Macherey-Nagel test strips for sulfite, in one presentation.
91306
A semi-quantitative test strip for sulfite, dosed to strip oxygen out of water. Dip it, wait the stated time, and compare the pad against the colour scale on the tube.
In boiler feedwater sulfite exists to consume the dissolved oxygen that would otherwise pit the tubes. A residual has to remain, because zero residual means the oxygen won and the corrosion is already happening.
A strip gives a range, not a figure. That is enough for the great majority of decisions — dose or do not dose, discharge or hold, keep running or stop — and it gives the answer where the sample is, in about a minute, with no instrument to calibrate and nothing to carry back to a laboratory.
When a number has to be defensible — a regulatory report, a dispute, a limit you are close to — a photometer or a laboratory method is the right tool. The strip's job is to tell you whether you need one.
Sulfite is consumed by air as well as by the boiler; take the sample with as little splashing as possible.
Supplied in one presentation.