Reduction Potential and Photographic Developers; the Effect of Sulfite

REDCCTION POTENTIAL AND PHOTOGRAPHIC DET-ELOPERS; THE EFFECT OF SULFITE IN DEVELOPER SOLUTIONS’

R. M. EVANS

AND

W. T. HANSON, JR.

K o d a k Research Laboratories, E a s t m a n K o d a k Co., Rochester, New Y o r k Received October 18, 1956 I. POTENTIAL MEASUREMENTS I N PHOTOGRAPHIC DEVELOPERS

A . Theoretical considerations

If an electrode of a noble metal, such as platinum, is inserted in a solution containing both the oxidized and reduced forms of a thermodynamically reversible chemical system, the electrode attains a definite electrical potential. While the absolute potential of this electrode cannot be measured, it may be compared with that of another which has a known and constant value. I n order to accomplish this, an electrode is placed in a solution which has been thoroughly investigated and whose potential is known with respect to that of a “standard hydrogen electrode.” This known solution is then connected with the unknown solution by means of a glass tube containing a solution of an electrolyte, usually potassium chloride. The potential difference between the two electrodes may then be determined by determining the potential which it is necessary to apply externally to prevent a current from flowing if the circuit is completed externally. The value so found is characteristic of the ratio of the active oxidized and reduced forms of the agent in the solution being measured. It may be represented by the well-known equation Eh

=

[Red] Eo - RT - lnnF

[Ox]

where EI, is the difference of potential against the normal hydrogen electrode, R is the gas constant, T is the absolute temperature, F is the faraday, Eo is a constant characteristic of the system at unit activity of all components including [H+], n is the number of electrons in the reaction, and [Red] and [Ox] indicate the concentrations of the active portions of the reduced and oxidized forms, respectively2 (8). 1 2

Communication X o . 601 from the Kodak Research Laboratories. The notation throughout this paper will follow that of Clark (8). 509

T H E J O C R K A L OF P H Y S I C l L CHEMISTRY, VOL.

41,

KO.

4

510

R. M. EVANS AND W. T. HANSON, JR.

I n the case of a reaction in which a bivalent reduced form loses two electrons in passing over to a n unionized oxidized form, i.e., [Red=] - 2~ = [Ox]

(2)

the value of n is 2. Such a reaction is typical of a great many organic reducing agents, such as hydroquinone, and we may restrict surselves to this case without changing the essential relations involved in the following discussion. Since the active form in this case is the bivalent anion [Red-], it is apparent that its concentration will depend not only on the total amount of agent present but also on the extent of its dissociation. If the first and second dissociation constants (K1 and Kz)are known for the compound being studied, the hydrogen-ion concentration [H+]may be measured and the concentration of the active form calculated from the known total concentration of the reducing agent [ S R ] . Equation 1 may then be rewritten for reaction 2 as

and, in general, for all reactions involving two electrons as

If for any reason, such as low solubility of the reaction products or side reactions which remove them from solution, the concentration of the oxidized form is maintained reversibly a t a low value, the second term of equation 4 is maintained a t a high value and the system has a relatively great (negative) reduction potential. The action of oxidizing agents on such a system then has a tendency to decrease the total reduced form [S,] only without increasing the total oxidized form [So]correspondingly. I n the limiting case in which [So] is maintained entirely constant independently of both [E,] and [H+],it may be included in the constant EOand the equation rewritten as

While this equation represents a condition that is seldom if ever attained in actual solutions, it has been derived to show that a thermodynamically reversible system under suitable conditions may appear to depend only on the logarithm of the concentration of the reductant and the pH of the solution.

PHOTOGRAPHIC DEVELOPERS

511

An equation exactly similar to equation 4 holds for oxidizing solutions. For silver bromide as an oxidizing agent we may write

I n the presence of metallic silver in the solid state, the solution may be considered saturated with silver and this term combined with Eo. The concentration of all components is independent of pH, so that this term becomes zero. The concentration of Ag+, however, does depend on the Concentration of Br- in such a way that the product of the two is always constant. Since in the case of photographic deveiopment it is usually the concentration of Br- that is known rather than A&, we shall find it convenient to rewrite equation 6 in accordance with the above facts as

By combining equations 4 and 7 , we may write an equation for the difference in potential, AE, between an oxidizing agent (silver bromide) and a reducing agent (whose oxidation involves the transfer of the two electrons) as

The experimental data to be presented in this paper are intended to illustrate the application of this equation to photographic developers and to clarify the nature of some of the variables involved. If a small amount of silver bromide is introduced into an excess of a reducing solution having a more negative value than the silver bromide, reaction takes place. The concentration of Br- increases, [S,] decreases, and [SO] increases until AE is equal to zero and the final solution comes to equilibrium at a potential intermediate between that of the original silver bromide and the reducing solution. All the original silver bromide has been reduced to metallic silver and bromide ions except the small amount of silver ion necessary to satisfy the constant product with bromide ion necessary in all solutions. It will simplify the consideration of actual developers if we discuss at this point the reaction which will take place if, instead of an oxidizing agent, we add another reducing agent to the above solution. If this added reducing agent is the reduced forni of an oxidation-reduction system having a more positive potential than the system under consideration, then it cannot itself be reduced. The slight amount of its oxidized form which is always present will be reduced, however, until the potential of

512


this added system is just the same as the potential of the solution as a whole. This reaction will use up only an infinitesimal amount of the original reducing agent and consequently the resulting potential will be that of the original system. In many solutions containing organic oxidation-reduction systems, it is found that electrodes come to equilibrium rather slowly, and it is sometimes difficult to determine the exact point a t which equilibrium has been attained. I n such cases, it has been found convenient to add to the solution a small amount of the reduced form (in measuring reducing solutions) of a system having a more positive potential. From the reasoning of the preceding paragraph it is seen that this does not appreciably affect the true reduction potential of the system being measured and, if the added system is characterized by readily recognized electrode equilibria, it is found that measurements are greatly facilitated. Such added agents are known as “potential mediators.” -4 more complete discussion of their action may be found in Clark (9).

