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 following some of the links provided below or by browsing the Internet.
 Building a pH probe
A pH probe is device designed to measure the pH of a medium. The entire pH assembly consists of 2 parts. These are namely the sensor or probe part, and the meter. A nice description on how to build your own meter is provided in another part of this wiki.
You could probably run out and buy a pH probe if you wanted. The costs varies quite a bit depending on what you are looking for, but where would the fun be in that? Here we show you how, with a few simple household items, you could build you own pH probe at home using readily available materials.
 Glass electrode
The key to pH measurement is the use of some kind of barrier that allows H+ ions to flow across it. It turns out really thin glass does this very well. recently, there have been advances using fancy membrane materials to make pH probes as well, but we'll stick to the glass pH probe here.
I came 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 cell. 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.
To make the reference electrode, you will need:
1. A piece of plastic tubing or straw
2. Some silver wire (see above)
3. Potassium chloride (see above)
5. Agar Agar - from the supermarket
 How to build it
1. To make the glass electrode, you first need to remove all the coating and junk that they use to decorate it. This is accomplished boiling it in bleach for a couple of hours. You'll notice the the covering peel off. Let it cool, and carefully take it out and scrape off the paint. If it is still difficult to remove, dunk it back in the bleach in and boil it some more. Eventually it will all come off. You should have a clear glass bulb. It must then be conditioned. I did this by soaking it in a dilute vinegar solution for a couple of hours, followed by dipping in dilute bleach solution for another couple of hours, and then rinsing it in water. When not in use, the glass bulb must be kept wet at all times. Just leave it in jar of water.
2. Now to make the rest of the probe I used a plastic pipette, but a piece of plastic tubing, glass tubing or plastic straw of the right size should work fine. Cut the tubing to the desired length and glue it to the open end of the glass bulb. This is basically the probe housing.
3. To make the silver chloride wire, simply dunk some silver wire into bleach for a few hours. I did this overnight. You'll notice the silver wire turn brownish. This is the coating of silver chloride (AgCl) that develops on the surface of the wire.
4. Insert the silver chloride wire into the probe, and fill it with saturated potassium chloride solution (KCl). To make a saturated solution, keep dissolving potassium chloride in water until no more disolves and you have some KCl crystals hanging around.
5. Thats pretty much it! you have a pH probe. I capped mine off with a rubber stopper to keep liquid in. And you should have something that looks like this:
The reference electrode. Making the reference electrode was much harder. Not because it was hard to make, but because there is very little written about it. A couple of nice posts on the web were helpful in helping me hack one together.
1. The big trick with the reference probe is the fact that it is also in contact with the medium of interest via a porous membrane of some sort that is not glass. Glass is used to measure pH. the reference electrode just provides a steady voltage for the measurement. Most commercial electrodes available use vycor tubing. This stuff is a little expensive and not the easiest thing to come by.
2. A cleaver alternative was to make a membrane out of agar agar and some potassium chloride.
To prepare this, dissolve up some agar agar in KCl solution. Cut a piece of plastic tubing, seal off one end and dip in the hot solution. Let cool and set. Then dip a small piece of cotton in silver chloride or salt solution, and insert it into the tube. Top off the rest of the tube with agar solution.3. Similarly to the pH probe, insert a piece of silver chloride wire into the the length of the tube but above the cotton piece. Let it sit for a while and the agar should solidify. You then have a functional reference electrode. It should look something like the complete probe shown in the image on the right.
 Things to keep in mind
1. The agar in the reference elctrode may not withstand harsh pH environments (very acid or very basic).
2. The agar melts at ~80C
3. Be careful with the type of tubing used to make these probes. Some plastics such as PVC may not survive harsh pH conditions.
 Interfacing and measuring
All you need now is to connect the two silver wires to a pH meter and you are good to go.
Do not be alarmed if at first you get really weird results. Depending on the materials you used, your cell may have slightly different characteristic from a standard pH probe. Not to worry. In order to take pH measurements, you need to calibrate your probe anyway.
 pH amplifier circuit
 List of components
 Calibrating a home-built glass electrode=
Calibration is accomplished by dipping your probe, in this case, both the glass probe and the reference probe in solution of known pH. Typically you would use what is called a buffer. A buffer is a special solution of a given pH that does not change easily. This allows for steady and reliable measurements. At home, you can improvise. For example, we know lemon juice has a pH of ~2, distilled water a pH of ~7, and bleach a pH of 13. You can use these as guides to calibrate your probe and you are good to go.
For instructions on building your own pH meter, check out the rest of the wiki!
3. Paper: Per-Odd Eggen and Lise Kvittingen, "Small-Scale and Low-Cost Electrodes for “Standard” Reduction Potential Measurements"
4. Paper: Rogerio T. da Rocha, lvano G.R. Gutz, and Claudimir L., "From Christmas Ornament to Glass electrode", Journal of Chemical Education, Volume 72 Number 12 December 1995