Is Thiourea Dioxide Stronger than Sodium Hydrosulfite?

Direct Answer

Direct Answer Under comparable conditions, yes, but the word stronger has to be split into three layers, otherwise it leads to the wrong conclusion.

1. Reduction potential

Industry material commonly quotes about -1230 mV for thiourea dioxide against about -1080 mV for hydrosulfite, a gap of roughly 150 mV. Note that potential varies with temperature, alkalinity, concentration and reference conditions, and published absolute values differ. Treat the figure as an order-of-magnitude guide rather than a number to guarantee in writing.

1. Equal reducing equivalents

On a pure-product, equal-reducing-equivalent basis, 1 kg of sodium hydrosulfite corresponds to roughly 0.62 kg of thiourea dioxide. That is the stoichiometric floor.

1. The real-process gap is wider

Our technical data sheet states that under optimum conditions thiourea dioxide is a strong reducing agent and at least three times more effective than hydrosulfite. The stoichiometric ratio is only 0.62, yet the practical effect reaches three times, and the difference comes from two things. First, hydrosulfite has a low effective utilisation rate in a hot alkaline bath: it keeps decomposing once dissolved and is continuously oxidised by air drawn into the liquor. Second, thiourea dioxide is a latent reducing agent with controlled release and proportionally smaller losses.

That is also why the dosage quoted externally should be the measured substitution ratio of about one third rather than the stoichiometric ratio.

But stronger does not mean it can be dosed freely

Strong reducing power is double-edged in stripping. Over-reduction shifts the shade and lowers fibre strength, and the effect is most visible on protein fibres and on certain disperse dyes. Start from a conservative dose and work down on the basis of a bench trial rather than cutting the recipe by a fixed factor.

Bottom line

Higher potential, lower equivalent dose and a practical effect up to three times that of hydrosulfite, but the extra reducing power has to be held in check by temperature and alkali.

Sources

1. 我司产品技术数据表《THIOUREA DIOXIDE Technical Data Sheet》（TDO-TDS，签发日期 2024-05-08）
2. 我司技术资料《Thiourea Dioxide vs Sodium Hydrosulphite》（2019-07-31）还原电位与用量对比，https://dasteck.com/News/99.html
3. US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en