Sodium Ascorbate

Sodium ascorbate (C₆H₇NaO₆; CAS 134-03-2) is the sodium salt of ascorbic acid — chemically equivalent to ascorbic acid plus an equivalent amount of sodium hydroxide, but supplied as a single shelf-stable salt.^[1] The practical difference is working pH and solubility: sodium ascorbate dissolves readily in water to give a near-neutral solution (pH ~7.5), where pure ascorbic acid solutions are strongly acidic (pH ~3). For developer mixing, this means sodium ascorbate can be added directly to a formulated developer without shifting pH — a meaningful convenience for reliable mixing.

Photographic uses

Interchangeable with ascorbic acid in essentially every hydroquinone-free developer formula, at the same molar concentration. The choice between the two is practical:

• Sodium ascorbate is preferred where mixing order matters (e.g., PC-TEA in glycol concentrate, where acid-form ascorbic acid would destabilize the TEA base).^[2]
• Ascorbic acid is preferred where a formula needs its acidic contribution for pH control.

Both appear in commercial formulations (Kodak Xtol uses sodium ascorbate; some DIY formulas use acid-form ascorbic acid) — the photographic result is indistinguishable.

Practical notes

Supplied as fine white crystalline powder, more hygroscopic than ascorbic acid — store tightly sealed. Solutions keep a few days at working pH; fresh for best results.

Substitution math: 1 g sodium ascorbate ≈ 0.89 g ascorbic acid on a molar basis. The sodium content of sodium ascorbate contributes to overall ionic strength but not to photographic chemistry.

Related compounds

Ascorbic acid — the acid form, see there for the full photographic chemistry discussion.