Silver and the Silver Ion

Ninety-odd elements were available and photography chose one. Not because silver is beautiful, and not because it was cheap — Robert Hunt’s 1854 manual records silver nitrate at five shillings an ounce against about two shillings a pound for bichromate of potash, which is potassium dichromate. Silver was chosen because it is the only element known to do four different things at once, and this page is about what those four things rest on: the behaviour of one small, positively charged ion.

Silver the element, and where its chemistry comes from

Silver is element 47, in group 11 between copper above it and gold below. OpenStax describes the yttrium-to-silver series as one in which ten d electrons are added successively, and lists copper, silver and gold among the elements whose d orbitals end up completely filled — which is why the IUPAC definition of a transition element, requiring partially filled d orbitals, technically excludes all three.

That full d shell matters more than it sounds. The ion this course is about, Ag⁺, is what is left when the single outer electron goes, and it keeps the filled shell underneath. A filled shell has no unpaired electrons to absorb visible light, which is why silver salts and silver complexes are white, pale yellow or colourless rather than the strong blues and greens of the middle of the d block. Every colour you will meet in a silver photograph comes from the metal, in particles, and not from the ion. That distinction is worth fixing now, because it explains the whole of the last section of this page.

Noble, but only nearly

The half-reaction that makes silver a noble metal is in every table of standard potentials:

Ag⁺ + e⁻ → Ag

OpenStax Appendix L: E° = +0.7996 V at 25 °C

A potential that positive means silver ion is comparatively easy to reduce and silver metal is comparatively hard to oxidise. Hydrogen ion, at 0 V by definition, cannot take an electron from silver, so silver does not dissolve in hydrochloric acid the way iron or zinc does. It needs an oxidising acid, which is why the industrial route to silver nitrate uses nitric acid and not any other.

But +0.7996 V is a statement about the free ion in water, and nothing else. Wrap ligands round that ion and the number changes completely. Appendix L gives the same silver as +0.22233 V when it sits in a chloride lattice, +0.373 V held by two ammonia molecules, +0.017 V held by two thiosulfate ions and −0.31 V held by two cyanide ions. That is a swing of more than a volt, and it is the single most useful fact on this page: it tells you that “silver” is not one chemical species with one behaviour, and that a developer which can reduce one form of it may be helpless against another.

The reason silver is not quite noble is sulfur, and the number that says so is the solubility product of silver sulfide: OpenStax Appendix J gives 1.6 × 10⁻⁴⁹. Nothing else on that page of the appendix is within thirty orders of magnitude. A compound that insoluble will form from almost any encounter between silver and a sulfide source, and the section on tarnish below is the consequence.

Ag⁺ and its ligands: what it grips and what it ignores

A ligand is a Lewis base — something with a lone pair to donate — and a metal ion that accepts it is a Lewis acid. OpenStax’s coordination chapter sets out the vocabulary: the atom that actually donates is the donor atom, and the number of donor atoms attached is the coordination number. What Part III established, and this page uses, is that silver’s characteristic coordination number in photographic chemistry is two, in a linear arrangement.

The interesting question is not whether Ag⁺ forms complexes but which ligands it prefers, and the formation constants answer it without any theory at all.

Ligand	Donor atom	Complex	Formation constant	Where photography meets it
Water	oxygen	—	no constant quoted here	the solvent; it barely holds silver at all
Chloride, Cl⁻	chlorine	[AgCl₂]⁻	1.8 × 10⁵	the reason a strong salt bath slowly clears a chloride print
Ammonia, NH₃	nitrogen	[Ag(NH₃)₂]⁺	1.7 × 10⁷	historical ammoniacal silver; prohibited from storage by Part II
Thiosulfate, S₂O₃²⁻	sulfur	[Ag(S₂O₃)₂]³⁻	4.7 × 10¹³	every fixer ever made
Thiocyanate, SCN⁻	sulfur	[Ag(SCN)₄]³⁻	1.2 × 10¹⁰	thiocyanate fixers and toners
Cyanide, CN⁻	carbon	[Ag(CN)₂]⁻	1 × 10²¹	the historical fixer this course studies and refuses to use

The silver constants come from OpenStax Appendix K except the thiosulfate one, which Part III takes from the coupled-equilibria worked example and which other sources give differently; that page owns the discrepancy.

