# How does pH affect the rate that CaCO3 dissolves in freshwater?

It is my understanding that calcium carbonate dissolves faster in lower pH environments. I am interested to know how higher PH levels affect the rate of dissolution. So I have broken this question down into 2 parts:

1. Starting from a neutral pH, how will each 1 unit increase in pH affect the rate of dissolution?
2. I believe that other factors likely play into this equation - for example, KH, GH and magnesium levels. How does this work?
• What do you mean by KH and GH? Commented Sep 8, 2022 at 18:24
• Aquarium talk: "There are two types of water hardness: KH and GH. KH measures an aquarium's carbonate hardness, or the degree to which it is able to buffer against pH fluctuations. GH stands for general hardness, which measures the number of dissolved calcium and magnesium ions. GH is crucial for the growth of certain fish species."
– Karsten
Commented Sep 8, 2022 at 18:37
• As solid phase is involved, it is much easier to evaluate thermodynamic equilibrium (involving pH and total dissolved carbonate forms) than kinetic rate. Commented Sep 9, 2022 at 4:12

Calcium carbonate dissolves by falling apart into ions (calcium cations and carbonate anions):

$$\ce{CaCO3(s) <=> Ca^2+(aq) + CO3^2-(aq)}$$

The rate of dissolution does not depend on pH. However, there is a back reaction, which depends on the concentration of the calcium cation and the carbonate anion. The carbonate anion concentration, in turn, depends on the pH because you have carbonate, bicarbonate and dissolved carbon dioxide in fast equilibrium:

$$\ce{CO3^2-(aq) + 2 H+(aq) <=> HCO3^-(aq) + H+(aq) <=> H2CO3(aq) <=> CO2(aq) + H2O(l) }$$

You can take a look at what happens to carbonate at different pH values here (it is called speciation).

The bottom line is that in the pH range above 10 or 11, pH does not matter for carbonate (although the calcium ion might precipitate as hydroxide). When you go more acidic than pH 10, for every pH step, about 10 times less carbonate exists because it turns into bicarbonate of carbonic acid/carbon dioxide. So the back reaction is slower, potentially speeding up the net forward reaction.

Whether the back reaction has an effect on the net rate of dissolution depends on what is in the water before you add the solid calcium carbonate. It also depends on how much you add.

Given two specific scenarios (the entire composition of the system), a quantitative answer would be possible.

• Thank you, Karsten for this detailed answer. So results of a water test, say from a local aquarium shop, would allow for a quantitative answer? I thought of posting with that info but refrained because on my main SE site (Webmasters) questions that only apply to one's specific situation are off topic for us. Seems that's not the case here. Commented Sep 10, 2022 at 15:13