Thursday, 27 October 2016

Calibrating a pH meter using buffers

  Determination of Shelf Life of Solutions in Laboratory

Learn how to determine the shelf life of volumetric, reagents and buffer laboratory solutions.


Incorrect strength or concentration of volumetric solutions, reagents and buffer solutions can alter the results of analyzed products. Solutions prepared for chemical analysis are not stable for a longer period.
The molarities of these solutions may change after a period of time. Shelf life of the solution depends upon the nature of the compound and the solvent also. Therefore, it is necessary to validate the stability period of these solutions individually in which these have to consume. Validation of shelf life is a mandatory GLP and regulatory requirement. There are different ways to assign the shelf life of the solutions; following are the simplest methods for the same.

volumetric Solutions:
Prepare the volumetric solution and allow standing. After 24 hours determine the molarity of solution in triplicate calculating the mean of the results and continue the determination at an interval of 3 days for 15 days. Determine the RSD of all 5 values. If RSD of all 5 values remains less than 1.0, shelf life of solution should be assigned 15 days otherwise RSD shall be calculated with first 4 values and shelf life should be assigned 12 days. It the RSD of first 4 values remains more than 1.0, RSD shell be calculated with first three values and shelf life of the solution should be assigned accordingly.


Reagent Solutions:
Reagents solutions, those are used in analysis as reactants and molarity is not calculated. These are validated on their performance. The Performance of these solutions is checked at an interval of 7 day for 1 month. Evaluate the performance of all 4 tests and assign the life accordingly.

Buffer Solutions:
Buffers are generally used for calibration of pH meters. pH of buffer solutions may change due to chemical degradation. The shelf life of buffer solutions must not be assigned more than 7 days.

Note: Solution should be stored under proper storage conditions otherwise results may alter giving improper shelf life. Validation data should be recorded and maintained for regulatory audits.

Calibrating a pH meter using buffers

A pH meter requires proper calibration in order to give accurate pH readings. The meter must accuratetely translate voltage measurements into pH measurements.

How a pH meter is calibrated to give accurate pH readings?

A pH meter is calibrated by immersing its electrode(s) into buffers (test solutions of known pH) and by adjusting the meter accordingly. Since pH measurements are affected by temperature, the temperature must remain constant during the time of the calibration. Modern pH meters have build-in thermometers and automatically correct their own pH measurements as the temperature changes.



What are the main steps for a generic  pH meter calibration?
The following procedures are used to calibrate pH meters (analytical chemistry):
·         A 2 or 3-point calibration : Two or three buffers solutions are used respectively. They are usually sufficient for initial calibration. After this initial calibration the meter can accurately measure pH values in between.
·         A 1-point calibration: A buffer with a pH close to the expected sample pH is used. The pH measurements are not as accurate as with the 2 or 3 point calibration.
Procedure for an 1-point calibration:
  1. Place the pH buffer solution (normally pH = 4.01 solution) into a small beaker. Place a magnetic stirrer and a temperature probe into the buffer solution in the beaker.
  2. Measure the temperature of the buffer solution if the pH meter does not have its own temperature probe. Adjust the temperature control of the pH meter according to the buffer’s measured temperature (remember pH measurements are temperature dependent).
  3. Remove the electrode protective cap(s). Rinse the electrodes and the temperature probe with distilled water using a squeeze bottle.
  4.  Dab dry the bottom of the glass bulb with a tissue paper. Do not wipe the glass bulb (scratches and static charges affect the electrode’s response).
  5. Place the electrode(s) and temperature probe into the well stirred pH buffer solution. The porous frit must be covered with the buffer solution.
  6. Adjust the slope/sensitivity control to read the true pH of the buffer solution (modern pH meters do this automatically).

Procedure for an 2-point calibration:
  1. Place the pH buffer solution (normally pH = 7.0 solution) into a small beaker. Place a magnetic stirrer and a temperature probe into the buffer solution in the beaker.
  2. Follow steps 2-6 of the 1-point calibration.
  3. Follow steps 1-6 of the 1-point calibration using the pH = 4.01 buffer solution
  4. Keep repeating steps 2 and 3 until practically no adjustments are required
Procedure for a 3-point calibration:
  1. Follow steps 1-6 of the 2-point calibration procedure
  2. Use a third pH buffer, whose pH value is as close as possible to the suspected pH of the sample, and follow the 1-point calibration.
The third point is used to confirm that the pH meter has been calibrated correctly.