Read the donor-atom column. Sulfur, nitrogen and carbon donors bind silver hard; the oxygen donor does not. Nitrate, sulfate and water are all oxygen donors, and silver ion wanders about among them essentially unattached — which is exactly why silver nitrate solution behaves as a source of free silver ion and why water alone will never fix a print.

⚗The name for the pattern, and what this course can and cannot source

Chemists call an ion like Ag⁺ — large, easily polarised, low in charge — a soft Lewis acid, and call sulfur, phosphorus and carbon donors soft bases, with the rule of thumb that soft prefers soft. The label is standard, and the course uses it because it is the vocabulary the emulsion literature is written in.

Be clear about the evidence, though. The course has not read a source that classifies silver ion on a hard–soft scale; what it has read are the formation constants above, and those are measurements. The pattern is therefore stated here as an experimental fact with a conventional name attached, not as a theory the course can vouch for. If you only ever remember the measurements you have lost nothing.

What follows for fixing, and for Level D

Two practical consequences, both of them arriving later in the course and both decided here.

Fixing works because thiosulfate is a sulfur donor. Silver bromide’s solubility product is 5.0 × 10⁻¹³ and thiosulfate’s formation constant is 4.7 × 10¹³; multiply them and you get a combined constant of about 24, which is a reaction that goes. Water offers nothing to multiply by. Part XI turns that into clearing times and capacities.

Cyanide was attractive for exactly the same reason, and more so. A formation constant of 10²¹ makes potassium cyanide a faster and more thorough fixer than hypo, which is why it was standard for wet collodion plates that hypo cleared badly. It is also why several historical processes are classified Level D in this course: the chemistry that makes cyanide good at the job is not separable from the chemistry that makes it lethal, and Part XXVI studies those processes without performing them.

Silver nitrate: the one soluble salt

Everything silver in this course starts in the same bottle, because silver nitrate is the only silver compound photography uses that dissolves freely in water. HSDB, through PubChem, gives 245 g per 100 g of water and 122 g in 100 cm³ at 0 °C. Compare that with silver bromide’s 0.135 milligrams per litre and you have a ratio of roughly a thousand million to one between the salt you can pour and the salt you cannot.

The reason is the anion. Nitrate is an oxygen donor that silver does not grip, and — as Part III’s solubility rules put it — every nitrate dissolves. So nitrate is simply the handle: the anion chosen because it lets silver be weighed, dissolved, measured and delivered to the place where it will be turned into something insoluble.

Three characters in one bottle

It is an oxidiser. The harmonised European classification carries H272, may intensify fire. CAMEO records that although silver nitrate does not itself burn, it accelerates the burning of combustible material — which is why paper and cloth soaked in it and left to dry are a fire risk rather than merely a stain.

It is corrosive. The same harmonised entry carries H314, causes severe skin burns and eye damage, and a third of notifiers add H318, serious eye damage. That single line is why Part II puts sealed splash goggles rather than safety glasses on the bench for every silver operation in this course.

It is reduced by almost anything organic, and by light. Towler’s 1864 manual already knew the fused sticks blacken; CAMEO says the material turns black on exposure to light or organic material. That is the whole of the next paragraph, and it is also the whole of photography.

The stain, in one equation

Ag⁺ + e⁻ → Ag

The reduction that makes the stain, the image, and the tarnish

A published kit instruction states the mechanism in plain words: a dilute spill on skin gives a brown to brown-black stain, and “the color is due to silver metal bound to the protein of the skin and cannot be washed off”. The electron comes from the protein; light speeds the same reduction along. The practical trap — that the reaction is slow, so the mark appears long after you wiped the drop away — is Part II’s, and the rule that follows is Part II’s too: rinse at once, because you cannot use the appearance of the stain to tell you at the time whether you were splashed.

