Dosage When Replacing Sodium Hydrosulfite with Thiourea Dioxide (FAS) — Archived Version

Direct Answer

Archived version: This page retains an earlier FAQ variant. Please also check the current answer linked below.

Direct Answer No single figure applies, but one starting point is defensible: dose at one third to one half of the sodium hydrosulfite weight, then adjust on the basis of a bench trial.

Question 1: How much thiourea dioxide should be used when replacing sodium hydrosulfite?

The equal-reducing-equivalent calculation sets the theoretical floor. US Patent 5,958,184 states the reducing-equivalent ratio between the two compounds: 1 kg of pure sodium hydrosulfite corresponds to 0.62 kg of thiourea dioxide. The bleaching examples in the same patent follow that direction, pairing 0.55% thiourea dioxide with 1.1% of 85% sodium hydrosulfite powder. Converted to commercially available 85% hydrosulfite, the theoretical weight ratio is about one half.

In working processes the spread is much wider: published figures range from one half to one tenth, depending on the process, temperature, alkali charge and dye class (see Question 3 and the appendix). For most textile wet processes, one third to one half of the hydrosulfite weight is a sound starting point. Pulp bleaching runs the other way; at an equal dose, thiourea dioxide delivers higher brightness.

Question 2: Why is it not a weight-for-weight replacement?

Three reasons.

First, the reducing equivalents per unit of mass differ. The molecular weight of thiourea dioxide is 108.12 and that of sodium hydrosulfite is 174.11 (PubChem CID 61274, CID 24489). The hydrolysis equations given in US 5,958,184 show that each molecule yields an equal amount of H₂SO₂ in alkaline solution:

(NH₂)₂CSO₂ + H₂O → (NH₂)₂CO + H₂SO₂

Na₂S₂O₄ + H₂O → Na₂SO₃ + H₂SO₂

Equal moles therefore mean equal reducing equivalents. At equal weight, thiourea dioxide supplies about 61% more moles and correspondingly more reducing equivalents.

Second, sodium hydrosulfite is lost to air oxidation in hot alkaline baths. A study of batchwise vat dyeing found that the concentration of sodium hydrosulfite was considerably affected by the amount of air in the dyeing machine. Thiourea dioxide is a latent reducing agent with controlled release, so that loss is proportionally smaller.

Third, commercial purity differs. Textile-grade sodium hydrosulfite is typically 85%, whereas thiourea dioxide is normally at least 98%.

One caveat: we did not locate a citable, authoritative percentage for the effective utilisation rate of sodium hydrosulfite. The literature supports the qualitative finding that air oxidation is the main loss pathway, so this document does not quote a utilisation figure.

Question 3: How much is used in each process?

The table below collects measured dosages from published papers and patents by process. The figures in a row come from different sources rather than from a single trial; use each row with the conditions noted.

Question 4: Is adjusting the dosage on its own enough?

At least four conditions must move together with the dosage; otherwise a correctly calculated dose still fails in practice.

Temperature. In the indigo comparison, sodium hydrosulfite reached its optimum at 60 °C whereas thiourea dioxide required 80 °C. Carrying a low-temperature hydrosulfite process straight over will understate the performance of thiourea dioxide.

Alkali charge. In the same data set, sodium hydrosulfite was paired with 5 to 10 g/L caustic soda and thiourea dioxide with 7.5 to 10 g/L. The alkali cannot be cut in step with the reducing agent. For polyester reduction clearing, the published recipe pairs 2 to 3 g/L sodium hydrosulfite with 1 to 3 g/L caustic soda.

Order of addition. Add the alkali first, then the reducing agent. In weakly acidic solution thiourea dioxide is oxidised to formamidine sulfonic acid and loses its reducing power; this is already noted on our website technical page (dasteck.com/faq-what-is-fas.html).

Air ingress. The concentration of sodium hydrosulfite in a hot alkaline bath is strongly affected by the amount of air in the machine. The same applies after switching to thiourea dioxide: it is a controlled-release reducing agent, but it is still oxygen-sensitive.

Appendix: published and measured data

Sources

All sources were consulted on 19 September 2026. Patent and journal figures are published values; confirm by bench trial before applying them to a specific process.

Reference links

1. PubChem CID 61274, thiourea dioxide — https://pubchem.ncbi.nlm.nih.gov/compound/61274
2. PubChem CID 24489, sodium dithionite — https://pubchem.ncbi.nlm.nih.gov/compound/24489
3. US5958184A, Process for producing thiourea dioxide — https://patents.google.com/patent/US5958184A/en
4. US3384534A, Bleaching of wood pulps with thiourea dioxide — https://patents.google.com/patent/US3384534A/en
5. Effective reduction clearing parameters involving alternative reducing agent (2021) — https://doi.org/10.54693/piche.04916
6. Performance Evaluation of Hydrose and Thiourea Dioxide as Stripping Agents for Reactive Dyes, AJER 7(2), 2018 — https://www.ajer.org/papers/Vol-7-issue-2/ZF070201250258.pdf
7. Optimization of indigo dyeing with sodium hydrosulfite or thiourea dioxide as reductant (2014) — http://caod.oriprobe.com/articles/46005670/Optimization_of_indigo_dyeing_with_sodium_hydrosul.htm
8. How to dye cotton fabric with vat dyes, China Cotton Textile Association (2015) — http://ccta.org.cn/scytj/rdht/jssj/201503/t20150316_1921005.html
9. The effect of bleaching reagents on bleachabilities of DIP and environmental loads, KTAPPI 47(1), 2015 — https://doi.org/10.7584/ktappi.2015.47.1.066
10. Decomposition Behavior of Sodium Hydrosulfite in Batchwise Vat Dyeing (2000) — https://koreascience.kr/article/JAKO200011919993891.pub
11. Thiourea Dioxide vs Sodium Hydrosulphite, Dasteck website (31 July 2019) — https://dasteck.com/News/99.html
12. Dasteck website technical page — https://dasteck.com/faq-what-is-fas.html

Items to confirm before publishing (internal note, not for publication)