THE IRREVERSIBILITY OF PHOTOGRAPHIC DEVELOPMENT I N ELON SOLUTIONS CONTAINING SODIUM SULFITE‘
ANGUS E. CAMERON Kodak Research Laboratories, Rochester, New York Received February 18, 1858
In a preceding publication in this Journal (2) the writer has presented evidence that the platinum electrode potentials which R. M. Evans and W. T. Hanson, Jr., (3) of these Laboratories measured in photographic developer solutions were not oxidation-reduction potentials but were functions of the partial pressure of oxygen with which the solutions were more or less in equilibrium. The explanation given by them for the existence of an oxidation-reduction potential in the presence of sodium sulfite, which can remove the oxidized forms of photographic developer chemicals by converting them into the reduced form of the monosulfonates, was that this sulfonation process was reversible and that there ’was thus maintained a small and essentially constant concentration of the oxidized form of the developing agent. Sheppard (6) and, more precisely, Beukers (1) and Reinders ( 5 ) have shown that a reversible oxidation-reduction system, such as mixtures of ferric and ferrous ions in the form of organic complexes, could act either as a reducer or as an oxidizer upon a predeveloped photographic image. Evans and Hanson have attempted to show that this is also true for organic developers in the presence of sulfite. Within the limits of stability of the oxidized forms, some, a t least, of the developer chemicals give rise to reversible oxidation-reduction systems. Thus it was shown by Evans and Hanson that in solutions of amidol and oxidized amidol there was a definite reduction in density upon the high potential side of the equivalence point. However, in the presence of sodium sulfite the existence of any reversible oxidation-reduction system must be questioned. The data given by Evans and Hanson for amidol with slightly less than 1 mole of sodium sulfite per mole of reducing agent and in the presence of 8 g. of potassium bromide per liter would appear to indicate that a marked discontinuity occurred in the density-potential curve a t the LLequivalence” point and that the reduction in density occurring in solutions showing more positive potentials is, a t best, very small, Communication No. 656 from the Kodak Research Laboratories. 629 THE JOURNAL OF PHYEICAL CHEYISTRY, VOL. 42, NO. 6
630
ANGUS E . CAMERON
Elon* was selected as the developer for the investigation reported here in order to avoid troubles caused by staining of the emulsion layer. The oxidation-reduction potential of the compound was known, as well as its “oxygen” electrode potential behavior in the presence of sodium sulfite. EXPERIMENTAL
Motion picture positive film, 30. 1301, was used for the experiments. Strips of 35-mm. film were exposed on a model 2B sensitometer, giving a series of twenty-one increasing exposures on each strip. These strips were developed for 5 minutes in D-16 (Eastman Kodak formula) at 18°C. with air agitation, rinsed in 1 per cent acetic acid, and thoroughly washed and dried in the dark. These strips were then numbered and slit lengthwise; one half of each strip was fixed in non-hardening alkaline sodium thiosulfate solution, washed, and dried. The fixed, or control strips, were then read with a Capstaff densitometer and the corresponding fixed and unfixed strips were immersed in the developer solutions for 15.5 hours with air or nitrogen agitation at 20°C. in the dark. The glass developing tubes of half-liter capacity were jacketed and water from a 20°C. thermostat was pumped through the jackets. The film strips were suspended from the rubber stopper closing the top of the tube. Purified nitrogen or air was introduced through a perforated glass agitator tube a t the bottom. A mater-sealed trap permitted escape of gas from the tube. When nitrogen agitation was used, the solutions were swept out with nitrogen for an hour before the film was immersed. The nitrogen was purified by burning out the oxygen present with ammonia over a heated platinum catalyst. Ammonia was introduced by passing the gas over solid ammonium carbonate. Carbon dioxide was removed with soda lime before the gas entered the furnace tube. Excess ammonia was removed with a sulfuric acid wash. After 15.5 hours in the developer solutions the strips were removed, washed briefly, immersed in an acid fixing and hardening bath for 5 minutes, washed thoroughly, and dried; the densities were then read on both strips. The results have been recorded as changes in density and have been plotted as a function of pH of the developer solutions. It is apparent that this is equivalent t o plotting the first derivative of the density, and that an equivalence point of development will be shown by the reversal of sign of the change in density. The solutions were read both before and after development with a glass electrode and platinum electrodes in atmospheres of controlled oxygen content. I n many cases readings were taken with a silver electrode. The glass electrode was standardized against the hydrogen elec2
Elon is a trade name for p-methylaminophenol.
