Whether lime titrations or pH control is belter depends, I think, on the level of control. If the level was down around 0. 1 lb per ton of solution, titration is a bit difficult without a highly skilled man, which is not always available. Around a pH of 11 or 11.5, a pH meter is sensitive and it is easier to see differences on a meter. The pH meter is not sensitive at pH 12 or higher, and in that range, titration is better. When you come down to instrumentation, titration cannot be used and something else, that responds to hydroxyl ions has to be used. In principle, a pH meter with electrodes can be used, and a conductivity probe design was very successful.
The diffusion rates of cyanide and oxygen ions are not very different. Therefore you can forget about diffusion rates and talk about concentrations. If they were exactly the same they would cancel each other out. If say, the diffusion rate of cyanide was twice as fast as oxygen, then you would have to say that you have to have twice as much oxygen as cyanide to take care of the slower diffusion. One oxygen molecule will remove the electric charges which are produced by more than one cyanide ion. One oxygen molecule dissolved in solution will take care of between four and eight cyanide ion charges .
More oxygen present than this does no good because the rate will then be controlled by the rate of diffusion of the cyanide.
Temperature affects the diffusion rates. High temperatures are not advantageous since they give rise to high cyanide consumption and high solution fouling. Increased agitation also affects the diffusion rates. A stagnant layer exists around the surface of the mineral and the thickness depends on the rate of agitation.
As the cyanide concentration is increased, other undesirable reactions are also increased which produce constituents which affect the dissolution rate of gold. As the solution “fouls”, the dissolution rate diminishes. The rate of gold dissolution is not very temperature dependent but some of the other reactions are temperature dependent. The rate depends on how fast you can change gold from gold metal, with no charge, to the gold ion.
Titration gives you the buffered effect or potential of the solution which the pH does not give you and therefore titration might be preferred. The pH gives you the activity of the hydroxyl ions at any particular time. A potential source of hydroxyl ions might be released as you add acid in the conventional titration for lime and therefore, I believe that the pH is more accurate to use.
A buffered solution is one which will retain its pH even though you add acid or alkali to it. The solution contains a mixture of slightly ionized material but the majority of it is not. As some of the ions are removed when an acid is added, more compound becomes ionized thereby remaining almost a constant pH. If undissolved Ca(OH)2 exists in solution and an acid is added, more Ca(OH)2 will go into solution.