How Development Works

Part IV left development at a sentence: a developer is an electron donor with enough potential to reduce silver at a silver speck and not enough, in any reasonable time, on a bare crystal. That sentence is correct and it is not yet useful, because it contains two quantities — enough potential and reasonable time — and gives neither a number.

This page supplies what can be supplied. By the end of it you will be able to say what potential the silver in a film actually sits at, calculate how far a gram of potassium bromide moves it, convert a rate ratio into an energy in kilojoules per mole, and work out how much bromide a developer takes on per gram of silver it reduces. You will also know exactly which number this course cannot give you, and why its absence is the most instructive thing on the page.

The reaction, written three ways

Development is one electron arriving at one silver ion, repeated a few hundred million times per crystal. Write it as the half-reaction and it is the same one that made the print-out image in Part I:

Ag⁺ + e⁻ → Ag

The reduction the developer pays for

In a crystal the silver ion is not in solution. It is already in the lattice, and what leaves is the halide:

AgBr(s) + e⁻ → Ag(s) + Br⁻

Chemical development, per formula unit: the silver stays where it was, the bromide goes into the bath

The electrons come from the developing agent, which is oxidised. Hydroquinone gives up two of them and becomes quinone, and because a developer is alkaline it is cleanest to write that half-reaction with hydroxide rather than with free protons:

C₆H₆O₂ + 2 OH⁻ → C₆H₄O₂ + 2 H₂O + 2 e⁻

The oxidation that pays for it, written for an alkaline bath

Add the two together and the whole of chemical development is one line:

C₆H₆O₂ + 2 AgBr + 2 OH⁻ → C₆H₄O₂ + 2 Ag + 2 Br⁻ + 2 H₂O

Two silver ions reduced per molecule of hydroquinone consumed

Three things are worth reading off that equation before any chemistry is done to it.

The stoichiometry is fixed and it is one to one. Every silver atom made releases exactly one bromide ion. Nothing about the developer, the temperature or the agitation changes that ratio; it follows from the formula unit. Halide release is therefore not a side effect to be managed but a quantity you can compute from the silver alone.

The alkali is consumed. Two hydroxide ions disappear for every two silver atoms made, which is why a developer’s pH falls as it works and why the buffer matters more than the starting pH. The alkali page of this part owns that arithmetic.

The oxidised agent stays in the tank. Quinone does not evaporate and it is not inert. What becomes of it decides whether the developer keeps, whether the negative stains, and whether the solution is still worth using tomorrow — the subject of sulfite-and-preservation.

🧮How much bromide, per gram of silver

Take the relative atomic mass of silver as 107.87 and of bromine as 79.90.

n = 1.000 g ÷ 107.87 g/mol = 9.271 × 10⁻³ mol

Moles of silver in one gram

One bromide ion per silver atom, so the same 9.271 millimoles of bromide are released:

m = 9.271 × 10⁻³ mol × 79.90 g/mol = 0.741 g

Mass of bromide released

Every gram of silver developed puts 0.74 g of bromide ion into the bath, which is the bromide content of 1.10 g of potassium bromide (molar mass 119.00). Hold that number: it is the reason a used developer behaves like a fresh one with restrainer added, and the second half of this page computes what it does to the driving force.

What the course cannot do is convert that into films. It would need the silver coating weight of a modern film, and the only coating weights in the corpus are Ware’s 1 to 1.6 g per square metre for enlarging papers, which is a different material. Kodak’s XTOL sheet gives capacity from the other end — about fifteen 135-36 or 120 rolls per litre at full strength — without saying how much silver that is. The conversion is left open rather than guessed at, and development-kinetics picks it up as an exhaustion question.

The silver couple is not one number

Ask what potential a developer has to beat and the obvious answer is the one in every textbook table: the silver ion to silver couple at +0.7996 V, which is where OpenStax Appendix L puts it. That answer is wrong in a way that matters, and the reason is the whole of complex formation restated as electrochemistry.

There is almost no free silver ion in a film. The silver is in a crystal, and the crystal’s solubility product fixes how much of it is in solution at any moment. Bind the ion and you lower its appetite for an electron. Part III’s ladder shows the consequence for the thiosulfate complex, which drops the couple from +0.80 V to +0.02 V. The same argument applies to a lattice, and it can be worked exactly.