🕰How it is made, and why that is a description and not a procedure

Kodak’s 1928 primer gives the industrial route in three sentences: the first step is to dissolve metallic silver in nitric acid, the silver replacing the hydrogen of the acid to form silver nitrate, with the liberated hydrogen decomposing a further portion of the acid; the salt is then crystallised out as colourless transparent plates. Photographic grade has to be extremely pure, because scrap silver carries copper and lead, so it is recrystallised. Kodak added, not without pride, that it made its own and was the largest maker of silver nitrate in the world, using about one-eleventh of all the silver mined in the United States — the Mint alone using more.

This course describes that and never instructs it. The reagent is nitric acid, whose harmonised classification is H272, H314 and H330, fatal if inhaled; CAMEO describes it as giving reddish-brown vapours and being very toxic by inhalation. Dissolving a metal in it is an operation for a fume cupboard and a trained operator, and there is no home version. Silver nitrate is bought.

Why the halides come down and stay down

Part III did the algebra, so here is only the shape of it. A solubility product is the largest value the product of the two ion concentrations can reach before solid appears, and for the three photographic halides OpenStax Appendix J gives:

Salt	Ksp at 25 °C	What it means in the darkroom
Silver chloride	1.6 × 10⁻¹⁰	clears fastest in a fixer; the halide of contact papers
Silver bromide	5.0 × 10⁻¹³	the working compromise; films and enlarging papers
Silver iodide	1.5 × 10⁻¹⁶	clears slowest of all; a trace additive, never the whole emulsion

Between chloride and iodide that is about a thousandfold difference in dissolved silver ion. Add a soluble halide to a silver nitrate solution and the ion product instantly exceeds Ksp, so solid falls out until the product is back down to it. Kodak’s primer describes what that looks like in a beaker: if the solutions are at all concentrated the silver bromide comes down as a thick, curdy precipitate. You will see exactly that in the next experiment.

📷The photography

The chain from this number to the print is short. A big Ksp means a fixer clears the paper in a minute; a small one means a film needs several, and an iodide-rich emulsion needs longer still. When Part XI tells you to double the clearing time and use that as your fixing time, the reason is in this table.

Silver sulfide, from tarnish to toner

Silver’s one real vulnerability has a solubility product of 1.6 × 10⁻⁴⁹, and Ware writes the reaction in its simplest form:

2 Ag + S → Ag₂S

Ware: sulfur as a mild oxidant towards silver

Any sulfur source will do it. Atmospheric hydrogen sulfide is the usual one; so are the sulfur-bearing side groups of proteins, which is how albumen prints yellow from the inside. Kodak’s 1924 primer names the sensitivity precisely for unprocessed material: a very small amount of hydrogen sulfide converts enough silver bromide to spoil photographic materials — which is why sulfide toners and unopened paper never share a cupboard.

Ware’s account of the damage is the one to hold on to, because it is not a simple story of loss. Small amounts of sulfide can enrich an image; an excess destroys it, replacing the strong plasmon absorption of metallic silver with the far feebler spectrum of colloidal silver sulfide. And the damage is one-way: a sulfided image cannot be brought back with an ordinary developer, whose redox potential is nowhere near negative enough, though a very powerful reducing agent such as borohydride will do it.

⚗Preview: the same reaction, done on purpose

Sepia toning bleaches the image back to a silver halide and then converts it deliberately to silver sulfide. The Image Permanence Institute records the conservation logic: converting the image silver to a more stable compound such as silver sulfide was recommended for chemical stability, and in practice was and is rarely done. So the reaction that ruins a print by accident is the reaction that protects it on purpose, and the difference is control. Part XX owns toning.

