# Solubility and pH

I was reading about antacids. It stated that in earlier times $$\ce{NaHCO3}$$ and a mixture of aluminium and magnesium hydroxide were used as antacids. In excess $$\ce{NaHCO3}$$ can make the medium alkaline but as

metal hydroxides (say, magnesium hydroxide) are insoluble thus they do not increase the pH above neutrality.

I am unable to understand how solubility is related to pH. Please explain.

When magnesium hydroxide dissolves in water, it increases the hydroxide ion concentration in solution:

$$\ce{Mg(OH)2(s) <=> Mg^2+(aq) + 2 OH-(aq)}$$

When hydroxide reacts with the $$\ce{H+}$$ given off by acids, it gets consumed:

$$\ce{OH-(aq) + H+(aq) <=> H2O}$$

The reason magnesium hydroxide stops dissolving is that there is too high a concentration of magnesium and hydroxide ions in solution (the reverse reaction is as fast as the forward reaction, and the system is at saturation or, more generally speaking, at equilibrium). Now if the second reaction happens as well, this will lower the hydroxide concentration, allowing more magnesium hydroxide to dissolve.

• This is related to LeChatelier's principle, to certain coupled reactions in Biochemistry, and to the common ion effect. – Karsten Theis Jan 21 at 18:31

Your stomach pH is in the range of 1 to 3 (Wikipedia). If it goes into the very acidic rage and you develop heartburn, antacids can be used to raise the pH and soothe the irritation.

Antacids aren't used to raise the pH above 7, just to move it above 1 or 2. The amount of NaHCO3 you would ingest is so small that it would not do more than this, and even tho its pH in plain water is 8.5, it is quite harmless.

Magnesium hydroxide is only very slightly soluble, but it will raise the pH of plain water to about 9; still pretty harmless, but it will dissolve in strong acid and neutralize it. The amounts ingested are small enough not to raise the stomach pH much, and it won't hurt the esophagus when going down. Aluminum hydroxide is similar.

Very strong and very soluble alkalies would burn the esophagus going down, so are not used. Antacids don't need to bring the pH into the alkaline range, just to increase the pH enough to be less irritating. Commercial proton pump inhibitors can decrease stomach acid by up to 99% (equivalent to about 2 pH units). That's all that's needed.

The pH is the negative decadic logarithm of the H+ concentration in the solution. If you have a salt (like NaHCO3) that can dissolve, it can form OH- or H+ during that process. Therefore the solution can become acidic or basic. If a salt is not soluble, nothing will happen.