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The pH meter
A pH meter is an electronic device used to measure the pH of a liquid.
A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body.
“The calomel reference electrode consists of a glass tube with a potassium chloride (KCl) electrolyte which is in intimate contact with a mercuric chloride (HgCl) element at the end of a KCL element. It is a fragile construction, joined by a liquid junction tip made of porous ceramic or similar material. This kind of electrode is not easily 'poisoned' by heavy metals and sodium.
The glass electrode consists of a sturdy glass tube with a thin glass bulb welded to it. Inside is a known solution of silver chloride (AgCl) buffered at a pH of 7.0. A silver electrode with a silver chloride tip makes contact with the inside solution. To minimize electronic interference, the probe is shielded by a foil shield, often found inside the glass electrode.
Most modern pH meters also have a thermistor temperature probe which allows for automatic temperature correction, since pH varies somewhat with temperature.” 
“The meter circuit is no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance, approximately 20 to 1000 MΩ, of the glass electrode probes typically used with pH meters. The inverting amplifier converts the small voltage produced by the probe (+0.0591 volt/pH) into pH units” 
How it works
A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode and an unknown liquid outside.
“Potential measured by pH meter is a sum of all potentials present in the system. Putting aside junction potentials that can be present in the experimental setup, we are left with three sources of electromotive force. First builds up on the glass electrode, thanks to different activities of the H+ ions on both sides of the glass. Second source is the glass electrode silver wire covered with AgCl and immersed in the solution of chlorides, and third is the reference electrode - silver chloride or calomel, depending on the application.” 
Thus the real potential measured is sum of three potentials:
From Nernst Equation:
Where, E''0contains all constants mentioned above and in the Nernst equation section.
BEFORE MEASURING THE pH OF ANY SUBSTANCE, READ THE CLEANING AND CALIBRATION SECTIONS OF THIS ENTRY!
How to calibrate it
Before measuring the pH of any substance, the pH meter needs to be calibrated. To do so, we use 3 different standard solutions with pH 4.00, 7.00 and 10.00 respectively. For calibration, we pour some of these standard solutions into 3 different 10-mL tubes.
First, rinse the probe with copious amounts of distilled water. Dry it following the procedures of the cleaning section below. Then, press the calibrate button and insert the probe into the first tube. Once it is ready, adjust the reading to the actual pH (4.00, 7.00 or 10.00).
Repeat the same process for the other two standard solutions and you will be set to go.
How to clean it
The cleaning of the probe is very simple, but it needs to be correctly done.
First and foremost, make sure that the residues- tap-o-ware is right below the probe.
Then, rinse the probe with copious amounts of distilled water. To dry the probe, use optical wipes. Just make the wipe and the surface of the probe to be in contact with one another. Let it sit like that for a little bit and the water will get absorbed by the paper. Repeat the process as many times as necessary.
NEVER rub the wipe against the glass, since this is really fragile.