Formalin, which is permanently non-acid and only slightly alkaline, close to pH9, may be prepared by the addition of borax till a good red colour is shown with phenolphthalein, or a slaty blue with thymol blue, when added to the diluted formalin.

Distillation of formalin from solid magnesium carbonate gives an acid product, which is at pH4·4.

Formalin neutralised with sodium hydroxide becomes acid on standing, the change being more rapid in sunlight than in the dark. A reaction of pH5·2 was reached in fifty-six days in light, the initial value being pH8·0. Commercial formalin, “ 40 per cent,” may be as acid as pH2·8.