Hello, I am not a chemist but a physicist, so I hope you will forgive me for the wrong terms..
I have started making some Madder Lake (used by painters and luthiers), by precipitating aluminium hydroxide from a K2CO3 + AlK 2(SO4) 12H20 solution. I ignore the presence of the madder roots, which contributes mainly alizarine, in the initial K2CO3 solution and of water impurities. I have also read that it is the calcium salts present in these impurities that allows AlOH3 to bind with alizarine, but cannot find a precise reference for that statement (cited in J Michelman, Violin Varnish, pag 85, freely available on-line).
I have done some stoichiometry to evaluate the correct proportion of the ?two initial salts, but my chemistry knowledge lacks... Could someone check my ?estimated reaction (in water)?
2 (AlK 2(SO4) 12H20) + 3 (K2 CO3) => 2 (Al (OH)3) + 4 (K2 SO4) + 18 H2O + ?3 (H2 CO3)
that gives a mass ratio of approx. 2.29 parts alum/ 1 part potassium ?carbonate.
1) The above formula looks numerically correct, but is it what is really happening?
2) As AlOH3 is insoluble (in absence of acid or strong alkali), and H2CO3 escapes, as it will mainly be in the form CO2 + H2O, I also suspect that if (1) is the correct reaction, then it will go wholly from left to right.
?3) as the solution is kept between 30 and 40C, I suppose that all the ?CO2 will leave the solution, is then the result a neutral ?solution?
Note that point (3), that is neutrality, would give me a better way of evaluating the amount of alum to add to the solution, as after filtration of the potassium ?carbonate/madder root solution, I do not really know how much carbonate ?has been discarded with the root pulp.
Finally, does anyone know how AlOH3 sequestrates dye molecules from a solution?