Can Potassium Carbonate be used to reduce Calcium hardness in water

Question

I have a solution of KCl 20g/l (2%) and 500mg/l Ca, To reduce the hardness, Na2CO3 is normally used, but can I use K2CO3 instead?

Answer 1

I know nothing about drilling mud.

Solutions of both $\ce{Na2CO3}$ and $\ce{K2CO3}$ will precipitate calcium carbonate.

$$\ce{Na2CO3 ->[H2O] 2Na+(aq) + CO3^{2-}(aq)}$$ $$\ce{K2CO3 ->[H2O] 2K+(aq) + CO3^{2-}(aq)}$$ $$\ce{Ca^{2+}(aq) + CO3^{2-}(aq) <=>[H2O] CaCO3 v}$$

Since there is already some $\ce{KCl}$ in solution this would slightly reduce the solubility of the potassium carbonate. However since potassium carbonate is more soluble than sodium carbonate anyway taht wouldn't seem to be a problem.

Solubilities are typically given as grams/100 ml of water as in the Wikipedia table.

\begin{array}{|c|c|c|}\hline \pu{Temp }^\circ\pu{C} & \ce{Na2CO3} & \ce{K2CO3} \\ \hline 0 & 7 & 105 \\ \hline 10 & 12.5 &109 \\ \hline 20 & 21.5 &111 \\ \hline 30 & 39.7 & 114 \\ \hline 40 & 49 & 117 \\ \hline 50 & - & 121.2 \\ \hline 60 & 46 &127 \\ \hline 70 & - & - \\ \hline 80 & 43.9 & 140 \\ \hline 90 & 43.9 & 148 \\ \hline 100 &45.5 & 156 \\ \hline \end{array}

However the important consideration here is moles per liter.

$$\pu{500 mg }\ce{Ca^{2+}}\pu{/L} = \dfrac{\pu{0.5 grams/L}}{\pu{40 g/mole}} = \pu{0.0125 moles/L}$$

The solubility product for calcium carbonate is $3.3\cdot10^{-9}$.

$$K_{\mathrm{sp}} = 3.3\cdot10^{-9} = \ce{[Ca^{2+}][CO3^{2-}]}$$

where the brackets indicate moles per liter. So $\pu{0.0125 moles/L}$ of $\ce{CO3^{2-}}$ would precipitate the calcium. You might want to add say 20% excess to allow for the fact that the calcium concentration might be just more or less $\pu{500 mg/L}$.

Other considerations are the extra water from the potassium carbonate solution which would dilute the mud. Also how to get the potassium carbonate solution mixed evenly with the mud.

It is strictly a guess, but sodium carbonate is probably preferred over potassium carbonate due to price.

\begin{array}{|c|c|c|}\hline & \ce{Na2CO3} & \ce{K2CO3} \\ \hline \pu{grams/mole} & 106.0 & 138.2\\ \hline \% \ \ce{CO3^{2-}} & 56.6 & 43.4 \\ \hline \end{array}