From "Amateur Photographer" Sept 25 1946:
"The sustance known as Meritol consists of a compound or mixture in equimolar parts of para-phenylenediamine and pyrocatechin.
The preparation is carried out as follows according to Patent Specification No.466,626 of Jun 1st, 1937:-
100 lb para-phenylenediamine is dissolved in 80 gallons of a 10 per cent solution of sodium bisulphite with the aid of heat, 2 lb decolourising carbon added, the mixture well stirred for 10 minutes and filtered hot; to this a hot solution of 100 lb of pyrocatechin in 20 gallons of a 10 percent solution of potassium metabisulphite is added; on cooling the new compound separates out in beautiful colourless prismatic crystals which melt at 110 degrees C and are remarkably stable in the air.
Similarly by substituting an equivalent quantity of pyrogallol or chlorquinol for the pyrocatechin in the above example the corresponding compounds are readily obtained."
I mention this out of interest only,NB some of these substances are highly toxic, don't try this at home!