The history of measuring the acidity of liquids electrically began in 1906 when Max Cremer in his studies of liquid interfaces [1] (interactions between liquids and solids) discovered that the interface between liquids could be studied by blowing a thin bubble of glass and placing one liquid inside it and another outside. It created an electric potential that could be measured. This idea was taken further by Fritz Haber (who invented the synthesis of ammonia and artificial fertiliser) and Zygmunt Klemsiewicz [2] who discovered that the glass bulb (which he named glass electrode) could be used to measure hydrogen ion activity and that this followed a logarithmic function.
The Danish biochemist Soren Sorensen then invented the pH scale in 1909.
Because the resistance in the wall of the glass is very high, typically between 10 and 100 Mega-Ohm, the glass electrode voltage could not be measured accurately until electron tubes were invented. Later still, the invention of field-effect transistors (FETs) and integrated circuits (ICs) with temperature compensation, made it possible to measure the glass electrode voltage accurately. The voltage produced by one pH unit (say from pH=7.00 to 8.00) is typically about 60 mV (milli Volt). Present pH meters contain microprocessors that make the necessary corrections for temperature and calibration. Even so, modern pH meters still suffer from drift (slow changes), which makes it necessary to calibrate them frequently.
Improvements have also been made in the chemistry of the glass such that pollution by salt and halogen ions could be halted. The reference electrode, which traditionally used silver chloride (AgCl) has been superseded by the kalomel (mercurous chloride, HgCl2) electrode which uses mercuric chloride (HgCl) in a potassium chloride (KCl) solution as a gel (like gelatine). But electrodes do not have eternal life and need to be replaced when they drift unacceptably or take unusually long to settle.
[1] Cremer M (1906): Z. Biol, 47, 562
[2] Haber F and Z Klemensiewicz (1909): Z. Physik. Chem., 67, 385
 The
pH meter measures the electrical potential (follow the drawing clock-wise
from the meter) between the mercuric chloride of the reference electrode
and its potassium chloride liquid, the unknown liquid, the solution inside
the glass electrode, and the potential between that solution and the silver
electrode. But only the potential between the unknown liquid and the solution
inside the glass electrode change from sample to sample. So all other potentials
can be calibrated out of the equation. |
The glass electrode consists of a sturdy glass tube with a thin glass bulb welded to it. Inside is a known solution of potassium chloride (KCl) buffered at a pH of 7.0. A silver electrode with a silver chloride tip makes contact with the inside solution. To minimise 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.
| Water is THE most important and miraculous substance
on Earth. Its molecules H-O-H form a boomerang shape with the O- end slightly
negative and the H2+ end slightly positively charged. These charged boomerangs
are attracted to one another, forming islands of cohesion, such that water
forms a liquid at temperatures where life thrives, whereas it should really
have been a very volatile gas like hydrogen sulphide (H2S) which has almost
twice its molecular weight. At the surface of Earth, water occurs in solid
form (ice), liquid (water) and gaseous form (steam or water vapour). In
cold areas all three phases co-exist.
Water is also unique in that it is both an acid (with H+ ions) and a lye (with OH- ions). It is thus both acidic and basic (alkaline) at the same time, causing it to be strictly neutral as the number of H+ ions equals that of the OH- ions. Because of its strong cohesion, only few water molecules dissociate (split) in their constituent ions: hydrogen ions (H+) and hydroxyl ions (OH-). Chemists would insist that H+ ions are really H3O+ ions or hydronium ions. Knowing that one molar of water weighs 18 gram (1+1+16), which equals 18ml, and that this quantity contains a very large number of molecules [1], only 0.1 millionth (10-7) mol are dissociated in one litre of water (pH=7). [2] The potential difference between the inside of the glass electrode and the outside is caused by the oxides of silicon in side the glass: Si.O- + H3.O+ = Si.O.H+ + H2.O Once the ionic equilibrium is established, the potential difference between the glass wall and the solution is given by the equation: E = R x T / ( F x ln( a )) Where E= electron potential (Volt), R= molar gas constant 8.314 J/mol/ºK, F= Faraday constant 96485.3 ºC, T= temperature in ºKelvin and a= the activity of the hydrogen ions (hydronium ions). ln( a )= the natural logarithm which converts to the decimal logarithm = 2.303 x log( a ) The combination R x T / ( 2.303 x F ) is approximately 0.060 V (60 mV) per tenfold increase in hydrogen ions or one pH unit. The pH range of 0 to 14 accounts for hydronium activities from 10 to 1E-14 mol/litre. One mol of water weighs 18 gram. A pH=7 corresponds to hydronium activity of 1E-7 mol/litre (1E-7). Because log( 10-7 ) = -7, the pH scale leaves the minus sign out. Even though modern pH glass electrodes have seen major improvements, they still don't like some substances low in H+ ions, like alkali hydroxides (NaOH and KOH), pure distilled water, etching substances like fluoride, adsorbing substances like heavy metals and proteins. Most modern pH meters have inbuilt temperature sensors to correct temperature deviation automatically to give values as if these were taken at a standard temperature of 25ºC. The readout is not influenced by temperature at pH=7.00 but outside this by 0.003 per ºC. Thus a pH taken at 5ºC (20º away from 25ºC), showing 4.00 must be corrected downward by 0.003 x 20 x 3.00 = 0.18. Likewise a pH value of 10.00 must be corrected upward by this amount. |
Note that a pH probe kept moist in an acidic solution, can influence results when not rinsed before inserting it into the test vial. Remember that a liquid of pH=4 has 10,000 more hydrogen ions than a liquid of pH=8. Thus a single drop of pH=4 in a vial measuring 400 drops of pH=8 really upsets measurements! Remember also that the calibration solutions consist of chemical buffers that 'try' to keep pH levels constant, so contamination of your test vial with a buffer is really serious.
[1] Avogadro's constant is 602,213,670,000,000,000,000,000 (602.214 billion trillion) or 6.02E23, named in honour of Amedeo Avogadro. One mole of a chemical substance contains this number of molecules. Amedeo Avogadro (1776-1856) was an Italian physicist. He proposed in 1811 his famous hypothesis, now known as Avogadro's law. The law stated that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Avogadro also distinguished between an atom and a molecule, and made it possible to determine a correct table of atomic weights.
[2] On the Seafriends web site we frequently use the exponential notation E, such that 2.34E-4 means 2.34 x 10-4.
| 0 | 5% Sulphuric acid, H2SO4, battery acid. |
| 1 | 0.1 N HCl, hydrochloric acid (1.1) |
| 2 | Lemon juice. Vinegar (2.4-3.4) |
| 3 | wine (3.5-3.7) |
| 4 | Orange juice. Apple juice (3.8). Beer. Tomatoes. |
| 5 | Cottage cheese. Black coffee. Rain water 5.6. |
| 6 | Milk. Fish (6.7-7). chicken (6.4-6.6). |
| 7 | Neutral: equal numbers of hydrogen and hydroxyl ions. Blood (7.1-7.4). Distilled water without CO2, after boiling. |
| 8 | Sea water (8.1). Egg white. |
| 9 | Borax. baking soda. |
| 10 | Milk of magnesia |
| 11 | Household ammonia |
| 12 | Photographic developer |
| 13 | Oven cleaner |
| 14 | Sodium lye NaOH, 1 mol/litre. |
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