Why silver, and not another metal

Four requirements, and the argument is that no other element meets all four.

1. Photosensitive compounds that are also insoluble. The silver halides absorb light and release an electron, and they are insoluble enough to stay where they are put. A photosensitive salt that washed out of the paper would be useless.
2. A latent image that survives. The cluster of a few atoms that light makes must last from the exposure until the developer arrives — minutes, or months. That is a demanding stability requirement and it is the subject of two later pages in this part.
3. Developability: an enormous amplification. The cluster must be able to catalyse the reduction of the entire crystal around it, so that the energy of a few photons is repaid a huge number of times over. Ware gives one measure of the gap being bridged: to make silver you can see by light alone, the exposure has to be increased by a factor of the order of a million over what forms a latent image.
4. An inert final image. The end product is metallic silver, which is stable enough to survive in albums for a century and a half — its one weakness being the sulfide chemistry above.

Iron gives you the cyanotype: photosensitive, printable, permanent enough — and no latent image and no amplification, so it prints out slowly and needs a strong light. Chromium gives you the dichromate processes: photosensitive, no amplification. Gold and platinum give beautiful and durable images but are reduced by iron salts that light has already changed, so again the light does the whole job. Silver is the only element in the course that supplies all four, and that is the reason a course about photographic chemistry is mostly a course about one metal.

Why silver images come in colours

Here is the fact that surprises people: the black of a developed print and the red-brown of a salt print are the same substance. Both are metallic silver. The difference is particle size and shape.

Ware’s account is quantitative. Print-out silver — the kind light makes directly — has particles in the 10 to 100 nanometre range, smaller than the wavelength of visible light. Developed silver is micron-sized bundles of filaments. The Getty atlas states the same contrast from the conservator’s side: photogenically formed particles are much smaller than chemically developed ones, and their colour follows their size.

Particles that small have a colour because light drives collective oscillations of their conduction electrons — surface plasmon resonance. For most metals that absorption lies in the ultraviolet, but for copper, silver and gold the way the metal’s dielectric function varies with frequency puts a sharp band in the visible. Ware notes that calculations for 10 nm particles of most metallic elements show colour to be a rare property: apart from those three, unreactive metals are grey or brown in the nanoparticle state.

Silver hydrosols: particle diameter against the colour you see

1. 110–20 nm — transmits yellow, scatters blue
2. 225–35 nm — transmits red, scatters dark green
3. 335–45 nm — transmits purplish-red, scatters green
4. 450–60 nm — transmits violet, scatters yellow-green
5. 570–80 nm — transmits dark blue, scatters yellow ochre
6. 690–100 nm — transmits light blue, scatters red-brown
7. 7120–130 nm — transmits grey-green

Calculated by Wiegel for spherical silver hydrosols and confirmed experimentally, as reported by Ware; the figures are his, the ladder is drawn. Real photographic silver is neither spherical nor alone in water, so read the direction and not the exact hue.

Two things besides size move the colour

What the particles sit in. Raise the refractive index of the surrounding material and the absorption band moves to longer wavelengths. Ware uses this to explain the violet of “sunned” silver chloride: particles of about 10 nm trapped inside the chloride lattice, whose refractive index of 2.071 shifts a band normally near 390 to 400 nm all the way to 550 nm — where the eye is most sensitive. The practical warning he draws from it is startling. Fix such a print in thiosulfate, and you dissolve the chloride matrix away; the silver is left in water instead of a high-index solid, the band snaps back to 400 nm where the eye barely responds, and the image goes from lilac to a very pale yellow with a drastic loss of apparent density.

Whether the particles touch. Linear aggregation of spheres splits the plasmon mode and adds a long-wavelength band, so a yellow 10 to 20 nm sol becomes red-brown on aggregating.