 

Calibration


 
. In calibration, the zero point and sensitivity (span point) of the pH meter are accurately adjusted using a pH standard solution. Nowadays, most of the operations explained here are performed automatically. Since some operations are unique to the pH meter, they must be performed in accordance with the instructions given in the operation manual provided.
  1. Attach the necessary sensor (electrodes etc.) to the pH meter main unit, then turn the main unit on.
  2. Prior to measurement, wash the sensor with pure water three times or more, and wipe it with clean, soft paper (such as tissue paper).
  3. Put some neutral phosphate standard solution (pH 6.86 at 25°C) into a beaker, and insert the detecting element into the solution.
    Adjust the temperature compensation dial of the pH meter main unit to the temperature of the neutral phosphate standard solution. In many of the latest pH meters, this operation is completed automatically simply by pressing a button.
    Use the adjustment dial of the pH meter main unit to set its scale to the pH value corresponding to the temperature of the neutral phosphate standard solution (pH 6.86 at 25°C). In many of the latest pH meters, this operation is also completed automatically simply by pressing a button. Formerly, the adjustment dial was known as the zero-adjustment dial (asymmetric potential difference adjustment dial).
    ● This operation corresponds to the correction for region “a” (zero-point adjustment) shown in the attached schematic diagram, which outlines zero-span calibration of the pH meter.
  4. Take the detecting element out of the neutral phosphate standard solution, wash it with pure water three times or more, and wipe it with clean, soft paper.
  5. When the pH of the sample is expected to be acidic (pH 7 or lower), put some phthalate standard solution (pH 4.01 at 25°C) or oxalate standard solution (pH 1.68 at 25°C) into a beaker, and insert the detecting element into the solution.
    Use the span adjustment dial to set the scale of the main unit to the pH value corresponding to the temperature of the standard solution (phthalate standard solution: pH 4.01 at 25°C; oxalate standard solution: pH 1.68 at 25°C).
    When the pH of the sample is expected to be alkaline (pH 7 or higher), use borate standard solution (pH 9.18 at 25°C) or carbonate standard solution (pH 10.02 at 25°C) and perform the same operation to set the scale of the main unit to the pH value corresponding to the temperature of the standard solution.
    In many of the latest pH meters, these operations are also completed automatically by pressing a button.
    ● This operation corresponds to the correction for region “b” (span adjustment) shown in the attached  schematic diagram, which outlines zero-span calibration of the pH meter.
  6. Repeat operations 2) to 5) until the indicated pH value agrees to within ±pH 0.05* with the pH corresponding to the temperature of the pH standard solution.
    * These operations must be repeated until the indicated pH value agrees with the pH corresponding to the temperature of the pH standard solution to within ±pH 0.02 for type I pH meters and ±pH 0.1 for type III pH meters.