🧮Deriving the potential of a silver halide electrode, and checking the method against a published value

The half-cell AgBr | Ag is the ordinary silver electrode with the silver ion concentration set by the crystal. In the standard state the bromide concentration is 1 mol/L, so the solubility product gives the silver ion concentration directly: [Ag⁺] = Ksp. Put that into the Nernst equation in the convenient form OpenStax gives, with the constants folded in at 298 K and one electron transferred:

E = E°(Ag⁺/Ag) + 0.0592 log [Ag⁺]

Nernst, for a one-electron silver electrode at 25 °C

and therefore

E°(AgX/Ag) = 0.7996 + 0.0592 log Ksp

The standard potential of a silver halide electrode

Check the method before trusting it. OpenStax Appendix J gives silver chloride a solubility product of 1.6 × 10⁻¹⁰, whose logarithm is −9.796:

E° = 0.7996 + 0.0592 × (−9.796) = 0.7996 − 0.580 = +0.220 V

Silver chloride, computed

Appendix L lists the silver chloride electrode at +0.22233 V, measured. The computation agrees to three millivolts, which is as close as two independent tables have any right to be. So the same arithmetic can be run on the halide photography actually uses.

Crystal	Ksp (Appendix J)	log Ksp	E°(AgX/Ag)
Silver chloride	1.6 × 10⁻¹⁰	−9.796	+0.220 V (tabulated +0.22233)
Silver bromide	5.0 × 10⁻¹³	−12.301	+0.071 V
Silver iodide	1.5 × 10⁻¹⁶	−15.824	−0.137 V

Appendix L carries no entry for silver bromide at all, which is why this course computes one rather than quoting one, and says so. The figures are standard-state values at 25 °C, and the assumptions are the ordinary ones: ideal behaviour, unit activity for the solids, and a bromide activity taken as its concentration.

Read the table and the design problem changes shape. A developing agent does not have to beat +0.80 V. It has to beat about +0.07 V, and if the emulsion carries iodide, some of the silver is held below zero. That is a far gentler requirement, and it is why ordinary organic molecules — a phenol, an aminophenol, a sugar acid — can do the job at all.

The ordering also lines up with what Part III established from the other end, because it is that result, rewritten. The same property that makes silver iodide the least soluble of the three halides makes it the hardest to reduce and the hardest for a fixer to take away; the same property that makes silver chloride the most soluble makes chloride papers the quickest to clear. One question — how tightly does the lattice hold the silver ion — answering three.

What a gram of bromide is worth, in volts

The standard-state figure assumes 1 mol/L bromide, which no developer has. Put the working bromide concentration back in and the potential of the silver electrode moves with it:

E = E°(AgBr/Ag) − 0.0592 log [Br⁻] = 0.071 − 0.0592 log [Br⁻]

The silver electrode in a real bath

Every tenfold rise in free bromide costs the developer 59 millivolts of driving force. That is the common-ion effect of Part III, converted into the currency the developing agent is paid in.

What the restrainer does to the driving force

• E of the AgBr | Ag electrode

Show the numbers behind this plot

A single straight line falling from left to right, showing the potential of the silver bromide to silver electrode against the logarithm of the free bromide concentration in the bath. At the far left, where the bromide is only the seven times ten to the minus seven molar that a saturated solution of silver bromide supplies in pure water, the potential is plus 0.436 volts. The line falls by 59 millivolts for every factor of ten in bromide, reaching plus 0.249 volts at a millimolar, plus 0.190 volts at ten millimolar, plus 0.131 volts at a tenth molar, and plus 0.071 volts at one molar, which is the standard-state value. Three points are marked on the line. The leftmost, at minus 6.15 on the axis, is a bath with no added bromide at all, such as freshly mixed D-76. The middle one, at minus 2.38, is the half a gram per litre of potassium bromide in Kodak DK-50. The rightmost of the three, at minus 1.47, is the four grams per litre in Kodak D-19b, a high-contrast developer. The teaching point is the vertical distance between the first and the last: adding four grams of potassium bromide to a litre lowers the potential the developing agent has to overcome by about 0.28 volts, roughly a quarter of a volt, purely by suppressing the free silver ion concentration, and it does so before any question of adsorption on the grain surface is raised.