B. Experimental technique By the use of a suitable potential mediator, it has been found possible to measure the reduction potentials of photographic developer solutions directly. The apparatus used in the work to be described below was of the conventional type for oxidation-reduction measurements. As a standard reference potential the “saturated calomel electrode” was chosen because of its low temperature coefficient. This electrode consists of platinum foil immersed in a solution which is saturated with respect to mercury, mercurous chloride, and potassium chloride. At 20°C. it maintains a constant potential of $0.250 v. with respect to the “standard normal hydrogen electrode.” All potential measurements reported in this paper will be in terms of differences with respect to this saturated calomel electrode and are given negative values when the system in which measurements are being made is either a more powerful reducing system or is less easily reduced than this reference system. The solution of the standard electrode was connected to that being measured by a salt bridge consisting of a glass tube filled with saturated potassium chloride solution. The end of the tube which was immersed in the solution being measured was partially closed by means of a cap fitted to the end by a ground-glass j oint , Bright platinum foil electrodes with a surface area of around 2 cmU2 were inserted into the unknown solution immediately before measurements were made. The unknown solution was kept at a temperature of 20°C. during all measurements. The difference of potential between the two electrodes was determined by opposing an opposite potential with a Leeds and Korthrup student potentionieter and determining the potential


513

at which no current flowed in the circuit by means of a high-sensitivity current galvanometer. All measurements were made in air with all parts of the equipment at room temperature except the solution being measured. When the electrodes were not in use, they were kept in alcohol. This was found necessary in order to maintain proper surface conditions. It was found experimentally that potassium ferrocyanide would serve as a satisfactory potential mediator for photographic developers. This compound is the reduced form of an oxidation-reduction system having an E; of approximately $0.20 v., and this potential is independent of the value of pH between 4 and 12.5. It is available commercially in a sufficiently pure form for the purpose and is relatively stable in water solution. From the considerations of section A above, it may be shown that additions of small amounts of this compound to a developer solution have no appreciable efYect on the true potential of the solution. In practice, it was found that the factor limiting the amount which could be added without appreciable change was dilution rather than chemical reaction. I n all cases the procedure of making a measurement was as follows: Approximately 150 cc. of the developer was placed in a water-jacketed glass beaker and 0.5 to 1 cc. of N/10 potassium ferrocyanide was added. The platinum foil electrode was rinsed with distilled water before immersing it in the solution whose potential was t o be measured. The solutions were constantly stirred by a glass paddle during all measurements, and the readings were independent of the rate. Balancing of the potential was commenced immediately on insertion of the electrode. The potential customarily fell very rapidly at the start and then tapered off gradually to a JT ell-defined equilibrium. This equilibrium was maintained anywhere from 30 seconds to one half-hour, depending on the nature of the solution, and then drifted slowly to more negative values. Tests with many electrodes of different metals and under varying conditions showed that this plateau was characteristic and repeatable for a given solution, and its value has been taken throughout this work as the true electrode potential for the solution under consideration. There appears to be a close analogy between the action of our electrodes and those reported by Clark in his work on indophenols. I n his work he observed the same type of plateau and used it as the true equilibrium potential. He gives an interesting discussion of the phenomenon in his first paper on the subject (7). KO solutions of developers at any value of pH have been encountered n hich cannot be read by the above method, although some alkaline solutions not containing sulfite Are characterized by an additional continuous drift due t o aerial oxidation. T-alues for typical mixed developers are presented in table 1 to show the range of potentials encountered in commercial solutions. The values given are those actually measured against

514


the saturated calomel electrode a t room temperature and with the developer at 20°C.3 All values given are uncertain in the third place. All solutions were mixed with Eastman tested chemicals as supplied to the trade. The pH values recorded in table 1 and used throughout this paper were obtained by means of a glass electrode measured against the same calomel electrode used for the oxidation-reduction potential work. By careful calibration and frequent checking for drifts, it was found practicable to measure the p H values of all developer solutions up to a value of 12 to 12.5. Sulfite and sodium ion errors have been neglected, but represent a small error which is constant for a given environment. The values as given are believed to be correct to within 0.1 pH. TABLE 1 Reduction potentials for typical mixed developers FORMULA'

D-1 D-9 D-16 D-72 D-76 D-76d D-82

TYPE

PH

Pyro Caustic hydroquinone Positive MQ Paper MQ Borax MQ Buffered borax RZQ High energy MQ

9.6 12.5 9.0 9.9 8.1 7.8 10.3

IREDUCTION

POTENTIAL

-0.259 -0.253

* T h e formula designations refer to those published by the Eastman Kodak Company in its trade pamphlets. For the collected formulae, see Elementary Photographic Chemistry, published by the same company.

The nature of the dependence of potential on the concentration of the main constituents of the developer solution is shown by figures 1, 2, and 3. I n figure 1 the potential is plotted against the pH of a solution containing 10 g. of amidol (Eastman hcrol) and 20 g. of sodium sulfite (anhydrous) per liter. The pH was varied by the addition of either concentrated hydrochloric acid or 1 N sodium hydroxide solution. Above a pH of 11.0, air oxidation of the amidol is so rapid, even in the presence of sulfite, that potential measurements could not be made accurately. Figure 2 shows the relationship between the potential and the logarithm of the concentration of the amidol in grams per liter. These measurements were made in the presence of 20 g. of sodium sulfite per liter and at a pH of 7.9. Since the addition of varying amounts of amidol caused fairly large changes in the pH of the solution, it was necessary to readjust this to 7.9 by the addition of small amounts of 1 N sodium hydroxide. I t is seen

* All data given in this paper will be those found under these same conditions.


