Basicity concept

Chromium salt basicity used in leather tanning
Chromium (+3) in a solution has a strong attraction for hydroxyl ions (OH-). The reaction of chromium with the OH may be written as a three steps reaction as it takes on the 1st, 2nd and the 3rd hydroxyl group. The tendency of the 1st step of reaction to take place is very strong, and even at pH 2(a concentration of OH- in the solution of 10-12) the chromium will hold the1st OH group. As the pH is raised (the concentration of OH- increase) the 2nd OH group enters into the reaction with chromium. This takes place between pH 3 and pH 4. Near pH 4 the 3rd oh enters into the reaction in order to complete the reaction with the 3rd OH group it is necessary to raise the pH to about 8 to 9 and to bring the temperature of the solution to the boiling point.
The percent of the primary valence bonds of the chromium in solution accompanied by OH groups is called the basicity of the solution.
[ Cr ]+3 + OH- <……> [ Cr- OH] ++ 33 % basicity
pH 2.0 and below – approximately
[ Cr- OH] ++ + OH- <………> [ Cr(OH)2]+ 66 %
pH 2.0 – 4.0 approximately
[ Cr(OH)2]+ + OH- <……> [ Cr(OH)3]0 ppt.
pH 4.0 – 8.0 or 10 may require heat
the chromium salt used in chrome tanning usually basicities between 33 % to 66 %.