Series	log of the free bromide concentration in mol/L	Potential of the AgBr | Ag electrode, volts
E of the AgBr | Ag electrode	-6.50	0.46
E of the AgBr | Ag electrode	-6.00	0.43
E of the AgBr | Ag electrode	-5.00	0.37
E of the AgBr | Ag electrode	-4.00	0.31
E of the AgBr | Ag electrode	-3.00	0.25
E of the AgBr | Ag electrode	-2.00	0.19
E of the AgBr | Ag electrode	-1.00	0.13
E of the AgBr | Ag electrode	0.00	0.07

Every point on this line is arithmetic from two published numbers — OpenStax Appendix J's solubility product for silver bromide and Appendix L's potential for the silver ion couple — through the Nernst equation at 25 °C. It is a thermodynamic statement about the silver, not a measurement of any developer, and it says nothing whatever about how fast anything happens.

Two consequences, and both are picked up later in this part. A developer with bromide in it from the start, like Kodak’s D-19b, is holding its own silver couple a quarter of a volt lower than a bromide-free one — the restrainers-and-antifoggants page is about what that buys. And a developer with no added bromide puts itself there as it works: 0.74 g of bromide ion per gram of silver, computed above, is enough to move a fresh bath a long way down this line within its working life. That is seasoning, and it is why a developer’s first film and its fifteenth are not developed in the same solution.

The number this course does not have

Now the other electrode. What potential does metol sit at, in a developer, at pH 8.6?

No source in this course’s corpus answers that question, for metol or for any other developing agent. Not the 1907 monograph, not Kodak’s primers, not a manufacturer’s sheet. The gap is worth being precise about, because the literature that would close it exists and this course has not read it.

What the corpus does hold is one number, and it is instructive largely because it does not transfer. Mike Ware, writing about clearing baths for platinum prints, gives ascorbic acid a redox potential varying from −0.283 to −0.066 V as the pH varies from 2 to 7, citing Borsook and Keighley’s 1933 measurement, and adds that this makes it more effective as a reductant in acidic conditions. Set that beside a developer and three things go wrong at once. The pH range stops two units short of where any film developer works. The couple was characterised to reduce iron(III), not silver. And Ware’s trend — more reducing as the solution gets more acid — runs opposite to what every photographic source says about developer activity, which rises with alkali. That last conflict may not even be a conflict, since photographic “activity” is a rate and a redox potential is not, and this page has already spent a section on the difference; but it is exactly the sort of thing that must be settled by reading rather than assumed. The course will not resolve it by extrapolating a curve past its measured range and calling the result photography. One sourced number, quoted for what it is and not stretched.

⚗Kodak's reduction potential is a bromide-tolerance scale, not a voltage

The 1928 primer talks constantly about the “reduction potential” of a developing agent and ranks the five it sold — hydroquinone lowest, then Athenon, then pyro, then Kodelon, then Elon highest — but it never prints a volt. Read carefully, the primer says what it means: “it is found that the higher the reduction potential of a developer, the more bromide is required to produce a given effect.” The quantity being ranked is how much restrainer it takes to hold the agent back.

That is an operational definition, and given the plot above it is a good one: bromide moves the silver electrode down a known 59 millivolts per decade, so measuring how much bromide an agent tolerates before its behaviour changes is measuring, indirectly, how much margin the agent had. It is a titration of the driving force using the only reagent available.

It is also, deliberately, not a rate. The primer is explicit that reduction potential “alone does not determine the speed with which the developer develops the image, because this depends chiefly upon the rate at which the developer diffuses into the film and on the quantity of developing agent and other substances in the developer.” Its analogy is an engine’s power rather than a car’s speed: on the level a small engine keeps up, and the difference shows on the hill. The hill is bromide, or cold, or exhaustion.

Sheppard and Mees had reached the same conclusion twenty years earlier and stated it as a warning. They credit Bredig with pointing out the importance of the reduction potential as a function of a developer, and then add that there is no strict proportionality between potential and reaction velocity, offering the analogy of a current, a voltage and a resistance in between that is difficult to define and harder to measure. That resistance is where photography lives.

The cluster as an electrode: three currents, one rate

The electrochemical picture is the one that makes selectivity intelligible, and Part IV drew it. What this page adds is that the speck is not carrying one current but three, in series, and that all three must run at the same rate or the process stops.

