# pKa value using a pH meter

Similar to the Potentiometric estimation, a different electrochemical estimation method is, using the pH Meter to calculate the concentration of H+ Ion, and thus pH of solution or pKa of acid and similar quantities, so Let’s discuss one Experiment,

(It would be preferred if you read the article about Potentiometric titration before, since they are related.)

Determination of pKa value of weak acid using pH meter

Instrument: pH meter

Apparatus: burette, 100ml beaker, straw, and electrode assembly

Solutions: weak acid and sodium hydroxide

Electrodes:  glass electrode (indicator) – calomel electrode (reference) assembly

the figure shown is a glass electrode.

Now before going into the experiment, we need to know what mathematical formula or tool is going to be helpful to us, coincidently this time also we are play with a ratio and a formula,

The one formula is derived by some approximation and a lot of ionic chemistry (discussed later in lecture 3.000) but here we are suppose to use it for estimation,

pH = pK+ log([salt]/[acid])

Now, just for recapturing some of older memories         pKa = -log(Ka

Where Ka is the dissociation constant, or we can say is the equilibrium constant of the reaction below

HA ——– > H+ + A

If we look at the equation carefully, we need the pKa value then we need salt = acid,

At that point the pH = pKa   so we just need to know what is the pH when the salt and acid concentration are equal.

Here we have a problem.

We cannot find the point at which the salt and acid are equal, so we calculate the point where acid is just consumed i.e.

We can calculate the twice volume value.

Procedure:

1. Pipette out 25ml of the given weak acid into a beaker. Immerse the glass electrode – calomel electrode assembly into it.
2. Connect the electrodes to pH meter and measure the pH. For the explanation, how the pH meter work or how do an electrode measure on a specific element to change its E0 value go to Lecture 1.008 Ion-selective electrode.
3. Now add NaOH from burette in increments of 0.5ml with constant stirring and measure the pH after each addition of NaOH.

Again here we will be using that same ratio thing as we did in the Potentiometric estimation, the related reaction is

HA + NaOH ————– >    Na+A  +  H2O

And in simultaneous there is another reaction of dissociation of acid,

HA < ——– > H+ + A     (move in reverse on adding NaOH)

So as we consume more and more HA (by adding NaOH) the H+ on concentration decreases.

An so the pH increases, But the amount of change in the pH, increases when it comes near to equivalence point and then again start to decrease so we can calculate the Equivalence volume of the titration.

Once we do that, we will plot the graph of pH vs. Volume of NaOH and from that we can get, whatever is the pH reading at half the equivalence point is the pKa value of given acid.

Experimental points to remember –

Now there are some points we need to understand from the process we observed happening in beaker.

1. The Equivalence point can be found near the inflection point in the second graph, if you can remember that the slope of the graph is actually like the double derivative of pH with respect volume of NaOH, this information could be used for rough estimation and will be much clearer once calculus is taught.
2. To get more accurate result we need to take smaller amount of NaOH, at least near the half equivalence point.
3. The glass electrode must not be touched or brought out of the beaker, reason being the surface of glass electrode is what is calculating the E value so, disturbing it will affect the value, so straw should be used to mix the added base.

Abhishek Kumar jha

(Chemistry  at Utkarshini)