515

that a linear relation exists over the entire range studied. Figure 3 shows the relationship found when the sulfite concentration is varied while the p H and amidol concentration are held constant (pH, 6.1; amidol concentration, 10 g.per liter). The first small addition of sulfite causes a very decided drop in potential to more negative values, and above 20 g. per liter there is little further change. It should be mentioned that the literature of photography contains many scattered references to potential measurements by electrochemical

means. They are found for the most part in the work of Bancroft, Bredig, Sheppard, and Nietz. I n more recent times there have been papers by Abribat and by Faerman and Bogdanov describing work which involves potential measurements, but, as far as can be determined, this is the first time in which the potentials of mixed developers containing sulfite have been measured and their dependence on the constituents of the solution investigated ~ysteniatically.~ 4 The authors desire a t this point t o acknowledge the work done by Mr. W. H. Bahler of these laboratories in the development of this technique.

516

R. M. EVANS AND TV. T. HANSON, JR.

C. The reversibility of development Sheppard and llees (18), in their fundamental work on photographic developers, demonstrated as early as 1906 that development with ferrous oxalate complexes is a reversible process. Their experimental method consisted in varying the ratio of ferrous to ferric complexes present in a

FIG.2, Relation between potential and amidol concentration. Sodium sulfite, 20 g. per liter; pH = 7.9.

given solution, keeping the bromide constant, and determining the effect the mixture had on both a latent image and a developed and fixed image. I n certain solutions the predeveloped image was found to bleach, and in other solutions the latent image was developed. Bromide ion was found to affect the results, On the basis of certain theoretical considerations similar to those of the first section of this paper, they investigated a series [Fe"+l was varied in a kno\vn manner. of solutions in which the ratio [Fe++] [Br-]


517

They found that as this ratio was varied, all solutions in which it had a d u e greater than approximately 700 were oxidizing agents for the silver image and would convert it to silver bromide. All those in which the ratio was less than 700 were found to be reducing agents for the latent image. The sensitivity of the method was rather low because of the slowness of the reactions involved, and they came to the justifiable conclusion that, within experimental error, as the value of this ratio increased, the solution ceased to be a developer, passed through an equivalence point at which no action took place, and became an oxidizing agent for a silver

GMS. OF SULFITE

PER LITER

FIG.3. Effect of sodium sulfite o n the potential of an amidol solution. Amidol, 10 g . per liter; pH

=

6.1.

image in a continuous manner. As a matter of fact, their published data do not show this. They do not appear to have been able to find two solutions whose ratio values had a difference of less than ten which would show the two effects. Nost of their data shows a ratio difference of a t least thirty between the highest value a solution could possess and still develop, and the lowest it could have and still oxidize a silver image. The above ratio of Sheppard and Mees may be rewritten in terms of the potential of such a solution and gives rise to equation 8 of the first section above. Their proof of the reversibility of development accordingly may be stated as follows: 911 solutions containing ferrous and ferric oxalate

518

R. M. EVANS AND W. T. HAKSON, JR.

complexes in the presence of soluble bromides act as oxidizing agents for a silver image if the potential of the solution is higher than a certain value, and as reducing agents for a latent image in silver bromide if the potential is less than a somewhat lower value. It remained for Reinders (15) and his student Beukers ( 5 ) , nearly thirty years later, to clear up the discrepancy between the limiting values by the application of actual potential measurements to similar solutions. They found that by the use of suitable ratios of ferrous and ferric complexes with oxalates, malonates, citrates, etc., a series of solutions having a wide range of potentials could be obtained. These potentials were independent of p H over a rather wide range and were readily measurable. They were able to obtain a total range of potentials with the various salts of from -0.050 v. to $0.650 v. on the hydrogen scale. (Approximately -0.300 v, to +0.400 on the scale used in this paper.) The technique followed by these investigators was as follows: Pairs of identically exposed strips, one of which had been developed in a metol developer to a density of approximately 1.5 and washed but not fixed, and the other of which had received no treatment, were placed in each of a series of the above solutions. Each set was then agitated for four to five hours in closed vessels to insure complete development unaffected by aerial oxidation. After fixing the strips the density of each was read, and the values plotted on a potential vs. density graph. A standard strip, which received the metol predevelopment but which was then fixed without further treatment and its density determined, served as the control from which the density of all the predeveloped strips was known. The accompanying figure (figure 4), taken from Beuker’s thesis ( G ) , shows the nature of the results obtained in a series of solutions containing malonate complexes. It is seen that the silver image in silver halide which was obtained by predevelopment is completely and continuously reversible about a single potential. Each curve of the figure represents a dserent exposure, and it is seen that the value of this equivalence point potential is independent of the original exposure and predeveloped density to within experimental error. The developer is, accordingly, reversible in the same sense, acting either as an oxidizing or reducing agent depending on its potential relative to that of the silver image. The curves for the latent images, however, show that a greater reduction potential is necessary to initiate development than is required to continue a development that has already been started. It is this discovery which explains the fact that Sheppard and Nees were unable to obtain the same value for the equivalence point when they tried to approach it from the two directions by different methods. Reinders and Beukers ascribe this difference to the solubility of the silver of the latent image, and obtain a value sixteen to fifty-three times 17.