Three transport paths meeting at one speck

1. 1Electrons: agent to speck, then through the metal — the only path that needs the speck to be a conductor
2. 2Silver ions: through the lattice to the metal boundary — interstitial ions, mobile because the lattice carries Frenkel defects — Part IV
3. 3Bromide: out of the crystal, through the gelatin, into the bath — one ion per silver atom made, without exception
4. 4Oxidised agent leaving — quinone here; what happens to it next is the preservative’s problem
5. 5The slowest path sets the rate — three processes in series, and agitation only touches the two ends in solution

Drawn to show which species moves where, not to scale and not from a micrograph. The speck is a few atoms across and the crystal about 0.2 micrometres; at that ratio the speck would be thinner than these lines.

Two of those paths are not chemistry at all in the ordinary sense. The interstitial silver ion that path 2 delivers is the same mobile ion that made the latent image possible; a silver halide crystal is a solid ionic conductor, and without that unusual property development would have to eat the crystal from the outside rather than growing a filament from within. Path 3 is diffusion through swollen gelatin, which Part III owns and which acutance-adjacency-and-compensation turns into a picture.

The series relationship is the practical content. If the slow path is path 1 at its solution end, the developer is transport-limited and agitation changes the time. If the slow path is the surface reaction itself, the developer is reaction-limited and agitation changes nothing but temperature does. Real developers move between the two as they are diluted, which is why a manufacturer’s times for stock and for 1+3 are not related by a simple factor.

Discrimination is a rate ratio, and a rate ratio is an energy

Here is the sentence Part IV finished on, and the arithmetic it deserves. An unexposed crystal is not undevelopable; it is slow. The speck does not change the thermodynamics, since the potential of the bath is the same everywhere in the tank. It changes the rate.

How much? The honest answer is that this course has no measured ratio to quote. But the question can be inverted, and inverting it is more instructive than a figure would be, because the Arrhenius relation turns any rate ratio into an energy.

🧮What selectivity costs, in kilojoules

Two processes differing only in their activation energy have rates in the ratio exp(ΔEₐ/RT). Rearranged, and at 20 °C where RT = 8.314 × 293 = 2,436 J/mol:

ΔEₐ = RT ln(ratio) = 2.436 kJ/mol × ln(ratio)

The barrier difference behind a rate ratio

Rate ratio, image to fog	ΔEₐ at 20 °C
100	11.2 kJ/mol
1,000	16.8 kJ/mol
10,000	22.4 kJ/mol

Each factor of ten in selectivity costs 5.6 kJ/mol, and that is the whole scale of the effect.

Put it beside the energy that started the process. A blue photon at 450 nm carries hc/λ = (6.626 × 10⁻³⁴ × 2.998 × 10⁸) ÷ (450 × 10⁻⁹) = 4.41 × 10⁻¹⁹ J, which is 266 kJ per mole of photons. A thousandfold discrimination between an exposed grain and its neighbour is therefore worth about six per cent of one photon’s energy.

Assumptions, stated. The two processes are taken to differ only in barrier height and not in their pre-exponential factor, which is the standard simplification and is certainly not exactly true when one route runs on a metal surface and the other has to nucleate its own. The ratios in the column are illustrative, not sourced. What survives those caveats is the scale: photographic selectivity is a small-energy phenomenon, of the order of a few kilojoules per mole, which is why it is so easily lost to a few degrees of warmth or a slightly stronger alkali.

Sheppard and Mees pinned down what the speck actually changes, and it is not the rate of reduction once development is running. Their microscopy and kinetics together indicated that the reactivity conferred by exposure lay rather in the starting of development, by curtailing an induction period, than in a change of the rate of reduction. Exposure does not make a crystal reduce faster. It makes it start sooner, and a crystal that starts sooner at a fixed finishing time has more time in which to finish.

They also established that the threshold is not a property of the film alone. Working out why the characteristic curve shifts when development is retarded, they concluded that the density obtained represents only that minimum necessary to render the grain developable for a particular development energy. Change the developer and you change which clusters count as clusters. That is the origin of the distinction between film speed and effective film speed that the course’s terminology keeps apart, and it is why development-kinetics treats speed as a property of a combination rather than of a film.

Fog is discrimination lost, and it is lost in three different ways

If selectivity is a rate ratio, fog is what happens when the ratio falls. The 1928 primer separates three mechanisms, and they have different remedies, which is the practical reason to keep them apart.

