# Homework Help: Measuring the pH of an acid

1. Nov 8, 2005

### Just some guy

Hi,

I've got this question where I have to describe two methods of measuring the pH of a substance. I've already got the one using electrochemical potentials, but I'm a bit stumped on the second method.

What I've said is to carry out a titration with an alkali until the solution is neutralised. One can then work out the concentration, and if the acid fully dissociates, it's pH. However this is a bit of a fudge as it doesn't work for anything but strong acids. Is there a better way to do this?

Also, I have a second question where I have to devise an experiment to measure the eqilibrium constant Ka of a weak acid. I've written that a number of solutions of the acid could be made, each with a different concentration (increasing linearly from, say, 0.01moldm^-3 to 1moldm^-3), and then dumping some methyl orange in the substance and seeing at which concentration the colour just changes to orange. This substance should be at a pH of 4, so the concentration of H+ ions can be determined and if the concentration of the original substance is already known (this could be measured from a titration I guess) then Ka can be determined. But again this is a bit of a manky method because methyl orange switches at a range of pH values, not just 4. Does anybody have a better method?

Cheers,
Just some guy.

2. Nov 8, 2005

### GCT

it works for a weak acid/base and strong acid/base titration, you don't want the Ka to be too small though.

One the second issue,you have the right idea, you'll just need to find an appropriate indicator use titration to find the end point data as well as half end point data, at the half end point pH=pKa, you can also deduce the molar mass of the weak acid and work out some additional correlational data (you'll need to research this yourself) to support your conclusion.

3. Nov 9, 2005

### Just some guy

Thanks for the help

I have another question though, I have to measure the standard electrode potential of silver from a solution of silver nitrate. If I measure the standard electrode potential of the silver nitrate what do I do next?:yuck:

4. Nov 9, 2005

### GCT

If I'm understanding you correctly, from what I remember, you'll need to construct a galvanic cell with silver nitrate and a zero standard EMF component (I'll let you figure out what that is).