Can thiourea dioxide be blended with sodium hydrosulfite?

Direct answer

Thiourea dioxide and hydrosulfite can be blended, and in practice often are. The point is not a vague best of both worlds; it solves two specific problems: holding the redox potential inside the process window, and restoring stripping power on heavy shades. What matters is that the ratio is set by the objective, not by habit.

1. Two objectives and the direction of the blend

2. Parameters that must be re-set when blending

1. Alkali: both reducers need alkali, but the consumption pattern changes in a blend, so it must be re-established

2. Temperature: thiourea dioxide releases fully only above 70 C, so it contributes little in a low-temperature process

3. Addition order: alkali first, then the reducers, dissolved separately, to avoid local over-concentration

4. ORP control: a blend still needs online monitoring; top up on the potential reading, not on the ratio

3. Three common mistakes

• Blending at a fixed ratio: the ratio should change with process, depth of shade and machine; it is not a constant
• Topping up as if a single reducer were in use: the two are consumed at different rates, so top up from the ORP reading
• Ignoring odour and effluent after blending: the higher the hydrosulfite share, the more noticeable the sulphur smell and effluent load

4. Bench trial suggestion

Run three or four blend ratios (for example thiourea dioxide at 20%, 40%, 60% and 80%), keeping everything else constant, and compare:

• The potential and how fast it decays
• Depth of shade and hue
• Stripping result and fibre strength

Sources

The blending directions are working ranges from our technical materials and published literature. The actual ratio must be established by bench trial on your own process.