Chemical fog is the agent developing crystals that carry no developable cluster. Kodak states the design constraint that fails here — the agent must be a sufficiently strong reducer to reduce the exposed silver salt and at the same time must not affect that which has not been exposed — and names the commonest cause: too much alkali. The quantity of alkali governs the energy of a developer, and if too much is present the developer will tend to produce chemical fog. The lever is the restrainer, or less alkali, and restrainers-and-antifoggants owns it.

Aerial fog is fog produced when film wet with developer meets air, and the primer records it as a particular vice of Elon-hydroquinone developers, with motion-picture positive film developed on a reel especially susceptible. Its remedy is the strangest instruction in the book and the most revealing: add about five per cent of old developer to the freshly mixed developer, which the primer says is more effective than raising the bromide. The oxidised developer, it suggests, is itself acting as an antifogging agent. A developer, in other words, contains its own restrainer after it has been used a little, which is another face of seasoning.

Dichroic fog is not a discrimination failure at all but a symptom of the wrong mechanism running. The primer describes negatives developed in a bath containing an excess of sulfite, or hypo, or ammonia showing a fog that looks yellowish-green by reflected light and pink by transmitted light, and gives the cause: dissolved silver salts are reduced to metallic silver in a very fine state of subdivision, particularly in the shadows, where no bromide is liberated during development. Note the condition. It appears where development is not happening, because that is where no released bromide is present to suppress the free silver. Fine-grained emulsions are recorded as most susceptible. Dichroic fog is a warning that the bath has been asked to dissolve more silver than it can quietly redeposit, and solvent-action-and-grain returns to it.

Chemical development, physical development, and the image that survives fixing

Chemical development takes its silver from the crystal the speck sits on. Physical development takes it from the solution: the bath itself carries silver ions, which plate out on the specks.

Sheppard and Mees offer the demonstration that settles what the latent image is made of, and it is worth knowing because it looks impossible. Expose a plate, fix it so that every trace of silver halide is dissolved away, and then treat it with silver nitrate plus an acid reducer — and an image appears. They report Eder’s finding that fixation removes the image-forming remnant only up to about four to ten times the threshold exposure, so a well-exposed plate still develops after fixing. There is nothing left in the gelatin but the specks, and the specks are enough to nucleate silver arriving from outside.

Between the two mechanisms sits the case that matters in an ordinary darkroom, and it is not exotic: put a silver halide solvent in a chemical developer and both run at once. Sulfite is that solvent, and it is in the bottle anyway. The dichroic fog above is that mixture misbehaving; the fine grain of D-76 is the same mixture behaving. sulfite-and-preservation separates the jobs and solvent-action-and-grain follows the silver.

2 Ag⁺ + C₆H₆O₂ → 2 Ag + C₆H₄O₂ + 2 H⁺

Physical development: the silver arrives from the solution and the crystal contributes nothing — written with hydroquinone for continuity, where Sheppard and Mees's own test used silver nitrate with an acid reducer

Reading the reaction at three scales

The same equation says something different depending on how far back you stand, and being able to move between the three is most of what this part is teaching.

In the grain. A filament of metallic silver grows out of a speck, fed by silver ions from inside the crystal and electrons from outside it. The crystal loses volume as the metal gains it, and what remains at the end is filamentary silver occupying the space the crystal filled. If a solvent is present, some of that silver has travelled through the solution on the way and the filament is stubbier. Grain is decided here.

In the emulsion. Bromide accumulates locally, faster where development is heaviest, and diffuses outward. Fresh developer diffuses inward, more slowly where the gelatin is already busy. The alkali is consumed where the reaction is fastest. Every one of those gradients is a difference between one part of the negative and the part next to it, which is exactly what an edge effect is. Sharpness and compensating behaviour are decided here.

On the negative. Crystals that started early are finished; crystals that started late are part-developed; crystals that never started contribute nothing but fog. How those starting times are distributed across the exposure scale, integrated over the development time, is most of what the characteristic curve records. Contrast and effective speed are decided here, and Part XIII measures them.

Three scales, one reaction, and no lever that touches only one of them. That is the honest summary of the whole part, and it is why reading-a-developer-formula asks you to predict five properties at once rather than one.