pH meters



pH meters
by Chris Woodford. Last updated: April 29, 2015.
If it turns pink, it's acid I think—you probably learned that useful phrase once upon a time, along with the second half of the same rhyme: "and if it turns blue, it's an alkali true." Measuring acids and alkalis (bases) with litmus paper is something pretty much everyone learns how to do in school. It's relatively easy to compare your little strip of wet paper with the colors on a chart and figure out how acidic or alkaline something is on what's called the pH scale. But sometimes that's too crude a measurement. If you keep tropical fish, for example, or you're a gardener with specimens that like soil of a certain acidity or alkalinity, getting things wrong with the litmus risks killing off your prized pets or your plants. That's why many people invest in a meter that can measure pH directly. What are pH meters and how do they work? Let's take a closer look!
What is acidity?
If you're interested in measuring acidity, it helps if you know what it is before you start! Most of us have only the faintest idea what an acid or an alkali really is. We know it's a substance that can "burn" our skin (though it's a chemical burn, not a heat burn), but that's about it. What's even more confusing is that we can safely eat some acidic things (lemons, for example, contain citric acid) but not others (drinking a chemical like sulfuric acid would be extremely dangerous).
Photo: Some acids, such as lemon juice, are perfectly safe to handle; others will burn your skin and can do painful, permanent damage.
Acids and alkalis are simply chemicals that dissolve in water to form ions (atoms with too many or too few electrons). An acid dissolves in water to form positively charged hydrogen ions (H+), with a strong acid forming more hydrogen ions than a weak one. An alkali (or base) dissolves in water to form negatively charged hydroxide ions (OH−). Again, stronger alkalis (which can burn you as much as strong acids) form more of those ions than weaker ones.
What does pH actually mean?
The pH (always written little p, big H) of a substance is an indication of how many hydrogen ions it forms in a certain volume of water. There's no absolute agreement on what "pH" actually stands for, but most people define it as something like "power of hydrogen" or "potential of hydrogen." Now this is where it gets confusing for those of you who don't like math. The proper definition of pH is that it's minus the logarithm of the hydrogen ion activity in a solution (or, if you prefer, the logarithm of the reciprocal of the hydrogen ion activity in a solution). Gulp. What does that mean?
It's simpler than it sounds. Let's unpick it a bit at a time. Suppose you have some liquid sloshing about in your aquarium and you want to know if it's safe for those angelfish you want to keep. You get your pH meter and stick it into the "water" (which in reality is a mixture of water with other things dissolved in it). If the water is very acidic, there will be lots of active hydrogen ions and hardly any hydroxide ions. If the water is very alkaline, the opposite will be true. Now if you have a thimble-full of the water and it has a pH of 1 (it's unbelievably, instantly, fish-killingly acidic), there will be one million times (10 to the power of 6, written 106) more hydrogen ions than there would be if the water were neutral (neither acidic nor alkaline), with a pH of 7. That's because a pH of 1 means 101 (which is just 10), and a pH of 7 means 107 (10 million), so dividing the two gives us 106 (one million). There will be 10 million million (1013) more hydrogen ions than if the water were extremely alkaline, with a pH of 14. Maybe you can start to see now where those mysterious pH numbers come from?
Photo: The pH scale relates directly to the concentration of hydrogen ions in a solution, but not in a simple linear way. The relationship is what we call a "negative exponential": the higher the pH (lower the acidity), the fewer the hydrogen ions—but there are vastly fewer ions at high pH than at low pH.
Suppose we decide to invent a scale of acidity and start it off at very acidic and call that 1. Then something neutral will have far fewer (one millionth or 10−6 times as many hydrogen ions) and something alkaline will have fewer still (that's one 10 trillionth, or one 10 million millionth, or 10−13 times as many). Dealing with all these millions and billions and trillions is confusing and daft so we just take a logarithm of the number of hydrogen ions and refer to the power of ten we get in each case. In other words, the pH means simply looking at the (probably gigantic) number of hydrogen ions, taking the power of 10, and removing the minus sign. That gives us a pH of 1 for extremely acidic, pH 7 for neutral, and pH 14 for extremely alkaline. "Extremely alkaline" is another way of saying incredibly weakly acidic.
How does a pH meter work?
If you're using litmus paper, none of this matters. The basic idea is that the paper turns a slightly different color in solutions between pH 1 and 14 and, by comparing your paper to a color chart, you can simply read off the acidity or alkalinity without worrying how many hydrogen ions there are. But a pH meter somehow has to measure the concentration of hydrogen ions. How does it do it?
An acidic solution has far more positively charged hydrogen ions in it than an alkaline one, so it has greater potential to produce an electric current in a certain situation—in other words, it's a bit like a battery that can produce a greater voltage. A pH meter takes advantage of this and works like a voltmeter: it measures the voltage (electrical potential) produced by the solution whose acidity we're interested in, compares it with the voltage of a known solution, and uses the difference in voltage (the "potential difference") between them to deduce the difference in pH.