519

as great as that for massive silver. It is not within the scope of this paper to discuss this interesting point, although the dependence of the threshold potential for development on the amount of the exposure is particularly suggestive. No measurements are reported by Beukers on conventional formulae containing organic developers. Predeveloped strips mere tested in buffer solutions having p H values ranging from 7.0 to 8.0 and saturated with quinhydrone. I n these solutions he obtained a reversible equivalence point in the same manner as for iron salts. The value of this point was found to be about 0.025 v. higher than before (apparently owing to differences in the bromide-ion concentrations of the solutions). He does not report any latent-image development over the potential range studied (+0.299 v. to +0.178 v. on the hydrogen scale). Only two solutions were tested which contained both sulfite and quinhydrone. I n one of these a t a pH value of 7 he found no change in density, while an increase was observed in the other which had a pH of 8.

+ so

-

YOO 750

zoo as0

300

mrllLvoLt

FIG.4. Equivalence point curves (from Beukers)

Since it has been found in the present work that, by using the experimental technique described above, the reduction potential of organic photographic developers can be measured, it becomes possible to extend the work of Reinders and Beukers to include that type of developer. The approach has followed that of Beukers rather closely. The greater part of our work to date has been done with amidol, and only the results obtained with this agent will be reported here. The ease with which amidol will develop a latent image at pH values as low as 4 makes it particularly suited for this kind of study, but this property is by no means essential. Entirely analogous data have been obtained using elon and hydroquinone. In order to cover a sufficient range of potential to study both the oxidizing properties of the developer and its latent-image development characteristics, a spread of nearly 0.400 v. was found necessary. Since in high concentration the primary oxidized forms of organic developers are not stable in solution, the potential range was covered for the most part by

520

R . M . E V A S S AND W. T. HANSON, J R .

varying the pH of the solutions. For the more positive solutions, however, the necessary pH was so low that the gelatin of the photographic material was attacked. I n these cases a slightly different method of decreasing the reduction potential was used. It was found that if an alkaline solution of amidol was agitated for about five minutes by means of a stream of air, the solution became very dark red, and on acidifying to a pH of around 2 had a potential of approximately $0.125 v. If this is mixed with an unagitated solution, or made slightly less acid, potential values more negative than +0.125 are obtained. The slight variation of the concentration of neutral salts caused by the addition of acids (hydrochloric and acetic) and alkalies (sodium hydroxide or sodium carbonate) in adjusting the pH has been neglected. A typical bromo-iodide negative emulsion, Eastman Par Speed No. 1201, was chosen for this work. A series of strips 35 mm. wide was exposed on a Type I1 B Eastman sensitometer to tungsten light corrected to a color temperature of 5400°K. This instrument gives a range of twenty-one light exposures, each with an intensity of 1.075 meter candles and with times increasing by a factor of .\/sIrom 0.004 second to 4.16 seconds. This series of strips was then developed uniformly for five minutes in an 1IQ developer (Eastman D-16 formula), washed thoroughly, and dried in the dark without fixing. As a standard for comparison, one strip in each series was removed and fixed and the densities carefully read. One of the above developed and dried strips and an identically exposed but untreated strip were then placed in each of a series of developers having potentials covering the range to be studied. The strips were supported on heavy celluloid to prevent overlapping and to give free access to the solutions. The developer solutions were contained in four-liter beakers which were covered with heavy paper to exclude as much air as possible, but no further protection was provided to eliminate the action of oxygen. Comequently the potentials of the solutions drifted somewhat during the course of a development. In every case the potential was read immediately after the development was completed, and this reading is the one that is used in the presentation of the data. The development was continued for from five to six hours with no agitation other than a frequent shaking of the beakers . Figure 5 shows graphically the results obtained using a developer containing 10 g. of amidol per liter, no sulfite, and no bromide. The measured potentials of the solutions a t the end of the development are plotted against the densities read for single exposure values after the films had been fixed in acid hypo, thoroughly washed, and dried. The full line curves give the values of the densities of the predeveloped strips corresponding to the step whose number is indicated in parentheses. The density value of the control strip removed and read after predevelopment

521


only is indicated by a short horizontal line across the curve, and a dotted line is carried down to the potential axis to show the value of the potential corresponding to a solution which would have produced no change in density. The densities read on the strips which had not been predereloped and which, accordingly, represent the density reached by a latent image for the given exposure and developer potential are shown as dashed lines. A heavy red stain was obtained on many of the strips of this series. In order to eliminate this from the density readings, the silver of the image was removed by ferricyanide and hypo after a careful determination of

POTENTIAL

AVERAGE

E.P.

= -o.asz

FIG.5 . Equivalence point curves. Emulsion No. 1201; amidol, 10 g. per liter

silver plus stain, and the stain alone iead in the same manner. This value was then subtracted from the sum of the two. A comparison of this figure with figure 4 shows that in all essential respects the results so obtained with amidol are the same as those found by Reinders and Beukers for development with ferrous complexes. It is interesting to note that if correction is made for the fact that their data are given in terms of a hydrogen rather than of a saturated calomel electrode, the two graphs show the same value for the equivalence point within 10 mv. As %-asshown in figure 3, the addition of sulfite to an amidol solution gives rise to a large shift in the potential of the solution toward more negatiye values. It was thought interesting to repeat the series of figure

522

R . M. EVANS AND W. T. HANSOh-, JR.

