Solid added to a litre rather than made up to a litre

Lifted from concentration and dilution, which names this as the most common mistake on its page and the easiest to avoid.

What you see

Nothing, in the bottle. This fault has no appearance at all until you go looking for it, and then it has exactly one — the finished volume is larger than the volume the formula names. A stock made “to 1 litre” that measures 1,050 ml was not made to a litre.

Downstream it shows as a solution that is consistently and slightly weak. A developer mixed this way is under strength by whatever fraction the solid’s own volume represents, so negatives come back a little thin and a little flat, reproducibly, and the fault survives every check aimed at the developer’s chemistry because the chemistry is correct and the concentration is not.

Likely causes

1. The water line of the formula was read as a description rather than an instruction. “Water to make 1 litre” tells you what the finished bulk must be; it does not tell you to measure a litre.
2. A litre was measured out first, into the mixing vessel, and the solid tipped in after it.
3. A hydrate was used and its water of crystallisation was not counted. Dissolving sodium thiosulfate pentahydrate in place of 100 g of the anhydrous salt puts about 57 g of water into the solution with it — roughly 57 ml — so adding a litre on top of that makes about 1,057 ml.

What is happening: the chemistry and physics

A dissolved solid occupies volume. Ten grams of a salt dissolved in water does not vanish into the gaps between the water molecules; the solution ends up larger than the water it started as, by an amount that depends on the substance and on how much of it there is.

So the same weighed mass gives two different concentrations by two different procedures. Dissolve 10.0 g in part of the water and top up to the 100 ml graduation and you have 10 g in 100 ml, which is 10 % w/v. Tip 10.0 g into a measured 100 ml and you have 10 g in something more than 100 ml, which is less than 10 % w/v.

The size of the discrepancy grows with concentration. It is unnoticeable in a 0.1 per cent solution and real in a 10 per cent stock, which is exactly why the habit is used at every strength rather than only at the strengths where it matters — deciding case by case is one more thing to get wrong.

Diagnostic questions

• What does the formula’s water line actually say? “Water to 1 litre”, or the period British “water to 20 ounces”, is an instruction to make up to volume. “In 1 litre of water” would be the other operation, and photographic formulae rarely mean it.
• Measure the finished volume in a graduate. Does it match the nominal volume, or exceed it?
• Is the discrepancy in the direction of weak? Adding to a volume can only ever dilute below the stated strength, never above it, so a solution that is too strong is a different fault.
• Was a hydrate substituted for an anhydrous salt at the same mass? That fault travels with this one and is its own entry.

Corrective action

You cannot correct it by adding more solid, because you no longer know how much water is in the vessel.

Two honest routes. Measure the actual finished volume and recompute the true concentration — mass divided by the volume you measured — then label the bottle with that figure and treat it as the stock it really is. Or discard it and remake, which is what a formula whose ingredients are cheap deserves.

What is not acceptable is to use it as though it were the nominal strength and let the error travel into every dilution taken from it.

Prevention

Dissolve the solids in about three-quarters of the final volume of warm water, then add cold water to make up to the final volume. That is not a laboratory nicety invented for this course — it is ILFORD’s own mixing instruction for its powder developers, and its stop-bath sheet goes further, having you rinse the measuring cylinder out into the mixing vessel before topping up, so that nothing is left behind in the glass.

Read the water line of every formula before you weigh anything. The British Journal Photographic Almanac explains its own house style in the same terms — its formulae are drawn up on the basis that the total bulk after the solids have dissolved is the figure stated — and the tell is the typography of the water line. Kodak was still writing “Water to make . . . 32 ounces / 1.0 liter” in 1928.