How does thiourea dioxide compare with other reducing agents?

Direct answer

Of the three reducing agents commonly used in textiles and paper, thiourea dioxide is the only one that combines strong reducing power with no formaldehyde and no irritating odour. Sodium bisulfite is far too weak, Rongalite (sodium formaldehyde sulfoxylate) is strong enough but contains formaldehyde, and hydrosulfite is strong but has clear safety and logistics drawbacks.

Comparison

Item	Thiourea dioxide	Sodium hydrosulfite	Sodium bisulfite	Rongalite
Reduction potential (approx.)	-1230 mV	-1080 mV	about -300 mV (weak)	strong, close to hydrosulfite
Typical use	Reduction clearing, stripping, machine cleaning, vat dyeing	Traditional reducing agent	Light reduction, dechlorination	Discharge printing
Odour	Odourless	Irritant sulphur dioxide	Faint	Releases formaldehyde on decomposition
Formaldehyde	None	None	None	Yes
Storage and transport	Sealed, cool, not above 25 °C	Water-reactive flammable gas; strict controls	Less demanding	Less demanding
End products	Urea + sodium sulphate	Sulphate	Sulphate	Formaldehyde + sulphate

Which one to choose

Replacing hydrosulfite for reduction clearing or stripping - thiourea dioxide, starting at about one third of the hydrosulfite dose
Very light reduction or dechlorination only - sodium bisulfite is cheaper
Discharge printing on an established recipe - Rongalite remains in use, but formaldehyde compliance has to be assessed
Black and grey stripping - blend thiourea dioxide with hydrosulfite at roughly 1:10

Bottom line

For replacing hydrosulfite, thiourea dioxide has a clear advantage in performance and safety. No single reducing agent is best in every process, so selection should follow the application.

Sources

Reduction potentials are quoted from our technical material and vary with bath conditions; the rest reflects established industry practice.

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How does thiourea dioxide compare with other reducing agents?

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