5 with solutions which covered the same range of potentials, but each of which contained 5 g. of sodium sulfite (anhydrous) per liter. The results of such a series are presented graphically in figure 6 . It is seen that the same type of family of curves is obtained as in the preceding case but that the slope of the curves for predeveloped images is less to the right of the equivalence point than before, although this comes a t the same value of potential. This change in slope and the cause of the drop in potential when sulfite is added to an amidol solution will be discussed more fully in part I1 of this paper. The rather large experimental errors found, par-

0 -0,350

-0.300

-0950

-0900

-0150

-0100

POTENTIAL

-0060

-0000

COO50

tOlOO

to 5 0

AVERAGE E . R ’ O . 0 3 9

FIG.6. Equivalence point curves. Emulsion No. 1201; amidol, 10 g. per liter; sodium sulfite, 5 g . per liter. ticularly in the values for the equivalence point, are due to a large extent to small fluctuations in the bromide-ion concentration of the solution. No bromide was added except that which washed out of the film after immersion in the developer. I n the subsequent series which are discussed below and in which added bromide was used smaller errors were encountered. Since equivalent results were obtained in the presence and absence of sulfite, all of the following experiments have been carried out using 6.25 g. of sulfite per liter. This eliminates the heavy stain and consequently greatly increases the accuracy of the density measurements. Referring to equation 8 of section A, it is seen that the addition of potas-


523

sium bromide to a developer solution should decrease the potential difference between the developer and the silver of the image by an amount

RT

equal to - In [ B r ] . At 20°C. R T / F has a value of approximately 0.06, F including the factor for the conversion to loglo, and accordingly for each time that the bromide-ion concentration is doubled the potential difference should decrease by about 0.018 v. T o test whether or not correspondingly lowered ralues of the equivalence point and the threshold potentials for development of the latent

image could be demonstrated experimentally, two series were run which were similar to the previous one but contained bromide. I n the first of these, for which the data are presented in figure 7 , solutions were employed each of which contained 10 g. of amidol, 6.25 g. of sodium sulfite, and 2 g. of potassium bromide per liter. I n the second series each solution contained 8 g. of potassium bromide per liter. The data are shown in figure 8. A comparison of the two figures shows that the equivalence points as well as the threshold potentials for latent-image development are separated by approximately 40 mv., in good agreement with the 36-mv. shift

524

R. 31. EVANS AND W. T. HANSON, J R .

FIG.8. Equivalence point curves. Emulsion No. 1201; amidol, 10 g. per liter; sodium sulfite, 6.25 g. per liter; potassium bromide, 8 g. per liter.

WENTIAL

-

AVERAGE E.P. -0.145

FIG.9. Equivalence point curves. Emulsion KO. 1301; amidol, 10 g. per liter; sodium sulfite, 6.25 g. per liter; potassium bromide. 8 g. per liter.


525

predicted by the equation. Figure 7 in comparison with figure 6 also shows a shift to more negative values, which is consistent with a greater bromide-ion concentration. To test the effect of a different grain size frequency distribution in the emulsion material on the value of the equivalence point, a series was also run on the same solutions as used for the data of figure 8 but using Eastman &lotion Picture Positive Film KO. 1301. The data are presented in figure 9. Within the experimental error the same value is found for the equivalence point. The strips were exposed to light having a color temperature of 3000'K. and an intensity of 27 meter candles, so that the position of the latent-image curves cannot be compared. They mere predeveloped in the same developer as the negative strips. From the foregoing discussion and data, it appears safe to conclude that organic developers both in the presence and in the absence of sodium sulfite are continuously reversible in the same sense as ferrous complex developers, and that the primary effect of the addition of potassium bromide to a developer is to decrease the potential difference between the developer and the image by an amount equal to that predicted by the application of the mass action lax. 11.

THE EFFECT O F SULFITE I N PHOTOGRAPHIC DEVELOPERS

A . Brief ri.sumS of the literature Sodium sulfite is the one chemical customarily found in all formulae for organic photographic developing agents. Its use as a protective against aerial oxidation dates back to 1882 and appears to have been suggested by Berkeley (4). In addition to this protective property, sulfite tends to prevent the formation of colored end products in the development reaction, and so has found wide use for many years in pyro developers. By varying the sulfite content of an alkaline pyro solution, the density of the brown stain image formed in the vicinity of the silver during development may be altered a t will or, a t high sulfite concentrations, practically eliminated. Recent writers have pointed out other properties of sulfite solutions, and we may summarize the matter as follows: Sulfite is an agent which (1)protects organic developers against aerial oxidation, ( 2 ) tends to prevent staining development products, (3) acts as a solvent for silver halides by the formation of complexes, (4) is a weak alkali, and ( 5 ) gives increased development under special conditions. In the present paper, experimental evidence will be presented to show that effects 1, 2, and 5 may be explained by the assumption of a single reaction mechanism from which new effects may be predicted. Th,: other two effects noted above will not be discussed here, but have been eliminated or held constant irl the experimental work.

526

R. M. EVANS APiD W. T. HANSON, JR.

The chemical reactions of sulfite in solution are the subject of an extensive literature which cannot adequately be reviewed here. We shall consider only a few of those papers which have a direct bearing on our subject matter. Sodium sulfite by itself in water solution is rapidly and completely oxidized by air to sodium sulfate. The rate of this oxidation is independent of the concentration of the sulfite but directly proportional to the rate of solution of oxygen in the mixture. Thus, the addition of alkalies to the solution will decrease the oxidation rate to a marked degree. The addition of slight amounts of substances called antioxygens, of which hydroquinone is an example, will also greatly prolong the life of the sulfite. Hydroquinone similarly, in neutral or alkaline water solution, is rapidly oxidized by air. I n this case increase in the alkalinity greatly accelerates the reaction, the reactivity of the compound apparently increasing more rapidly than the rate of solution of oxygen decreases. The end products of the oxidation are not fully known, but the work of Eller (10) has gone far toward showing that for hydroquinone as well as many other developing agents they are polymerized bodies formed from the corresponding quinones and higher oxidized forms. Eller has shown that they are of the group of compounds known as humic acids. The addition of a slight amount of sulfite to a water solution of hydroquinone will so decrease the rate of oxidation by air that it proceeds a t a much lower rate than it would have in either compound alone. I n fact it is found that if about 10 per cent of either compound is added to a solution of the other the rate of oxidation becomes the same in either case and the solutions are relatively stable. Considerable research has been undertaken to determine the nature of the reaction products when both compounds are present. Perhaps the earliest investigator to accomplish their isolation and identification was Pinnow (14), who found in 1912 that the chief product was hydroquinone monosulfonate. This he isolated and identified by comparison with the monosulfonate formed by direct sulfonation with concentrated sulfuric acid. He also found that hydroquinone mono- and di-sulfonates were formed when silver halides were the oxidizing agents. The reaction was assumed to take place in two steps, the hydroquinone first being oxidized to quinone and this then being converted to the monosulfonate by the sulfite under the influence of the alkali present. In explaining the mechanism by which this could take place, he reviewed the literature up to that time and gave references dating back to Carstaujen in 1877 and to Storch in 1893 to show that quinone in alkaline solutions containing sodium sulfite reacted a t once to form sulfonates of hydroquinone. Andresen (1) had previously suggested, in 1898, that sulfonates might be a reaction product of hydroquinone developers, but had not demonstrated their presence.


