pH and Density Changes
Date: Summer 2013
Does pH have a direct affect on specific gravity?
pH is a measure of hydrogen ion concentration or more precisely the hydronium ion (H3O+) concentration. A solution may contain a variety of species that are ionized besides the hydronium ion. The total concentration of these species and their molecular constitution governs the density. Therefore, density and pH are usually independently variable properties of a solution. However, in the case of any particular type of solution such as hydrochloric acid solutions, the density decreases with increasing pH until about pH=4, because such solutions contain decreasing amounts of hydrochloric acid. This type of behavior is exhibited in the lead-acid battery, where the concentration of sulfuric acid in the battery decreases as the battery is discharged, and the pH of the solution also increases slightly.
It's not exactly clear what you mean by "direct affect". pH in itself does not affect specific gravity, but most of the acids and bases that would cause a difference or change in pH will not have the exact same specific gravity as water, and thus cause a change in the specific gravity of any solution containing them (even if too small to measure easily over some of the pH range).
In general the answer is no. It only takes a few drops of a strong acid (or base) to alter the pH of water (7.00) by several orders of magnitude. The addition of a few drops of a strong acid (or base) to decrease or increase the pH by orders of magnitude.
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Update: November 2011