STUDY O F SOME WURSTER SALTS 117

therefore, n =I9 84 electrons

that is, two electrons were involved in each step. Variation of Eg with pH for both waves suggested that two electrona and

two protons were involved in the electrode reaction over the pH range 3 -9-9 -35. A graph of E* as a function of pH had a slope of 53 mV/pH unit for the anodic wave and 59 mV/pH unit for the cathodic wave (Figs. 11, 12).

It appears that even in solutions of high pH two protons are still involved in the electrode reaction.

E.M.F. (VOLTS U. S.C.E.)

Fig. 10.-Diaminodurene. Change of wave with time ; pH 4.6.

The anodic wave was due to the amine. Duroquinone was probably responsible for the cathodic wave as the Et a t pH 6.24 (-0.131 Vj was the same as that found for duroquinone (Page 1943), -0.140 V in a ~olution of apparent pH 6 -24. Yellow needles which separated from the sloIntion on &anding were shown to be duroquinone by the absorption spectrum.

(ii) Amperometrio TiSrations.-Amperometric titration of the amhe with bromine water showed that two equivalents of bromine were required for the amine and one equivalent for the Wurster salt to give the maximum cathodic current. Oxidation of the radical produced a current equal to that obtained by the complete oxidation of the same molar concentration of the amine.

This behaviour is in agreement with expectations as the duroquinone formed from the hydrolysis of the di-imine is perfectly stable.

118 J. A. FRIEND AND N. IZ. ROBERTS

From the polarographic behaviour of the radical it is clear that it does not exist as such in aqueous solution. The green colour formed at higher concentra- tions appears to indicate that the polymer is more stable than the radical. At all concentrations, however, only waves produced by the diamine and duro- quinone are observed. Duroquinone is formed by the hydrolysis of the unstable di -imine.

Fig. 11.-Wurster salt from diaminodurene. Half- - wave potential against pH ; anodic wave.

Fig. 12.-Wurster salt from diarninodurene. Half- wave potential against pH ; cathodic wave.

IV. D I ~ C U ~ ~ I O N One of the most striking features which emerge from the present studies

is the dramatic effect of a change of solvent upon the stability of Wurster salts. Some salts which Michaelis and hia collaborators found to be quite stable in methanol-acetic acid-water become much less so in acetate buffers. Wurster's -blue remains fairly stable, but its stability is much less than it is in distilled water, and is affected by the pH of the solution. It is noteworthy also that Wurstor's blue is quite labile in some organic solvents. 8olutions in hexanol are pink, and in pyridine and quinoline there is quite a rapid colour change (to be

POLAROGRAPHIC STUDY O F SOME WUFLSTER SALTS 119

expected because of the basic character of these solvents). The semiquinone formation constant for Wurster's blue, as deduced qualitatively from the separa- tion of the waves, is approximately 20,000 a t pH 4.5, much larger than the value (approximately 4) deduced by Michaelis from his titration curves.

The instability of the Wurster salt from p-phenylenediamine is confirmed, ' but from the polarogram, Michaelis' conclusion that this reflects the instability

of the fully oxidized quinonoid structure is support . Wurster's red also appears less stable in the buffer solutions studied than in the organic solvent.

The Wurster salt from p-diaminodurene presents an interesting situation. In Michaelis' work, in methanol-acetic acid, the Wurster salt and the di-imine compound were both sufficiently stable to produce oxidation steps in the titration with bromine. In the present work, it was not possible to detect the Wurster salt in solution, either by the polarogram, or by the absorption spectrum. The polarogram was that to be expected from a mixture of diamine and duroquinone, and the absorption spectrum showed no sign of the strong bands at about 450-480 mp characteristic of the free radical. Duroquinone and p-diamino- durene are both devoid of pronounced absorption in this region. The bands of duroquinone are at 432 mp ( ~ = 2 4 . 6 ) and 327 mp (~=470) in hexane ; in water the positions of the maxima are shifted slightly.

Where polarographic waves were obtained which could be compared, reduction potentials agreed quite well with Michaelis' values a t the smooth platinum electrode. This indicates that the species being reduced exhibit comparatively small over-voltages a t the mercury electrode. The agreement between the values of half-wave potentials obtained a t the dropping mercury electrode and the platinum microelectrode supports this conclusion. It appears that it would be worth while to persevere with attempts to study the polaro- graphic behaviour of Wurster salts in non-aqueous solvents, and some work is projected in this field. I t is also intended to study the electrical conductance of solutions of the salts in non-aqueous solvents in order that a more exact value of the diffusion coefficient may be obtained in the absence of possible hydrolysis.

V. REFERENCES ALBRECRT, A., and SIMPSON, W. (1955):-J. Amer. Chem. Soc. 77 : 4454. GATTERMANN, L. (1948).-" Laboratory Methods o f Organic Chemistry." pp. 314 et seq.

(Maomillan & Co. : London.) I L K O V I ~ , D. (1934).-Coll. Trav. Chim. Tch6cosl. 6 : 498. JULIAN, D., and RUBY, W . (1950).-J. Amer. Chem. Soc. 72 : 4719. KOLTHOFF, I . &I., and LINGAKE, J . J . (1941).-" Polarography." p. 45. (Interscience Publishers

Ino. : New Y o r k . ) LAITINEN, H., and KOLTROFF, I . (1941).-J. P h y s Chem. 45 : 1079. LCVALLE, J., GLASS, D., and V T ~ ~ s s ~ ~ ~ ~ ~ ~ , A. (1948).-5. Amer. Chem. Soc. 70 : 2223. MICHAELIS, L., and GRANICK, S . (1943).-J. Amer. Chem. Soc. 65 : 1747. MICHAELIS, L., and HILL, E . (1933).-J. Amer. Chem. Soc. 55 : 1481. MICHAELIS, L., SCHUBERT, M., and GRANICK, S . (1939).-J. Amer. Chem. Soc. 61 : 1981. MWLLER, 0. H . (1951).-" Polarographio Method o f Analysis." p. 44. (Chemical Education . Publishing Co. : Easton, Pa.) NEF, J . U . (1887).-Liebigs A n n . 237 : 3. PACE, J . E., and ROBINSON, F. A. (1943).-J. Chem. Soc. 1943 : 133.