Silver nitrate

Every light-sensitive crystal in this course begins in this bottle. Silver nitrate is the only silver compound photography uses that dissolves, and everything else silver does starts with an Ag⁺ ion that came from here.

In photography

It makes the sensitive salt. Kodak’s 1928 primer gives the pattern as the simplest kind of chemical reaction: two soluble salts change partners and the insoluble one leaves the solution.

AgNO₃ + NaCl → AgCl + NaNO₃

Double decomposition: Kodak's 1928 primer

AgNO₃ + KBr → AgBr + KNO₃

The same reaction, making the negative halide

Emulsion making is that reaction run inside warm gelatin, in the dark, with the silver nitrate added slowly: without the gelatin the precipitate would sink; with it, the crystals stay suspended.

How much of it decides what kind of photograph you get. For a material that is to be developed the primer’s rule is absolute: an excess of bromide or chloride and no free silver, because free silver fogs the whole surface as soon as it meets the developer. For a material that prints out the rule reverses — Solio, the chloride paper of its day, was made with an excess of silver nitrate, and that is what makes it darken fast enough to print in the sun.

⚗The chemistry

Reilly explains why that excess is necessary rather than merely helpful. Talbot found that with chloride and silver in equal amounts his papers were hardly light-sensitive at all, and settled on roughly six times more nitrate: a 2 to 4 per cent salting solution against a 12 per cent silver bath. Hermann Vogel gave the reason. As light dissociates each unit of silver chloride, the liberated chlorine unites with the silver nitrate present to form new silver chloride, which light breaks down in turn, and the cycle repeats. The excess silver is the halogen acceptor: take it away and photolysis stalls; supply it and the image builds to a real maximum density.

Elsewhere it appears in small quantities to give a bath the silver it would otherwise steal: Kodak’s Hypo Alum Sepia Toner T-1a carries 4.2 g of it with 4.2 g of table salt in 4 litres, because a fresh hypo-alum bath eats the highlights until it has some silver in it. It is also the silver half of Monckhoven’s intensifier, whose other half is cyanide, so the course records that as history and gives no procedure.

Properties

Colourless square tables, anhydrous, and neutral when carefully prepared: Towler’s 1864 account still describes what comes out of the packet. There is no hydrate and no conversion factor.

It is extravagantly soluble: 122 g in 100 cm³ of water at 0 °C, and 245 g per 100 g of water. Kodak’s 1928 table of stock-solution strengths, in ounces per 100 fluid ounces of saturated solution, puts it at 109 at 4.4 °C and 135 at 21.1 °C — the largest figure in that table. Nothing this course mixes comes near saturation, and there is no reason to warm the water.

It is not, by itself, light-sensitive. Ware states it directly: silver nitrate becomes so only in the presence of organic matter, being reduced to black silver by its light-induced oxidation of that substance — paper, sizing, skin protein, cotton. Towler gives the same fact from the other end: the fused lunar caustic sticks “frequently blacken by exposure to light, whilst pure crystallized nitrate of silver does not change”. The impurity supplied the electron.

Ag⁺ + e⁻ → Ag

One reduction: the image, the stain and the spoiled bottle

Kodak’s 1928 primer names it the outstanding example of a photographic chemical decomposed by light, notes that a solution changes much faster than the solid, and prescribes dark brown bottles for both. Grey specks, a brown tinge or a mirror on the glass mean silver has left solution, and the bottle is no longer the concentration its label claims.

Handling

The aggregated ECHA notifications held by PubChem classify it Danger, with the oxidiser, corrosion, health-hazard and environment pictograms. Of the 802 reports carrying hazard codes, 99.9 per cent give severe skin burns and eye damage, 99.8 per cent long-lasting aquatic toxicity, 97.4 per cent the oxidiser statement, a third serious eye damage and 11.7 per cent possible harm to the unborn child; the harmonised European entry, which is law rather than notification, carries H272, H314, H400 and H410. Two airborne limits agree on the number, and HSE’s is binding in Great Britain: EH40 Table 1 gives silver as soluble compounds, as Ag, a long-term limit of 0.01 mg/m³ against 0.1 mg/m³ for the metal, and the NIOSH Pocket Guide gives the same figure, an IDLH of 10 mg/m³, and symptoms of blue-grey eyes, nasal septum, throat and skin. That blue-grey is argyria, which Reilly records as permanent.

Why Level B. The course rubric names silver nitrate in its Level B criteria, and the substance meets three of them independently: corrosive at the concentrations actually handled, a weighed powder that must not be inhaled, and a reagent whose failure mode is a corrosive splash rather than a spoiled negative. The controls follow: splash goggles rather than glasses, an apron, eyewash within reach before the cap comes off, weighing slowly at bench level. It stops short of Level C because no source the course holds names a fume cupboard as the recognised control, and its waste is collectable for recovery.

⚠Safety

Goggles and gloves before the cap, and a dedicated spatula that has never touched a developer. Weigh into a tared glass or plastic vessel, never onto paper: paper is organic material, and this substance both stains and oxidises it. The incompatibility list is unusually consequential. CAMEO records silver acetylide from acetylene with ammonia, silver fulminate from alcohols, explosive mixtures with phosphorus and tin(II) chloride, and a silver nitride precipitate that exploded on stirring after an ammoniacal solution was treated with sodium hydroxide. Nothing loaded with it — absorbent, wipe, glove, cloth — is left in an open bin to dry.

Waste

Here a waste stream is also an asset. The aggregated classification gives very toxic to aquatic life with long-lasting effects in more than 99 per cent of notifications; Kodak’s J-52 lists silver among the effluent parameters municipalities most often regulate, mean limit 1.2 mg/L; and Kodak’s J-214 sets the United States federal line at 5 ppm of silver in a liquid waste, EPA number D011. Taking silver’s atomic weight as Wall’s 1912 table gives it, 108 against the compound’s 169.87, one spilled gram carries about 0.64 g of silver — enough to push half a cubic metre of water past that 1.2 mg/L limit.

Recovery is the precipitation reaction used deliberately. Reilly records that only 6 to 8 per cent of the silver in a sensitised sheet reaches the image, that most of the rest is in the first two changes of wash water, and that the workshop method is to add sodium chloride or sodium carbonate to the saved water, let the insoluble silver salt settle, decant and send the solid to a refiner — while warning that smelting and re-nitrating are not home operations. Collect it for the river, not the resale. ILFORD tells United Kingdom domestic users to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern, and they differ.

History

The salt is older than photography. Wall’s 1912 dictionary defines caustic as a term for corrosive or irritant chemicals and notes that nitrate of silver is known as lunar caustic — the fused sticks Towler describes, moulded to the size of a quill for surgeons to cauterise with.

Kodak’s two primers, four years apart, record how completely photography then took the compound over. The 1924 edition says the company made its own silver nitrate for purity, using about one twenty-fifth of all the silver mined in the United States; the 1928 edition repeats the sentence with the figure at one eleventh, second only to the Mint.

Ware supplies the beginning. Wedgwood, around 1797, made the first recorded contact images with silver nitrate on paper, and preferred white kidskin leather — which, Michael Gray has argued, worked better because tawing left chloride ions in the skin. What Wedgwood had was silver chloride, formed in place before anyone had written the reaction down.