What's it made of?
A typical pH meter has two basic components: the meter itself, which can be a moving-coil meter (one with a pointer that moves against a scale) or a digital meter (one with a numeric display), and either one or two probes that you insert into the solution you're testing. To make electricity flow through something, you have to create a complete electrical circuit; so, to make electricity flow through the test solution, you have to put two electrodes (electrical terminals) into it. If your pH meter has two probes (like the one in the photo at the top of this article), each one is a separate electrode; if you have only one probe, both of the two electrodes are built inside it for simplicity and convenience.
The electrodes aren't like normal electrodes (simple pieces of metal wire); each one is a mini chemical set in its own right. The electrode that does the most important job, which is called the glass electrode, has a silver-based electrical wire suspended in a solution of potassium chloride, contained inside a thin bulb (or membrane) made from a special glass containing metal salts (typically compounds of sodium and calcium). The other electrode is called the reference electrode and has a potassium chloride wire suspended in a solution of potassium chloride.
Artwork: Key parts of a pH meter: (1) Solution being tested; (2) Glass electrode, consisting of (3) a thin layer of silica glass containing metal salts, inside which there is a potassium chloride solution (4) and an internal electrode (5) made from silver/silver chloride. (6) Hydrogen ions formed in the test solution interact with the outer surface of the glass. (7) Hydrogen ions formed in the potassium chloride solution interact with the inside surface of the glass. (8) The meter measures the difference in voltage between the two sides of the glass and converts this "potential difference" into a pH reading. (9) Reference electrode acts as a baseline or reference for the measurement—or you can think of it as simply completing the circuit.
How does it work?
The potassium chloride inside the glass electrode (shown here colored orange) is a neutral solution with a pH of 7, so it contains a certain amount of hydrogen ions (H+). Suppose the unknown solution you're testing (blue) is much more acidic, so it contains a lot more hydrogen ions. What the glass electrode does is to measure the difference in pH between the orange solution and the blue solution by measuring the difference in the voltages their hydrogen ions produce. Since we know the pH of the orange solution (7), we can figure out the pH of the blue solution.
How does it all work? When you dip the two electrodes into the blue test solution, some of the hydrogen ions move toward the outer surface of the glass electrode and replace some of the metal ions inside it, while some of the metal ions move from the glass electrode into the blue solution. This ion-swapping process is called ion exchange, and it's the key to how a glass electrode works. Ion-swapping also takes place on the inside surface of the glass electrode from the orange solution. The two solutions on either side of the glass have different acidity, so a different amount of ion-swapping takes place on the two sides of the glass. This creates a different degree of hydrogen-ion activity on the two surfaces of the glass, which means a different amount of electrical charge builds up on them. This charge difference means a tiny voltage (sometimes called a potential difference, typically a few tens or hundreds of millivolts) appears between the two sides of the glass, which produces a difference in voltage between the silver electrode (5) and the reference electrode (8) that shows up as a measurement on the meter.
Animation (above): How ion exchange works.
Although the meter is measuring voltage, what the pointer on the scale (or digital display) actually shows us is a pH measurement. The bigger the difference in voltage between the orange (inside) and blue (outside) solutions, the bigger the difference in hydrogen ion activity between. If there is more hydrogen ion activity in the blue solution, it's more acidic than the orange solution and the meter shows this as a lower pH; in the same way, if there's less hydrogen ion activity in the blue solution, the meter shows this as a higher pH (more alkaline).
Making accurate pH measurements
For pH meters to be accurate, they have to be properly calibrated (the meter is accurately translating voltage measurements into pH measurements), so they usually need testing and adjusting before you start to use them. You calibrate a pH meter by dipping it into buffers (test solutions of known pH) and adjust the meter accordingly. Another important consideration is that pH measurements made this way depend on temperature. Some meters have built-in thermometers and automatically correct their own pH measurements as the temperature changes; those are best if fluctuations in temperature are likely to occur while you're making a number of different measurements. Alternatively, you can correct the pH measurement yourself, or allow for it by calibrating your instrument and making pH measurements at broadly the same temperature.
Who invented the pH meter?
Who do we have to thank for this clever stuff? First, Nobel-Prize winning German chemist Fritz Haber (1868–1934) and his student Zygmunt Klemensiewicz (1886–1963) developed the glass electrode idea in 1909. The modern, electronic pH meter was invented about a quarter century later, around 1934/5, when American chemist Arnold Beckman (1900–2004) figured out how to hook up a glass electrode to an amplifier and voltmeter to make a much more sensitive instrument.

