Mallory described the reagent now generally known as PTAH in 1897^[15]. PTAH stains tissues either reddish brown or blue depending on their type. This property of simultaneously staining two different colours is different from other haematoxylin reagents e.g. alum-haematoxylin. The role of phosphotungstic acid and the mechanism of staining is not fully understood. Interestingly the active component of haematoxylin is the oxidised form, haematin, although this rarely acknowledged in the literature which refer to haematoxylin staining. Phosphotungstic acid forms a lake with haematin^[16]. The make -up of the reagent is uncertain, examination of a year old sample showed there to be three coloured components, blue, red and yellow.^[17]. These were not identified. Some investigations of "model" systems, reacting various compounds e.g. amino acids, purines, pyrimidines and amines. with PTAH show that they give rise to different colours

Mallory described the reagent now generally known as PTAH in 1897^[15]. PTAH stains tissues either reddish brown or blue depending on their type. This property of simultaneously staining two different colours is different from other haematoxylin reagents e.g. alum-haematoxylin. The role of phosphotungstic acid and the mechanism of staining is not fully understood. Interestingly the active component of haematoxylin is the oxidised form, haematin, although this rarely acknowledged in the literature which refer to haematoxylin staining. Phosphotungstic acid forms a lake with haematin^[16]. The make -up of the reagent is uncertain, examination of a year old sample showed there to be three coloured components, blue, red and yellow.^[17]. These were not identified. Some investigations of "model" systems, reacting various compounds e.g. amino acids, purines, pyrimidines and amines. with PTAH show that they give rise to different colours.

In these reagents two or three basic dyes are used with phosphotungstic acid, in either a one step or multi-stage procedure. These reagents colour different tissue types different colours. Again the mechanism of staining is not fully understood. Some explanations include the proposal that phosphotungstic acid acts as a mordant to bind the dye to the tissue^[19] or that alternatively it binds to tissue blocking it to dye molecules
Adsorption onto tissue or the surface of viruses and its electron density are the bases of phosphotungstic acids action as a negative stain. This electron density arises from the presence of the 12 tungsten atoms which each have an atomic number of 74. The mechanism of the adsorption onto tissue has been proposed as being electrostatic rather than involving hydrogen bonding, as adsorption is not affected by pH