Lithium chloride

There is no chemical reason to salt a paper with lithium rather than sodium. There is a physical one, and it decided the composition of an entire class of nineteenth-century printing paper: common salt will not dissolve in a mixture of alcohol and ether, and this will.

In photography

It salted the collodion printing-out papers. A collodio-chloride emulsion is made inside collodion, which is nitrocellulose in alcohol and ether. The Getty Conservation Institute’s atlas sets out the procedure: one portion of collodion is mixed with silver nitrate and a little citric or tartaric acid, another with alcohol-ether soluble halides — usually calcium, strontium or lithium chlorides — and the two are combined under non-actinic light, so that silver chloride forms as microcrystals well dispersed through the collodion. Sodium chloride is not on that list, and its absence is the entire argument for this page.

Wall’s 1924 formulary states the trade’s preference outright: the best chlorides are those of strontium and lithium, and those two are the most satisfactory in practice. His collection of collodio-chloride formulas — Hanneke, Niederstadt, Valenta, Eder, Liesegang and a dozen more — uses lithium chloride in most of them, typically 2 to 3 g against 15 to 25 g of silver nitrate in a litre of alcohol and ether, with citric acid and a little glycerine.

!Which lithium chloride does a period formula mean?

Wall’s 1912 dictionary gives the compound as LiCl·2H₂O = 78.5 and describes the article as octahedral crystals, very deliquescent. His 1924 formulas call for “lithium chloride, cryst.”. The modern record for the substance is the anhydrous salt, LiCl, relative molecular mass 42.4, CAS 7447-41-8, and the course could not establish from any source it holds whether the dihydrate Wall describes is still a commercial article.

The gap between those masses is a factor of about 1.85 — the largest hydrate discrepancy anywhere in this encyclopaedia. Anyone reworking a collodio-chloride formula has to establish which salt was meant before weighing anything, and the course cannot do it for them. What can be said is that Wall’s own handling instruction makes the question sharper, not softer: he says the salt absorbs varying quantities of moisture, so even a jar of the correct article is of uncertain composition.

Its modern use is not photographic chemistry at all — it is humidity. Ware’s chrysotype practice gives a table of saturated salt solutions for constant relative-humidity enclosures at 20 °C, and lithium chloride is the driest entry on it: 15 per cent RH, from a saturated solution needing about 800 g per litre. Humidity is a variable in print-out — Ware’s argyrotype notes state that the colour of a silver print is determined by the size of the metal particles, which is affected by the humidity of the coating during the exposure, which is the silver page’s particle-size argument arriving from the weather. A box held at a known humidity is how that variable is controlled.

The lithium ion itself is still in current alternative printing for a related reason: Bostick & Sullivan’s Ziatype kit is built around a lithium palladium solution, and Ware’s account of why lithium salts behave as they do is on the same thermodynamic ground.

⚗Why a lithium salt gets warm and an ammonium salt gets cold

Ware’s platinum monograph uses this compound as the worked example. Dissolve lithium chloride in water and the solution warms: the dissolution is exothermic, with an enthalpy of −37.2 kJ/mol, driven by the strength of the bonds the small, highly charged lithium ion makes to four water molecules. Dissolve ammonium chloride and the solution gets cold, because the large, diffuse ammonium ion binds water only weakly and the lattice energy is not repaid.

The consequence Ware draws is about print-out rather than temperature. A cation that holds water tightly changes how much water a coating must absorb before its ions become mobile enough for a print-out process to work, and how readily the coating gives that water up again. Lithium and ammonium sit at opposite ends of that behaviour, which is why they are not interchangeable in a sensitiser even though both are monovalent cations that leave no residue.

Properties

LiCl, relative molecular mass 42.4, CAS 7447-41-8, deliquescent. Around 84 g dissolves in 100 g of water at 25 °C, rising to 128 g at 100 °C, which puts it among the most soluble salts here.

The property that earned it a photographic career is the non-aqueous one. About 25 g dissolves in 100 cc of ethanol at 30 °C and 42 g in methanol, and Wall records it as soluble in alcohol and ether. Sodium chloride is not. In a water-based salting bath common salt is cheaper and better; in collodion it simply will not go in, and the whole lithium-and-strontium preference of the printing-out literature is that one difference.

The second property, the deliquescence, is a liability rather than a feature — and Wall’s remedy of dissolving the packet in alcohol on the day it arrives is the correct response to any deliquescent reagent.

Handling

The European picture and the Japanese picture do not match, and the mismatch is recent. There is no harmonised CLP entry. The aggregated ECHA notifications, across two list numbers and more than a thousand company reports, give Warning with the irritant pictogram and nothing worse. Japan’s NITE-CMC classified it Warning in FY2010, added may damage fertility or the unborn child in FY2022, and in FY2025 revised it again to Danger, adding harm to breast-fed children and organ damage. Safe Work Australia also gives Danger.

HSE’s EH40 carries no entry for lithium chloride. It lists lithium hydride at 0.02 mg/m³ and lithium hydroxide at 1 mg/m³, and neither covers this compound; paragraph 6 says that absence from the list does not indicate that a substance is safe.

⚠Why Level B, and the one operation that is not

Read against the European notifications alone, this substance would sit at Level A: irritant, harmful if swallowed, nothing corrosive, nothing volatile. The rubric places it at Level B for two reasons that the notifications do not carry. The first is the reproductive-toxicity statement that a national scheme has now added twice in three years; the course takes the stricter view where the cost of being wrong falls on someone who is pregnant, and no current course procedure needs this salt, so nothing is lost by doing so. The second is Level B’s own criterion about fine powders that must not be inhaled, which a deliquescent, caking powder with a respiratory-irritation statement meets.

The one operation that takes it out of Level B is the one Wall already prescribed. Weigh the whole packet once, on the day it arrives, into a 10 per cent alcoholic stock, and dose by volume ever after. The powder is then weighed once or never — the same control the course applies to erythrosin — and what remains on the bench is a solution with a known strength and no dust.

Waste

Neither European aggregation gives this substance an aquatic hazard statement, and no photographic waste guidance the course holds mentions it. That is an absence of evidence rather than a permission, and in practice the decision is made by what else is in the bath: a collodio-chloride sensitiser carries silver, and silver decides the route. Check your local regulations; they govern, and they differ.

History

Lithium chloride’s photographic life begins and ends with one class of material. Collodio-chloride printing-out paper — the glossy, self-toning, sun-printed paper of the 1880s and 1890s — needed a chloride that would dissolve in collodion, and there were only three candidates. Wall’s 1924 formulary preserves the whole tradition in about six pages of formulas, and preserves its ending too: by the time he was writing, the process was already a specialism. When gelatin papers took the market, the requirement for an alcohol-soluble halide vanished with them.

What survived is the physical chemistry rather than the photography. A small cation that binds water hard is what you want for holding a box at a known humidity, or for keeping a palladium sensitiser workable, and that is where the element still earns its shelf space.