an electronic pH meter is made out of 3 main components; the pH probe, the voltmeter and the circuit.

Step 1: The PH Probe

The pH probe measures pH as the activity of hydrogen cations surrounding a thin-walled glass bulb at its tip. we will be making this in two parts.

glass pH probe
materials:
1.a small, thin glass bulb.(such as a Christmas ornament bulb, the smaller the better)
2. some epoxy glue
3. some silver wire
4. some bleach
5.some plastic tubing(plastic straw works well)
6. Potassium chloride (can be found as low sodium salt and in vitamin capsules)
7. Water
8.some vinegar

how to make it:
1. if using a christmas decoration you have to make sure that all the paint is scraped off and it is completely clean by boiling it in bleach for a couple hours. 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. It must then be conditioned. do 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. then to make the rest of the probe use a piece of plastic tubing, glass tubing or plastic straw (should be around the size of the opening of tthe glass bulb). Cut the tubing to the desired length and glue it to the open end of the glass bulb.
3. To make the silver chloride wire, simply dunk some silver wire into bleach for a few hours(even better over night). 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. Then use a rubber stopper to cap of the tube so that the liquid is kept in.

reference electrode
materials:
1. A piece of plastic tubing or straw
2. Some silver wire
3. Potassium chloride
4. Water
5. Agar Agar (gelatinous substance, obtained from algae can be found in supermarket)

how to make it:
1. the reference probe is also in contact with the medium of interest via a porous membrane(agar agar) 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.
2. the membrane is made 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.

Step 2: Voltmeter

simply use an elecronic voltmeter.

materials:
1.copper coil(very thin wire, can be found in many electrical devices)
2.wood
3.glue gun
4.flat pieces of plastic
5.small magnet
6.a piece of straw
7.sewing needle
8.2 small pieces of metal

how to make it:
1.if your copper coil is wrapped around a non-conductive material, great. if not un-wind the wire and wrap it around a non-conductive material.
2.to make the frame take a piece of wood about 6"x6". and glue 2 other pieces(about 1/2" thick) about 1/2 and inch apart standing up, this will support the bearings for the voltmeter.
3.Cut small narrow grooves in the pieces of plastic to serve as the bearings for the voltmeter and glue them to the bearing supports.
4.Glue a fourth piece of wood to the side of the bearing supports. Glue the coil on top of this piece so that the coil terminals are facing away from the bearing supports.
5.Attach a very small magnet to the bottom of the straw and push the sewing needle all the way through the straw near the end that has the magnet. Place the sewing needle in the bearing grooves that you made in Step 2 so that the coil will repel the magnet when a current goes through it
6.Glue the two pieces of metal at the ends of the needle to stabilize it and keep in the grooves. Now tilt the base of the platform so that the magnet comes to rest very close to the coil. When a current is applied to the coil, it will produce an electromagnetic current in the coil that will repel the magnet and make the top end of the straw swing to the side. When the current flow stops, gravity will pull the straw back to its zero point.

time to calibrate it!
to calibrate your voltmeter you will need to get a supply that you are sure gives a correct output of voltage. you will then connect your voltmeter to this supply and make a scale by marking where the straw is at each voltage. you can then fill in the rest by dividing the space equally.

Step 3: The Circuit

materials:
1.the probe you just made
2.the voltmeter you just made
3.operational amplifier(very cheap, $2)
4.wire
5.soldering iron
6.solder
7.2 9 volt batteries

how to do it:
1.look at the image above of the circuit and copy
2. connect the probe to the op amp(5) and the reference electrode to the negative end of one battery
3.connect the 2 batteries...1 battery's neg terminal to op amp(4), the pos of that bat to the neg of the other and the pos of the other to op amp(8).
4.connect op amp(6) to op amp(7).
5. connect voltmeter so that it measures the potential difference between op amp(6 ) and op amp(7)

and you are done! thanks for taking you're time to read my instructable!
please vote if you liked it!