527

The exact reason why this formation of a monosulfonate results in such a marked decrease in the aerial oxidation of the solution has not been found. It appears probable that by the reaction a chain mechanism, by which one oxidized molecule is able to cause the oxidation of others, is prevented. To explain this, however, it seems necessary to postulate that the first oxidation product produced by oxygen is more of the nature of a peroxide of hydroquinone than quinone itself. This assumption appears to gain some support from the fact that the humic acid end products in the absence of sulfites have been shown by Eller to have the general formula (CGH50&, but the whole subject is in need of investigation before the actual mechanism may be stated. I n the case of oxidation by silver halides, there appears to be little difficulty in assuming that quinone is the first product. I n 1933 and 1934 Seyewetz and Szymson (16, 19) at Lyons, and Lehrnann and Tausch (12, 20) at Dresden independently investigated the oxidation products of photographic developers. These workers succeeded in showing that in the case of both hydroquinone and elon, sulfonates were formed as oxidation products whether the oxidizing agent was air or silver bromide. Seyewetz and Szymson later showed (17) that either air or silver halide oxidation of hydroquinone, pyrocatechol, pyrogallol, pphenylenediamine, p-aminophenol, metol, glycine, o-aminophenol, o ,pdiaminophenol, and the sodium sulfonate of p-amino-a-naphthol (eikonogen) produced in all cases either the mono- or di-sulfonates of the original compounds. Since these represent nearly all the known types of organic developers, it seems safe to conclude that in practically every case the principal reaction product of the developing agent in photographic development consists of the sulfonated form of the developing agent itself. Since all of the above sulfonates are colorless, water-soluble compounds, it becomes immediately apparent how it is possible for sulfite to prevent the formation of staining end products. The fact that all stain is not removed, in the case of pyro for example, unless large quantities of sulfite are present, is explained by the findings of Tausch (20). H e showed that, in the case of both elon and hydroquinone in solutions containing sulfite, a certain proportion of the reaction products was humic acids and that this proportion was frequently as high as 5 per cent. The formation of sulfonates, therefore, is to be considered as the chief reaction but not the only one. It has been noted by several investigators, notably Andresen (3), that the addition of even slight traces of sulfite to an alkaline solution of such varying developing agents as hydroquinone, p-aminophenol, and metol has a tremendous effect on the maximum density obtainable by prolonged development. Thus in an alkaline solution of chlorohydroquinone he obtained a density of 1.67 with no sulfite present and 2.59 when 4 g. was added per liter. Nietz (13) also came to the conclusion that in a hydro-

528

R. M. EVANS k Y D W. T. H A S S O S . JR.

quinone-carbonate developer the maximum developable density (D,)for a given exposure increased as the concentration of sulfite in the solution was increased. This effect is not to be confused with an increase in development velocities. I n the present work we are concerned only with the effect of development which has been carried to the point a t which the density produced by a given exposure has reached its maximum attainable value. The cause of this increase may be seen by reference to figures 3 and 5 to 9 inclusive. I n figure 3 is shown the dependence of the potential of an amidol solution on the sulfite concentration. As sulfite is added to the solution, the potential a t first drops very rapidly and then more slowly. Figures 5 to 9 show the dependence of the maximum developable density on the potential of the solution. Since the density obtained for a given exposure vanes almost linearly with the potential, it follows that the addition of small amounts of sulfite should increase the density greatly, as has been found to be the case. The cause of the drop in potential will be discussed in the following section.

B. Generalized hypothesis for the action of sulfite in photographic developers From the considerations set forth above and in part I of this paper, it appears that in any conventional alkaline organic developer solution containing sulfite, oxidizing agents, such as air or silver halide, produce as a principal reaction product the monosulfonated form of the developing agent. This reaction is assumed to take place in two steps which may be different for the two cases. The intermediate product in each case, however, is an oxidized form of the developing agent, and in each case this form reacts with the sulfite to produce the reduced form of the monosulfonated compound. Owing to the substitution of an acidic group in the ring, the EOof the system (of which it is the reduced form) is more positive than that of the system from which it originated. The monosulfonates of hydroquinone and elon have been synthesized in these laboratories by 31r. L. A. Smith, following the methods of Pinnow and Tausch. Hydroquinone and elon were eliminated as impurities as far as possible, but no great attempt was made to obtain 100 per cent purity as regards inert material. The purity as obtained by analysis appears to be better than 90 per cent in both cases. Reduction potential measurements were made on these compounds in comparison with equivalent amounts of hydroquinone and elon in the same environment. The results obtained are given in table 2. The formula was as follows, except that small amounts of sodium hydroxide or hydrochloric acid were added where necessary to give the desired value of pH: elon, 5 g., or hydro-

