Introduction to pH
A pH measurement of a medium is a measure of how acidic or how basic it is. Lemon juice or vinegar for example are acidic, whereas bleach or sodium bicarbonate are basic (the opposite of acidic). pH can be imagined as a measure of the presence of hydrogen ions or H+ in the medium or the ability of the medium to produce H+ ions. An acid produces H+ ions whereas a base absorbs H+ ions. In more technical terms pH is defined as a negative decimal logarithm of the hydrogen ion activity in a solution. Using this definition, a pH value of 7 corresponds to a neutral solution, neither acid nor basic. A pH value < 7 corresponds to an acidic medium, and a pH value > 7 corresponds to a basic solution. Anyways, the reason you care about all this is that biological systems are very sensitive the pH of their environment. Whether or not fish survive in a lake or whether your fermentation goes well for your latest beer batch depends on the pH. So it it is kinda important. You can read a lot more about pH by googling around.
Building a pH probe
I can across a paper where they described how to make a home made pH probe using a Christmas glass ornament and some simple materials. This is a great way to make a pH probe on the cheap at home.
What you need
So what you'll need to make this thing are:
1. a Christmas glass bulb, the smaller, the better 2. Some household bleach 3. some plastic tubing, plastic straw or similar. 4. some epoxy glue or even better, aquarium glue 5. Some silver wire. I got some from my neighborhood hobby store. 6. Potassium chloride, this is available as low sodium salt or as vitamin capsules. Very easy to find.
In addition to the glass pH probe, you'll need a reference probe. The pH probe by itself is only a half cell. In order to complete the circuit, you'll need the other half of the cel. The trick with the reference electrode is that it must provide a steady voltage, so that the pH can be measured. Otherwise your measurements will jump all over the place and you will never get a result.
How to build it