Distilled Water

Distilled water (H₂O; CAS 7732-18-5) is the universal solvent of photographic chemistry — the only water type that should be used for mixing sensitive photographic solutions.^[1] Tap water contains calcium, magnesium, iron, chlorine, and (depending on region) fluoride and phosphate, all of which interfere with photographic chemistry: calcium and magnesium form scum in developers, iron stains alt-process prints, chlorine attacks silver halide emulsions, and phosphate disrupts developer pH buffering. Distilled water removes all of these; deionized water is an acceptable alternative for most purposes.

Photographic uses

• Mixing sensitive photographic solutions: Platinum/palladium printing, cyanotype, colour-process developer concentrates, hydroxylamine-based preservatives, and most alt-process sensitizer mixes all fail with hard tap water. Distilled is the only practical answer.
• Final rinse for film and paper: A brief distilled-water rinse after the final wash eliminates mineral spots from drying.
• Diluting concentrated developers to working strength: Commercial liquid developers are formulated to dilute with ordinary tap water, but sensitive workers use distilled for more reproducible results.

Practical notes

Distilled water is sold in grocery stores (1-gallon containers, typical price ~$1/gallon) or can be produced with a home distiller. Deionized water is chemically equivalent for photographic purposes; reverse-osmosis water is usually sufficient but retains some ions depending on the system's filter state. Do NOT use "drinking water" or "spring water" which have not been treated for mineral content.

Distilled water is mildly acidic in practice (pH 5–6) because it dissolves CO₂ from the air on standing. The acidity doesn't matter at photographic scales.

Related compounds

Not a meaningful category; distilled water is the photographic-grade form of common water.