529


quinone, 10 g., or the monosulfonates in equivalent amounts; sodium carbonate (anhydrous), 30 g.; sodium sulfite (anhydrous), 7 5 g. From the data it is seen that in a normal developer solution the potentials given by the sulfonates are definitely more positive than those found for the unsulfonated forms. It follows from the considerations on potential mediators in part I, section A above, that the sulfonates play no part in the potential of a developer solution, nor in the actual development of an image. For each molecule of sulfonate which is formed, however, a molecule of the oxidized form of the developer has been removed from the solution. The remaining form does affect developer potential directly, decreasing amounts of it increasing the reduction potential of the solution according to equation 4. Accordingly the formation of sulfonates tends to maintain the reduction potential of the solution against oxidizing agents. This effect has been reported by Faerman (4)and Bogdanov, who found that the effect of the presence of sulfite when metol was titrated with TABLE 2 Reduction potential measurements AQENT

~

PH

POTENTIAL

10.1 10.1 10 .O

-0 305 -0 286 -0 400 -0.345

zolts

Elon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elon monosulfonate., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroquinone.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroquinone monosulfonate.. . . . . . . . . . . . . . . . . . . . . . . . . . .

10 .O

potassium ferricyanide in the absence of oxygen wa5 t o oppose the normal increase in the potential of the solution as the metol was oxidized. It follows, therefore, that the primary action of sulfite in a developer solution i s that of an acceptor for the oxidized f o r m . Such an acceptor action has been formulated in electrochemical terms in part I, section A, where it was shown that if the concentration of the oxidized form were held at low values, the reduction potential of the solution no longer foilowed equation 4. In the limiting case in which the concentration of the oxidized form was held constant during oxidation of the solution, it was shown that the potential was then governed by the logarithm of the concentration of the reduced form alone, i.e.,

This is in fact what is found to be the case experimentally. Figure 2 shows that with constant pH and sulfite concentration the potential of an amidol solution varies linearly with the logarithm of the amidol concen-

530

R. &EVANS I. AND W. T. HAITSOR’, JR.

tration. From the nearly linear relationship shown above (figure 5, for example) to exist between the reduction potential and the maximum developable density for a given exposure, it should follow directly that this density for a given developing solution should vary linearly with the logarithm of the concentration of the developing agent. Unfortunately, a search of the literature has failed to rereal a study in which this effect is clearly shown. I n most cases which we have found the data are not admissible because the pH has not been held constant. There are many papers which show that development velocity has such a relationship t o concentration, but while this may be considered as contributory evidence, a study must first be made of the connection between potential and velocity before any conclusions may be drawn. The assumption that in the presence of sulfite the oxidized form of a developing agent is held a t almost constant low values for any given value of pH carries with it certain other necessary assumptions. It is first necessary to assume that the reaction between sulfite and the oxidized developer to form a sulfonate is reversible. For this assumption we have no direct evidence. However, from the fact that a potential can be read at all and the fact that this potential is dependent on the quantity of sulfite rather than merely being affected by its presence, the conclusion that an equilibrium exists appears to be inevitable. A second necessary assumption is that the developing agent itself and its first oxidation product are in reversible equilibrium. For this assumption we have the evidence presented in the first part of this paper. It was shown that as the reduction potential of a given developer solution was decreased, a point was reached at which it would act as an oxidizing agent for the silver of a partially predeveloped image. It may be considered that the reversibility of developers, at least in the first stages of the reaction, is well established, both by the evidence presented here and by that found in the literature cited. X further indication that both of the above assumptions are admissible is found in the sets of data for solutions which contained sulfite (see figures 6 to 9 inclusive). I n all of these sets it is found that the amount of oxidation which has taken place in the duration of the experiment is less than that found for solutions not containing sulfite. The densities, in fact, are found to vary only slightly as the potential changes but, if the above acceptor hypothesis is correct, since we are dealing here with very small quantities of oxidized form, this is exactly what would be expected. The fact that oxidation takes place at all is sufficient indication that both assumptions have some basis in fact. Until further studies have been made as to the rate of this oxidation, it is not safe to assume that sufficient time was permitted in these sets for complete equilibrium to take place. Calculations based on the observed potentials indicate that the quantity of oxidized developer is extremely low, and it does not appear

531


probable that six hours is sufficient under these conditions. Because of this uncertainty we cannot draw any conclusions from the difference in slope of the density lines on the two sides of the equilibrium point. Theoretical considerations indicate that in the presence of sufficient sulfonated form the final equilibrium should be the same as in developers of the same potential which do not contain any sulfite. The experimental verification of this, however, is extremely difficult, owing to the shifts of potential encountered when development is extended for long times. An interesting extension of the concept of sulfite as an acceptor can be made in the case of the coupler developers of Homolka (2). I n these solutions a coupling agent is present which has the property of uniting with the oxidized form of the developing agent as it is formed, precipitating an insoluble dye in the vicinity of the developing image. A typical reaction is as follows : 0 2

+H

z N O NH2

+ c>OH -

(colored precipitate)

It is apparent that this is the same type of acceptor action which we have postulated in the case of sulfite except that the steps of the reactions are probably quite different. We may write the sulfite reaction as

io2+ H2N

c>

-

NH2 + Na2S03

S03Na H2N c > N H 2

+ NaOH

(colorless, soluble) The analogy may be carried somewhat further. It is found that the amount of dye formed by a Homolka coupler developer, when a given

amount of silver halide is reduced, depends markedly on the ratio of coupling agent to sulfite in the solution, as would be expected in the case of tn7o acceptors competing for the same product. Also it is found that couplers will act in the same sense as sulfite in protecting the developing agent from aerial oxidation. It is thought that the mechanism is somewhat similar to the protective action of sulfite, i.e., the prevention of a chain reaction, but this has not been established definitely. Also, it appears that the same dye is formed by both oxygen and silver halides. I n customary developer solutions the effect of an acceptor action may be seen readily by considering the reactions taking place in the vicinity of a developing photographic image. I n the case of a ferrous oxalate developer in which no acceptor is present, the oxidized form of the developer builds up in concentration at the point in the gelatin where reduction of the silver halide is taking place. h two-way diffusion is then necessary t o carry