Where the silver plasmon band sits, and why the same silver looks different

• Silver particles in water (380–410 nm) — band near 390–400 nm; the sol looks pale yellow because the eye barely sees here
• Same particles inside silver chloride (530–570 nm) — band near 550 nm; the material looks lilac because the eye is most sensitive here

Band positions as Ware gives them; the widths are drawn for legibility and were not measured. The lesson is the direction of the shift: a higher-index surrounding moves the absorption towards the middle of vision, and taking that surrounding away moves it back. The coloured strip approximates where the visible spectrum falls and is a reading aid only; the wavelengths in the labels carry the information. The bands and curves are drawn to show the relationship, not measured.

📷The photography

You can now predict the direction of a tone shift before you read the box a paper comes in. Anything that makes the silver particles smaller or keeps them apart moves the image warm; anything that grows them into filaments moves it towards neutral black. A solvent developer, a warm-tone paper, a print-out process and a sulfide toner are four different ways of steering the same variable, and Parts VIII, XII and XX are where each is taken up.

Recap

• Ag⁺ has a filled d shell, so it is colourless; every colour in a silver photograph belongs to the metal, in particles.
• The standard potential of +0.7996 V describes the free ion only. In a chloride lattice it is +0.22 V, in the thiosulfate complex +0.017 V, in the cyanide complex −0.31 V. Same element, four different chemistries.
• Silver ion grips sulfur, nitrogen and carbon donors and ignores oxygen donors. That single preference is why thiosulfate fixes, why ammonia dissolves silver chloride, why cyanide was attractive and dangerous, and why water does nothing.
• Nitrate is the handle. It is the anion that lets silver be weighed and poured; the halides, whose solubility products run from 1.6 × 10⁻¹⁰ down to 1.5 × 10⁻¹⁶, are the anions that make it stay.
• Silver sulfide, Ksp 1.6 × 10⁻⁴⁹, is the one thing silver cannot resist. It is tarnish when it happens to you and toning when you do it on purpose.
• Silver was chosen because it does four things at once — photosensitive insoluble salts, a stable latent image, enormous developability and an inert final image — and no other element does all four.
• Print-out silver is 10 to 100 nm and coloured; developed silver is micron-sized filaments and black. The mechanism is surface plasmon resonance, and the variables are particle size, shape, aggregation and what the particles sit in.

Check your understanding

Question 1. You have 10.0 g of silver nitrate. How many grams of silver does it contain, and how many moles of Ag⁺ will it release?

• About 10.0 g of silver, and 0.0589 mol of Ag⁺
• About 6.35 g of silver, and 0.0589 mol of Ag⁺
• About 6.35 g of silver, and 0.108 mol of Ag⁺
• About 4.32 g of silver, and 0.0589 mol of Ag⁺

Show the answer and why

Answer: About 6.35 g of silver, and 0.0589 mol of Ag⁺

Silver nitrate has a molar mass of 169.873, so 10.0 g is 10.0 ÷ 169.873 = 0.0589 mol, and each formula unit releases one Ag⁺. Silver's share of the mass is its atomic weight, about 107.87, over 169.873, which is 0.635 — so 6.35 g. The useful habit is the second step: about 63.5 per cent of the mass of silver nitrate is silver, which is the number to reach for when you are costing an experiment or estimating what is going into the silver waste container.

Question 2. A test tube holds a white precipitate of silver chloride. Which of these will dissolve it, and why?

• Distilled water, because silver chloride is only sparingly insoluble
• Sodium chloride solution, because chloride is the common ion
• Sodium thiosulfate solution, because sulfur is a donor atom silver grips hard
• Dilute nitric acid, because it is an oxidising acid

Show the answer and why

Answer: Sodium thiosulfate solution, because sulfur is a donor atom silver grips hard

Thiosulfate removes silver ion into [Ag(S₂O₃)₂]³⁻ with a formation constant of 4.7 × 10¹³, so the solubility product can never be reached and the solid keeps dissolving. Water fails because the solubility product is 1.6 × 10⁻¹⁰. Sodium chloride is the trap: adding the common ion pushes the equilibrium the wrong way and makes silver chloride less soluble, not more — though at high chloride concentrations the [AgCl₂]⁻ complex, formation constant 1.8 × 10⁵, does eventually begin to redissolve it, which is a genuine but slow effect. Nitric acid attacks silver metal, not the halide.