Photo: How do you measure the pH of soils on Mars? Simple! You build a pH meter into a robotic space probe. The Mars Phoenix Lander space probe (left) used this built-in, mini chemical laboratory (right) to measure different aspects of the Martian soil, including acidity and metal concentrations. Photos by courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).

pH meter principles What is pH and how is it measured?

History
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


How a pH meter works
When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility. When a metal is brought in contact with a solution of salts or acids, a similar electric potential is caused, which has led to the invention of batteries. Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart. 
A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen. To complete the electrical circuit, also a reference electrode is needed. Note that the instrument does not measure a current but only an electrical voltage, yet a small leakage of ions from the reference electrode is needed, forming a conducting bridge to the glass electrode. A pH meter must thus not be used in moving liquids of low conductivity (thus measuring inside small containers is preferable).
 
schematic pH electrodesThe 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 calomel reference electrode consists of a glass tube with a potassium chloride (KCl) electrolyte which is in intimate contact with a mercuric chloride 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 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.
Caring for a pH meter depends on the types of electrode in use. Study the manufacturer's recommendations. When used frequently, it is better to keep the electrode moist, since moisturising a dry electrode takes a long time, accompanied by signal drift. However, modern pH meters do not mind their electrodes drying out provided they have been rinsed thoroughly in tap water or potassium chloride. When on expedition, measuring sea water, the pH meter can be left moist with sea water. However for prolonged periods, it is recommended to moist it with a solution of potassium chloride at pH=4 or in the pH=4.01 acidic calibration buffer. pH meters do not like to be left in distilled water.
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.

Measuring the pH of a Solution with a pH meter

The pH of a solution can be measured quickly and accurately with a pH meter (see Figure 1).


Figure 1: A digital pH meter


How does a pH meter work?

A pH meter has to somehow measure the concentration of the hydrogen ions [H+] in a solution. An acidic solution has far more positively charged hydrogen ions in it than an alkaline solution, so it has greater potential to produce an electric current under certain conditions - in other words, it is like a battery that can produce a greater voltage. A pH meter takes advantage of this and works like a typical voltameter: in brief, a pH meter consists of a pair of electrodes connected to a meter capable of measuring small voltages, on the order of millivolts. It measures the voltage (electrical potential) produced by the solution whose acidity we are interested in, compares it with the voltage of a known standard solution, and uses the difference in voltage (the potential difference) between them to calculate the difference in pH.
What are the parts of a pH meter?
A typical pH meter consists of two parts: i) one special measuring probe (a glass electrode) or two measuring probes that are inserted into the solution whose pH is required  and ii) an electronic meter that measures and displays the pH reading.
A glass electrode is in a sense two electrodes combined in one. It consists of a long glass tube with a thin walled glass bulb at the end. Special glass of high electrical conductance and low melting point is used for the purpose. This glass can specifically sense hydrogen ions  Hup to a pH ≈ 9 (with special glass electrodes pH ranges from 1-13 can be measured). The bulb contains 0.1 M HCl and a Ag/AgCl electrode (used as an internal reference electrode) is immersed into the solution and connected by a platinum wire for electrical conduct.


Figure 2: A glass electrode
The main advantages of the glass electrode are:
  • It can be used in the presence of strong oxidizing or reducing substances and metal ions
  • Accurate results are obtained in the range pH 1-9. However, by using special glass electrodes pH 1-13 can be measured
  • It is simple to operated. It can be attached to portable instruments and is used quite often in chemical, biological, industrial and agricultural laboratories
The main limitations of the glass electrode are:
  • It does not function properly in some organic solvents (i.e. ethanol)
  • It does not function properly above pH > 9 since it is sensitive to Na+ ions so a correction has to be made

In case that the pH meter has two probes (two electrodes): i) one of them is a glass electrode (has silver wire suspended in a solution of KCl that is contained in a special glass bulb coated with silica and metal salts) and ii)  the other is the reference electrode and has a KCl wire suspended in a solution of KCl (see Figure 3).

Figure 3: A scheme of a pH meter with two probes (electrodes). Where:  1 = electronic meter that displays pH values - converts voltage to pH,   2 = Glass electrode (silica glass and KOH solution),  3 = Silver electrode,  4 = Solution being tested,  5 = H+ ions,  6 = Reference electrode
When the probe(s) are immersed into the solution some of the H+ ions in the solution move toward the glass electrode (Figure 3, labeled as 2) and replace some of the metal ions

The pH scale
The values for pH make more sense when compared with that of known substances. Note that the pH scale is logarithmic and that each next value contains ten times less hydrogen ions. A pH=0 contains the most, and is highly acidic.
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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