532

R. &I. EVANS AND TV. T. HANSOX, J R .

away this product and bring in fresh developer before the image can come to equilibrium with the main body of the solution. Cnder such conditions the attainment of equilibrium will be slow and we should expect the reaction to proceed with little energy. With organic developers on the other hand, as oxidation proceeds, there is always plenty of sulfite present (frequently fifty times as much as the total developing agent), so that the oxidized form of the developing agent is immediately greatly reduced without the necessity of any outward diffusion process. Coupled with the diffusion in of fresh developer the total effect at the image is to maintain a potential throughout development which is only slightly lower than that of the solution as a whole. (Since Tausch has demonstrated that the sulfonates are not only not restrainers but in some cases-notably elonmay actually be used as developers, it should be noted that the severe restraining action indicated by the Eberhard and similar effects are to be ascribed to outward diffusion of the hydrogen bromide formed by the reaction and not to the oxidized developer, as is frequently stated.) We should further note at this point that in the case of organic developer solutions not containing sulfite, we should still expect development to proceed with greater energy than that of the ferrous complexes a t the same solution potential because of the ease with which the oxidized form of these developing agents can polymerize to form multiple quinoidal products of the humic acid type. This polymerization also removes the primary oxidation products, usually depositing a stain either in the vicinity of the image or throughout the gelatin, and so also tends to maintain the reduction potential against oxidizing agents. This section of this paper may be summarized as follows: It appears consistent with the known facts concerning the reactions of developing solutions containing organic developing agents and sodium sulfite in an alkaline environment that the primary effects of the sulfite are due to its action as an acceptor for the oxidized form of the developing agent. This action explains its protective properties against the oxygen of the air, its preventive action against the formation of staining products of development, and its augmenting action on the maximum developable density for a given exposure. SUMMARY

Part I 1. By the use of a suitable potential mediator it has been found possible to obtain consistent and repeatable measurements of the oxidation-reduction potentials of customary organic developing solutions. 2. By the use of this technique the reversibility of development when these solutions are used has been demonstrated. The facts observed by


533

Reinders and Beukers for ferrous complex development are demonstrated for organic developers both in the presence and in the absence of sulfite. 3. The potential of a developer solution is found to vary linearly with the logarithm of the concentration of the developing agent, and to depend on the sulfite concentration as well as on the pH value. 4. I t is found that if the variables under 3 above are held constant, the effect of Br- in the solution is that formulated by Sheppard and Mees, the potential difference between the photographic image and the developer solution decreasing by an amount equal to that predicted from mass action considerations.

Part I I 1. It is postulated that the effect of sulfite in a photographic developer is that of an acceptor for the oxidized form of the developing agent. 2. It is found that this hypothesis satisfactorily explains the protective and stain-preventive properties of the sulfite. It also explains the dependence of the potential of the solution on the logarithm of the concentration of the reducing agent, and on the sulfite concentration. The potential-sulfite relationship is shown to explain the increase in developable density caused by the addition of sulfite to a developer. 3. The postulated acceptor action of sulfite is shown t o have a close analogy in the action of the coupler in the Homolka coupler developers. The effect of the presence of sulfite in such a developer is also satisfactorily explained by the same reasoning. REFEREXCES (1) ANDRESEN, A I . : In Eder’s Ausfuhrliches Handbuch der Photographie, 6th edition, Vol. 3, Part 2 (1930). (2) Reference 1, pp. 18-9. (3) Reference 1, p. 33. (4) BERKELEY, H. B.: Phot. News 26, 41 (1882). (5) BEUKERS,hI. F. C. : Fotografische Ontwikkelaars, Thesis, Delft, 1934. (6) Reference 5 , p. 53, figure 13. (7) CLARK,W.hl.: Studies on Oxidation-Reduction. 111. Pub. Health Repts. 38, 933 (1923). (8) CLARK,W.&I.: The Determination of Hydrogen Ions, 3rd edition. The Williams & Wilkins Co., Baltimore (1928). (9) Reference 8, p. 391. (10) ELLER, W b r . : Brennstoff-Chem. 2, 129-33 (1921); Ann. 442, 160-80 (1925). (11) FAERMAS, G.: IXth Intern. Congress of Photography, pp. 195-206, Paris (1935); Science ind. phot. 6, 29-1 (1935). E.: Phot. Korr. 71, 17-23, 35-8 (1935). (12) LEHMANS,E., A N D TAUSCH, (13) NIETZ,A. H.: The Theory of Development, p. 157. D. Van Kostrand Co., New York (1922). (14) P r x i o w , J.: Z. Elektrochem. 21, 380-8 (1915), and previous papers. (15) REISDERS,IT.: J. Phys. Chem. 38, 783-96 (1934); IXth Intern. Congress of Photography, pp. 345-6, Paris (1935).

534

.

R . M. EVANS AND W. T. HANSON, JR.

(16) SEYEWETZ, A., AND SZYMSON, S.: Bull. soc. franp. phot. 21, 71-81 (1934). (17) SEYEWETZ, A.,AND SZYMSOX, S.: Bull. soc. franp. phot. 21, 236-8 (1934). (18) SHEPPARD, S. E., AND MEES,C. E. K.: Investigations on the Theory of the Photographic Process, p. 97ff. Longmans, Green and Co., New York (1907). (19) S Z Y M S OS.: N , Thesis, Lyons, 1934. (20) TAUSCH, E.: Thesis, Dresden, 1934.

Reduction Potential and Photographic Developers; the Effect of Sulfite

Recommend Documents