The pH scale is a logarithmic measure of how acidic or alkaline a solution is, which is why every step on it represents a tenfold change in hydrogen ion concentration. This pH calculator converts between pH and concentration, works out the pH of a strong acid or base solution from its molarity, and gives you the matching pOH and hydrogen ion concentration.
What Is the pH Calculator?
pH is the negative logarithm of the hydrogen ion concentration, and because it is logarithmic it compresses an enormous range into a manageable scale. A pH of 3 has a thousand times more free hydrogen ions than a pH of 6, even though the numbers differ by only three.
This calculator converts in both directions, from a measured pH to a concentration and back, and adds the pOH, the hydroxide ion concentration and a plain-language description of the solution.
How the pH Formula Works
pH = −log10[H+], and pH + pOH = 14 at 25 degrees Celsius
The relationship with pOH holds only at 25 degrees Celsius, where the ionic product of water is exactly 1 times 10 to the minus 14. At other temperatures the neutral point moves, so a solution that is neutral at 20 degrees is slightly alkaline at 30.
For a strong acid that dissociates completely, the hydrogen ion concentration equals the molarity of the acid, so the pH is simply the negative log of that molarity. Weak acids dissociate only partially, so their pH is higher than this calculation suggests and requires an equilibrium constant.
Reading the Scale
A pH of 7 is neutral at room temperature, below 7 is acidic and above 7 is alkaline. Most of the scale in practice is crowded into the range 3 to 11, because that covers everything from lemon juice to household ammonia.
The factor row in the calculator is the most useful one for intuition. A pH of 2 is a hundred times more acidic than a pH of 4, so two units of pH is a factor of a hundred, and one unit is a factor of ten.
Measurement and Its Limits
A pH meter measures the activity of hydrogen ions rather than their concentration, so the readings drift with temperature, ionic strength and the age of the electrode. Calibration with buffer solutions at two points is essential, and a flat or drifting calibration is the usual cause of a suspect result.
Remember that pH is not the same as acidity. A weak acid such as acetic acid at pH 4 can still neutralise a substantial amount of base, because it holds a reserve of un-ionised acid that dissociates as the hydrogen ions are consumed.
Frequently Asked Questions
How do I convert pH to hydrogen ion concentration?
Raise ten to the power of the negative pH. A pH of 3 gives 0.001 mol/L, and a pH of 7.4 gives about 3.98 times 10 to the minus 8 mol/L.
What is the pH of a 0.01 molar strong acid?
A strong acid dissociates completely, so the hydrogen ion concentration equals 0.01 mol/L and the pH is negative log of 0.01, which is 2.
Why is neutral pH 7 and not 0?
Because the scale is logarithmic and is anchored at the hydrogen ion concentration of neutral water at 25 degrees Celsius, which is 1 times 10 to the minus 7 mol/L. The neutrality point moves slightly with temperature, so neutral water is 6.51 at 50 degrees.
What is the difference between pH and pOH?
They are complementary measures of the same equilibrium. pH measures acidity through hydrogen ions and pOH measures alkalinity through hydroxide ions, and at 25 degrees the two always add up to 14.
Can I use this for a weak acid?
Not directly. The simple calculation assumes complete dissociation. Weak acids such as acetic acid or citric acid need their dissociation constant and a quadratic solution, which gives a noticeably higher pH than the molarity alone would suggest.