63 1
PHOTOGRAPHIC DEVELOPMENT I N ELON SOLUTIONS
trode in solutions of the same sodium-ion concentration, 1.5 N in all cases, in which sodium sulfite had been replaced with sodium sulfate. The buffer salt employed was, in most. cases, potassium hydrogen phthalate. TABLE 1 Results obtained with the Jirst series of strips with nitrogen agitation 8 E- . of elon, 8 E . of potassium bromide, and 6.25 E. of sodium sulfit ?er liter PH
3.63 4.18 5.20 5.68 6.12 6.12
AD STEP
AD 12
-0.01 0.01 0.05 0.27 0.53 0.55
STEP
16
-0.01 0.00 0.04 0.10 0.40
0.39
AD
Eair
Eai,
FOG
BEFORE
AFTER
UdtS
$oris
Wll8
-0.01 -0.01 0.01 0.03 0.13 0.13
-0.046 -0.002 -0.065 -0.092 -0.'120 -0.113
-0.047
-0.080
-0.008
-0.082 -0.099 -0.138 -0.171 -0.180
__-
-0.087 -0.100 -0,144 -0.120
EAE
FIG. 1 FIG 2 FIG.1. Change in density of predeveloped strips, and platinum and silver electrode potentials in air, as functions of the pH of the developer solution; 8 g. of elon, 8 g. of potassium bromide, and 6.25 g. of sodium sulfite per liter. FIG. 2. Change in density of predeveloped strips, and platinum and silver electrode potentials in air, as functions of the p H of the developer solution; 8 g. of elon, 8 g. of potassium bromide, and 64 g. of sodium sulfite per liter.
Sulfuric acid was added when necessary to overcome the buffering effect of sodium sulfite in adjusting the pH t o the desired value. The 3.5N calomel half-cell and bridge were used as a reference half-cell.
632
ANGUS E. CAMERON
This combination was assigned a value of 0.2502 volt, against the normal hydrogen electrode a t 20°C. For conwnience in comparing the results obtained and reported by Evans and Hanson, these potentials have been recorded as “observed potentials” and have not been converted t o the hydrogen scale. The saturated calomel haif-cell employed by Evans and IIanson is assigned a value of 0.2488 volt at 20°C. TABLE 2 Measurements similar l o those of tabZe I , but made with a tenfold greater concentration of sodium sulfite 8 g. of elon. 8 g. of potassium bromide, 2nd 64 g. of sodium sulfite per liter AD STE1’
4 01 4 57 4 67’ 5 00 5 32* 5 13 5 75 5 80 5 93*
6 00 6 6 6 6 6 6 6
7 7
7 5
9
10
11 35
sot 70t 75 85t 05t 25t 56 50 40
0 0 0 0 0 0
AD
12
01 02 01 01
08 13 n 23 0 43 0 38 0 44 0 50 0 46 0 44 0 34 0 37 0 5i 0 41 0 61 0 41 0 52 0 81 I 07
AD
STEP 16
rob
-0.01 0.04 0.02 0.01 0.05 0.07 0.10 0.13 0.19 0.19 0 33 0.23 0 37 0.32 0.33 0.68 0.42 0.50 0.48 0.44 0.82 0.74
-0.01 0.01 -0.02 0.01 0.00 0.0: 0.01 0.02 0.03 0.03
____
ad18
aults
uolts
0 .on6 -0.035 -0.052 -0.064
0,002 -0.041 -0.060 -0.077 -0.102
-0.093 -0.097 -0,090 -0.m -0.106 -0.117 -0.117 -0,139
-0,090
-0.117 -0.09-1 -0,104 -0.106
0.14
0.02 0.35 0.92 1 00 0.50 1 . 13
2.39 1.34 0.70 0.75 0.67
-0.112
-0.124 -0,110 -0.138 -0.335 -0,165 -0.154
--0,129
-0,134 -0,166 -0.138
-0,177 -0,173 -0.162
- 0.18.k -0.181 -0.178 -0,177 -0.207 -0.208 -0.210
-0.291 -0.325
* Air agitation.
t Potassium dihydrogen phosphate buffer.
The first series of strips was developed in solutions of the following composition, sulfuric acid or sodium hydroxide being added as necessary t o adjust the pH to Ihc desired value: 8 g. of elon, 8 g. of potassium bromide, 6.25 g. of sodium sulfite (anhydrous), 20.2 g. of potassium acid phthalate, 106.5 g. of sodium sulfate (anhydrous), and water ts make 1 liter. The results of this series with nitrogen agitation are given in table 1. These data a p p e x graphically in figure 1! Tvhere the change in density and the
633
PHOTOGRAPHIC DEVELOPMENT I N ELON SOLUTIONS
potentials are plotted as functions of the pH of the solutions. Similar measurements with a tenfold greater concentration of sodium sulfite are
FIG.3 FIG.4 FIG.3. Change in density of predeveloped strips, and platinum and silver electrode potentials in air, as functions of the pH of the developer solution; 40 g. of elon, 8 g. of potassium bromide, and 29.2 g. of sodium sulfite per liter. FIG.4. Potentials of platinum and silver electrodes in air as functions of the partial pressure of oxygen at pH 6.0 in a developer containing 8 g. of elon, 8 g. of potassium bromide, and 64 g. of sodium sulfite per liter.