Question 3. A salted-paper print made by printing out is red-brown; a developed enlargement on the same day is neutral black. What is the difference between them?

• The salt print contains a different silver compound
• The salt print contains residual silver chloride that has not been fixed out
• The two images are both metallic silver, but with very different particle sizes
• The developer adds a black dye to the image

Show the answer and why

Answer: The two images are both metallic silver, but with very different particle sizes

Both are metallic silver. Ware puts print-out silver at 10 to 100 nm and developed silver at micron-sized filament bundles, and the Getty atlas records the same contrast from the conservation side. Particles smaller than the wavelength of light show a surface plasmon absorption band whose position depends on their size, shape, aggregation and surroundings, and that band is what you see as a colour; filaments are far too large for the effect and simply absorb everything, which reads as neutral black.

Question 4. Why is nitrate the anion silver is supplied with, rather than sulfate or acetate?

• Because nitrate is the only anion that does not react with light
• Because every nitrate is soluble, and nitrate is an oxygen donor that silver ion does not hold
• Because nitrate makes the solution acidic, which stabilises silver ion
• Because nitrate has the lowest molar mass of the available anions

Show the answer and why

Answer: Because every nitrate is soluble, and nitrate is an oxygen donor that silver ion does not hold

The solubility rule that every nitrate dissolves is what makes silver nitrate weighable and pourable, and the reason it dissolves so freely is that nitrate is an oxygen donor — silver ion barely coordinates to it, so the salt in solution behaves as free Ag⁺ plus free NO₃⁻. That is exactly the property you want in a reagent whose whole job is to deliver silver ion to something that will take it away again. Silver nitrate is also very much reduced by light and by organic matter, so the first option is wrong on its own terms.

Question 5. A print that has been stored in a cheap cardboard box for ten years has yellowed and lost its deepest blacks. What has most likely happened, and can it be reversed with a developer?

• The silver has oxidised back to silver nitrate; a developer will restore it
• The silver has been converted to silver sulfide; an ordinary developer cannot reverse it
• The gelatin has yellowed; the silver is unaffected
• The print was under-fixed, and the residual halide has printed out

Show the answer and why

Answer: The silver has been converted to silver sulfide; an ordinary developer cannot reverse it

Sulfur from the enclosure and from the air converts image silver to silver sulfide, whose solubility product of 1.6 × 10⁻⁴⁹ makes the reaction all but irreversible in ordinary chemistry. Ware states the consequence directly: sulfided images cannot be restored by redevelopment because the redox potentials of photographic developers are not sufficiently negative, although a very powerful reducing agent such as borohydride will do it. Residual halide from bad fixing is a real defect too, and it also darkens on ageing — which is why a diagnosis needs the storage history and not just the appearance.

Question 6. Which single property of the silver ion explains BOTH why a fixer works and why the course refuses to store ammoniacal silver solutions?

• Its standard reduction potential of +0.7996 V
• Its preference for sulfur, nitrogen and carbon donor atoms over oxygen donors
• The insolubility of its halides
• Its filled d shell

Show the answer and why

Answer: Its preference for sulfur, nitrogen and carbon donor atoms over oxygen donors

The same donor-atom preference gives thiosulfate a formation constant of 4.7 × 10¹³, which is what dissolves unexposed halide, and gives ammonia a constant of 1.7 × 10⁷, which is what makes ammoniacal silver solutions easy to prepare and historically popular. The hazard is what those solutions do afterwards: they can deposit silver nitride, the "fulminating silver" of the old manuals, a contact explosive. The chemistry that makes a ligand useful and the chemistry that makes it dangerous are frequently the same chemistry, and a course that teaches only one of them is teaching half a subject.

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