Sodium chlorite or sodium hypochlorite? The difference that decides your process

Two or three times a year an enquiry reaches us asking for a price on "sodium chlorite bleach". Nine times out of ten the buyer means sodium hypochlorite, and the two are not interchangeable. It is worth spending two minutes on the distinction before it costs a container.

Sodium hypochlorite is NaClO. It is what is in household bleach, and it is what most municipal systems used for decades. It is cheap, it is widely available, and it works by direct chlorination.

Sodium chlorite is NaClO2, CAS 7758-19-2. On its own it is a relatively stable salt. Its industrial value is that, when activated with acid, it generates chlorine dioxide — and chlorine dioxide is the thing most processes are actually specifying.

The practical differences follow from that. Chlorine dioxide works across a wider pH band, so a process that drifts in pH does not lose its disinfection margin. It diffuses and penetrates better, which matters when you are trying to reach crevices and dead legs inside equipment rather than just treat open water. And because it oxidises rather than chlorinates, it produces far fewer chlorinated disinfection by-products — which is the reason waterworks moved to it in the first place, and the reason it appears in drinking water regulations rather than as a general-purpose bleach.

In textile and pulp bleaching the argument is different but points the same way: sodium chlorite bleaches mildly and over a longer period, so fibre damage is lower than with chlorine bleaches. A mill that switches usually does so because it needs whiteness without losing tensile strength, or because it is replacing a chlorine-containing bleach for environmental compliance.

So the question to answer before requesting a quotation is not "which is cheaper per tonne" — it is "what is my process specified to use, and what does my regulator accept". If the specification says chlorine dioxide generation, you need sodium chlorite. If it says hypochlorite dosing, we are not the right supplier and we will tell you so.

One more thing that trips people up: sodium chlorite is classified UN 1908 for transport as a solution, and it must not be carried with acids or reducing substances. Sodium hypochlorite has its own classification. If your freight forwarder has quoted on the wrong one, the booking will not survive the paperwork.

If you are unsure which your process needs, send us the specification sheet or the process description. We will read it and say which material it calls for, including when the answer is that you do not need us.