Recap

• Development is one electron per silver ion, and one bromide ion out per silver atom made. That ratio is fixed by the formula unit: 0.74 g of bromide ion per gram of silver, the equivalent of 1.10 g of potassium bromide.
• The silver a developer meets is not free silver ion at +0.80 V. Computed from the solubility product, the silver bromide electrode stands at about +0.07 V in the standard state, silver chloride at +0.22 V — a figure that matches the published one to three millivolts — and silver iodide at −0.14 V. Iodide holds silver hardest; chloride holds it most loosely.
• Bromide moves that potential by 59 mV per decade, so four grams of potassium bromide in a litre costs about a quarter of a volt of driving force, and a developer that starts bromide-free manufactures its own restrainer as it works.
• No source this course holds gives a redox potential for any developing agent at developer pH. Kodak’s “reduction potential” is a bromide-tolerance ranking, not a voltage, and it says so.
• The speck carries three currents in series — electrons through the metal, interstitial silver ions through the lattice, bromide out through the gelatin — and the slowest sets the rate. Agitation only reaches the two that end in solution.
• Selectivity is kinetic and small. A thousandfold rate ratio is about 17 kJ/mol at 20 °C, six per cent of the energy of the photon that made the speck. Each further factor of ten costs 5.6 kJ/mol.
• Exposure curtails an induction period rather than raising the rate of reduction, and the developability threshold belongs to the film and the developer jointly.
• Fog comes three ways: chemical fog from an agent or an alkali with too much energy, aerial fog from wet film meeting air, and dichroic fog from dissolved silver redeposited where no bromide has been released.
• Physical development builds the image from silver in the solution, and a fixed plate can still be developed that way, which is the cleanest evidence that the latent image is a nucleus rather than a store of material.

Check your understanding

Question 1. Using the solubility product of silver bromide, 5.0 × 10⁻¹³, and the standard potential of the silver ion couple, +0.7996 V, what is the standard potential of the silver bromide electrode, and why does it matter to a developer?

• About +0.80 V, because the silver is the same element either way
• About +0.07 V, because binding the silver in a lattice lowers the concentration of free silver ion and therefore lowers the potential, so an ordinary organic reducing agent is strong enough to develop
• About −0.14 V, which is the silver iodide figure
• It cannot be computed from a solubility product

Show the answer and why

Answer: About +0.07 V, because binding the silver in a lattice lowers the concentration of free silver ion and therefore lowers the potential, so an ordinary organic reducing agent is strong enough to develop

The silver halide electrode is the ordinary silver electrode with the silver ion concentration set by the crystal, so at unit bromide activity the Nernst equation gives E° = 0.7996 + 0.0592 log Ksp = 0.7996 − 0.728 = +0.071 V. The same arithmetic on silver chloride returns +0.220 V against a published +0.22233 V, which is how the method is checked. The consequence is the design of every developer: the agent has to beat about seventy millivolts, not eight hundred, and that is a requirement a phenol can meet.

Question 2. A litre of a bromide-free developer is used until it has reduced 2.0 g of silver. Roughly how much bromide has it taken on, and what does that do to the driving force?

• About 0.2 g, and nothing measurable
• About 1.5 g of bromide ion, near 0.019 mol/L, which lowers the silver electrode by roughly 260 mV compared with the bromide-free bath
• About 15 g, which would exhaust the developer immediately
• None, because the bromide stays in the film

Show the answer and why

Answer: About 1.5 g of bromide ion, near 0.019 mol/L, which lowers the silver electrode by roughly 260 mV compared with the bromide-free bath

One bromide ion leaves the crystal for every silver atom made, so 2.0 g of silver at 107.87 g/mol is 18.5 mmol, and 18.5 mmol of bromide is 1.48 g. In a litre that is 0.0185 mol/L, log −1.73. A freshly mixed bromide-free bath sits near log −6.15, where the free bromide is only what saturated silver bromide supplies; the difference of about 4.4 decades at 59 mV each is roughly 0.26 V, and all of it is accumulated by the developer on its own account. That is why manufacturers publish a capacity and a replenisher rather than trusting a developer to stay the same solution.

Question 3. Why does the course refuse to quote a redox potential for metol at pH 8.6, when it happily quotes one for ascorbic acid?