TABLE 3 Measurements in solutions codaining jive times the concentration of elon but with the ratio of elon to sodium sulfite the same as i n table 1 40 g. of elon, 8 g. of potassium bromide, and 29.2 g. of sodium sulfite per liter 4D
PH
STEP 12
3.60 4.93 5.15 5.87
0.01 0.07 0.13 0.54 0.60
AD 3TCP
AD 16
1’00
E,ir BEFORE
mils
5 87
0.01 0.03 0.03 0.41 0.38
0.00 -0.01 0.03 0.29 0.25
-0 007 -0 081 -0 076 -0 128 -0,123
I ~
Enir AFTEE
lolls
-0 -0 -0 -0
012 093 112 167
-0 161
volts
-0.100 -0,108 -0.115 -0.184 -0.185
given in table 2. The change in fog density has been omitted from the plot of thcse data in figure 2 t o avoid undue complication. A third series of strips was run in solutions containing five times the
634
ANGUS E. CAMERON
concentration of elon but with the ratio of elon to sodium sulfite mole to mole as in the data of table 1. These data are shown graphically in figure 3. DISCCTSSIOS
From an examination of the plotted data it is apparent that on the acid side of the pH a t which development continues there is no reduction in density. Estimating this point from the plotted data and reading the potentials of the “air” electrode at the same value of pH, the data in table 4 result. Sone of the potentials recorded is found to be in agreement with the value of -0 150 volt given by Evans and Hanson for amidol at this bromide concentration. The point of continuation of development is not markedly influenced by a tenfold increase in the concentration of sulfite, but a fivefold increase in the elon concentration appears t o cause a shift to a lower value of pH. The measurements made with air agitation show good agreement with those made with purified nitrogen. The control strips showed slight variations in density when read for the second time. Below pH 5 there TABLE 4 GRAVS Or CLON A A D OF SODIUM SULFITE PER LITER ~
8 g. of elon; 6 . 2 5 g. of sodium sulfite. . . . . . . . 8 g. of elon; 64 g. of sodium sulfite.. . . . ... 40 g. of elon; 2 9 . 2 g. of sodium sulfite. . . . . . , . . .
pH
1
1 I ,
5.0 5.0 4.8
E,,, BEtORE
Bolts
-0.050 -0.064 1 -0.067
E,,, AFTER
volts
-0.062 -0.081 -0.084
were small decreases in the second densities. The first set of readings n as used in determining the change of density in the predeveloped but unfixed drips. Considerable variation appeared in the potrntials of the same solution before and after development. This variation was not due to change in pH, because in every caw the agreement between solutions before and after development mas perfect or was within 0 05 pH unit. Figure 4 shows the behavior of the potentials before and after development as a function of the percentage of oxygen in the saturating atmosphere. The potential behavior of the platinum electrode after the film has been immersed in the solution for 15 5 hours is very much like that of a silver electrode. Kolthoff and ’IT’ang (4) h m e shown that gold and platinum electrodes in silver-ion concentrations greater than 0.01 M behave like silT er electrodes and are independent of the hydrogen-ion concentration or the presence of oxygen. In low concentrations of silrer ion, such as might exist in a developer solution of low reducing power in which silrer has been dis3olred
PHOTOGRAPHIC DEVELOPMEKT IN ELON SOLUTIONS
635
by the sodium sulfite, platinum electrodes show this same behavior. Solutions in which colloidal silver is present after development show a shift in the dependence of the platinum electrode potentials upon the partial pressure of oxygen. This shift to lower values of the potentials can be produced by addition of colloidal silver to a solution which has not been in contact with photographic emulsion and appears to be the result of the catalytic effect of the metal upon the rate of autoxidation of the elon. The potential dependence upon the logarithm of the partial pressure of oxygen remains a straight line. Examination of the plots of the silver electrode potentials as functions of pH shows that the silver-ion concentration decreases above the pH at which development is found to continue. It is perhaps not surprising that such is the case, for development in solutions of such low pH, compared to those usually used in photographic development, appears to be largely, if not entirely, due to reduction of silver ion present in solution with deposition of the finely divided silver upon the nuclei present in the predeveloped image. SUMMARY
1. Attempts have been made to determine a reversal point of photographic development in solutions of elon containing sodium sulfite. 2. A point of “continuation” of development is found which is not markedly shifted by a tenfold increase in sulfite concentration and which shows a slight shift toward lower values of pH with a fivefold increase in elon concentration. 3. The “air” electrode potential of solutions at the pH of “continuation” of development does not agree with the potential reported by Evans and Hanson for the reversal point of development in amidol solutions containing sodium sulfite. 4. I t is concluded that no reversible oxidation-reduction system exists in elon solutions containing sodium sulfite. REFEREKCES (1) BEUKERS, M. F. C.: Fotografische Ontwikkelaars, Thesis, Delft, 1934. (2) C4MERON, A. E.: J. Phys. Chem. 42, 521 (1938). (3) Evaxs, R. M., . ~ N DHANSON,JR., W. T.: J. Phys. Chem. 41, 509-34 (1937). (4) KOLTHOFF, I. M., AND WAXG, CHIN: J. Phys. Chem. 41, 539-44 (1937). (5) REINDERS,W.:J. Phys. Chem. 38, 783-96 (1934); I X t h International Congress of Photography, pp. 345-6, Paris (1935). (6) SHEPPARD, S. E.: J. Chem. SOC. 87, 1311 (1905).
