Modelling Mixtures of Acids and Bases

For simple mixtures of acids and bases, this online calculator lets you predict the pH of a mixture. Before starting the calculator, you must either know the concentrations of the acid and base in the total solution after mixing (see their example Case #1), or the concentrations and flowrates of the acidic and basic streams before mixing (see Case #2).

WebQC pH Solver

Let’s try this out! For example, say I have a mixture containing 0.1 mol/L of HCl and of NaOH:


HCL pKa=-7.0 c=0.1

NaOH pKb=0.2 c=0.1


[H+] = 1.0763310243078E-7

[OH-] = 9.2908220372361E-8

pH = 6.9680541413824

pOH = 7.0319458586176

So this stream would be very slightly acidic (ph<7), but very close to neutral.

pKa and pKb are “dissociation constants” which you need to look up in a reference. Where can you get pKa and pKb values? The Chembuddy program documentation is one source.

By the way, Chembuddy program is pretty convenient and not that expensive at all. They also have good documentation if you want to try these equations by hand.

For example, if you wanted to double-check the pHsolver or try work by hand, for “strong” acids and bases try Chembuddy’s Equation 11.13, or a simpler and almost as accurateEquation 14.2. In either case you will know all the variables in the equation except for the [H+], which you can solve by re-arranging the equation or by guess-and-check iteration:

Fast procedure for these equastions: Assume a value for [H+]. Substitute it in to the left and right side of the equation, and calculate the value of Left Side – Right Side. You want LS-RS=0. Keep iterating different values of [H+] until you’re there. If you use a computer program like Excel’s goal seek you might try multiplying by a million or more before running the goal seek, since you’re trying to get accuracy in the 10^(-14) range. Once you have a value of [H+] that makes LS-RS=0, then you can calculate the other variables such as pH = -log[H+].

If you have  just a single chemical to consider, or just want to look up some pKa and pKB values, the Wolfram Alpha pH Widget is another resource for you.

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