• Because metol is a salt and salts have no redox potential
• Because no source in its corpus reports one; the single ascorbic acid figure it holds was measured between pH 2 and 7 for reducing iron, and extending it to a silver developer at pH 8.6 would be extrapolation dressed as measurement
• Because potentials are irrelevant to development
• Because metol works by a different mechanism entirely

Show the answer and why

Answer: Because no source in its corpus reports one; the single ascorbic acid figure it holds was measured between pH 2 and 7 for reducing iron, and extending it to a silver developer at pH 8.6 would be extrapolation dressed as measurement

Ware gives ascorbic acid a potential varying from −0.283 to −0.066 V between pH 2 and 7, citing Borsook and Keighley in 1933, in the context of clearing iron stains from platinum prints. Three things stop that becoming a developer number: the pH range ends short of any film developer, the couple was characterised against iron rather than silver, and Ware reads the trend as making ascorbic acid a better reductant in acid, which is the opposite direction from the alkali dependence every photographic source describes. The honest position is a stated gap, and the practical consolation is that a potential would not have predicted development speed anyway.

Question 4. Two developers are compared. In the first, doubling the agitation shortens the time to a given contrast noticeably; in the second it makes almost no difference. What does that tell you about which of the three transport paths is slowest in each?

• The first is reaction-limited and the second is transport-limited
• The first has its slow step at the solution end — developer diffusing in or bromide diffusing out — while in the second the slow step is at the crystal surface, where stirring the tank cannot reach
• The second developer has more agent in it
• Agitation only affects temperature

Show the answer and why

Answer: The first has its slow step at the solution end — developer diffusing in or bromide diffusing out — while in the second the slow step is at the crystal surface, where stirring the tank cannot reach

The three paths run in series: electrons from the agent through the metal, silver ions through the lattice, bromide out through the gelatin. Agitation thins the boundary layer at the film surface and therefore speeds only the two paths that end in solution. A developer that responds strongly to agitation is transport-limited; one that does not is limited by something stirring cannot touch, and temperature is the lever there instead. Dilution moves a developer from one regime towards the other, which is why times for stock and for 1+3 are not related by a simple factor.

Question 5. A negative shows a fog that looks yellowish-green by reflected light and pink by transmitted light, worst in the shadows, on a fine-grained film developed in a bath carrying a great deal of sulfite. Which fog is it, and what is the mechanism?

• Chemical fog, from too much alkali
• Aerial fog, from the film meeting air while wet
• Dichroic fog: dissolved silver salts reduced to metallic silver in a very fine state of subdivision, worst where no bromide has been released to suppress the free silver ion
• Safelight fog

Show the answer and why

Answer: Dichroic fog: dissolved silver salts reduced to metallic silver in a very fine state of subdivision, worst where no bromide has been released to suppress the free silver ion

Kodak’s 1928 primer describes exactly this appearance for developers containing an excess of sulfite, hypo or ammonia, and gives the mechanism: dissolved silver is being reduced out of solution as very finely divided metal. The location is the diagnostic detail. It is worst in the shadows because that is where little development is happening, so no bromide has been liberated locally to hold the free silver ion down; and fine-grained emulsions are recorded as most susceptible because they present more surface for the solvent to attack. The remedy is less solvent action, not more restrainer.

Question 6. Selectivity between an exposed and an unexposed grain is worth about 17 kJ/mol at 20 °C if the rate ratio is a thousandfold. Why does that number make a developer run three degrees warm a real risk rather than a small one?

• Because warmth destroys the latent image
• Because the whole discrimination is a small energy, so anything that raises rates generally erodes it; a barrier difference of a few kilojoules per mole is not a large margin to spend
• Because gelatin melts
• Because bromide becomes insoluble when warm

Show the answer and why

Answer: Because the whole discrimination is a small energy, so anything that raises rates generally erodes it; a barrier difference of a few kilojoules per mole is not a large margin to spend

Each factor of ten in the image-to-fog rate ratio corresponds to only 5.6 kJ/mol at 20 °C, so the entire separation between a photograph and a grey sheet is a handful of kilojoules — around six per cent of the 266 kJ/mol carried by a blue photon. Development and fog are both chemical rates, and both rise with temperature; what matters is that a margin that small has little slack in it. That is also why the remedies are the ones that shift the two rates differently — a restrainer, a lower pH, a less energetic agent — rather than simply developing for less time.

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