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
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:
- 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.
- 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).
- Remove the electrode protective cap(s). Rinse the electrodes and the temperature probe with distilled water using a squeeze bottle.
- 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).
- Place the electrode(s) and temperature probe into the well stirred pH buffer solution. The porous frit must be covered with the buffer solution.
- 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:
- 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.
- Follow steps 2-6 of the 1-point calibration.
- Follow steps 1-6 of the 1-point calibration using the pH = 4.01 buffer solution
- Keep repeating steps 2 and 3 until practically no adjustments are required
Procedure for a 3-point calibration:
- Follow steps 1-6 of the 2-point calibration procedure
- 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.
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.
- Attach the necessary sensor (electrodes etc.) to the pH meter main unit, then turn the main unit on.
- Prior to measurement, wash the sensor with pure water three times or more, and wipe it with clean, soft paper (such as tissue paper).
- 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. - 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.
- 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. - 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.
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?
HistoryThe 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.
Measuring the pH of a Solution with a pH meter
 |
| 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 H+ up 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
- 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 |
| 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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