?pH Meter Resources.

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pH Meter Resources.

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).

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 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/°K, F= Faraday constant 96485.3 °C, T= temperature in °Kelvin 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 25°C. The readout is not influenced by temperature at pH=7.00 but outside this by 0.003 per °C. Thus a pH taken at 5°C (20° away from 25°C), 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, 0000 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.

Definition

The pH of a solution measures the degree of acidity or alkalinity relative to the ionization of water sample. Pure water dissociates to yield 10-7 M of [H+] and [OH-] at 25 oC; thus, the pH of water is neutral i.e. 7.

pHwater = - log [H+] = - log 10-7 = 7

Most pH readings range from 0 to 14. Solutions with a higher [H+] than water (pH less than 7) are acidic; solutions with a lower [H+] than water (pH greater than 7) are basic or alkaline.

pH Measurement

Measuring pH involves comparing the potential of solutions with unknown [H+] to a known reference potential. pH meters convert the voltage ratio between a reference half-cell and a sensing half-cell to pH values. In acidic or alkaline solutions, the voltage on the outer membrane surface changes proportionally to changes in [H+]. The pH meter detects the change in potential and determines [H+] of the unknown by the Nernst equation:

E = Eo + (2.3RT)/nF log {unknown [H+]/internal [H+]}

where: E = total potential difference (measured in mV); Eo = reference potential; R = gas constant; T = temperature in Kelvin; n = number of electrons; F = Faraday's constant; [H+] = hydrogen ion concentration.

pH Temperature Compensation

The pH of any solution is a function of its temperature. Voltage output from the electrode changes linearly in relationship to changes in pH, and the temperature of the solution determines the slope of the graph. One pH unit corresponds to 59.16 mV at 25 °C, the standard voltage and temperature to which all calibrations are referenced. The electrode voltage decreases to 54.20 mV/pH unit at 0.0 °C and increases to 74.04 mV/pH unit at 100.0 °C.

Since pH values are temperature dependent, pH applications require some form of temperature compensation to ensure standardized pH values. Meters and controllers with automatic temperature compensation (ATC) receive a continuous signal from a temperature element and automatically correct the pH value based on the temperature of the solution. Manual temperature compensation requires the user to enter the temperature of the solution in order to correct pH readings for temperature. ATC is considered to be more practical for most pH applications.

pH System

A successful pH reading is dependent upon all components of the system being operational. Problems with any one of the three: electrode, meter or buffer will yield poor readings

pH Electrodes

A pH electrode consists of two half-cells; an indicating electrode and a reference electrode. Most applications today use a combination electrode with both half cells in one body. Over 90% of pH measurement problems are related to the improper use, storage or selection of electrodes.

pH Meters

A pH meter is a sophisticated volt meter capable of reading small mill volt changes from the pH electrode system. The meter is seldom the source of problems for pH measurements. Today pH meters have temperature compensation (either automatic or manual) to correct for variations in slope caused by changes in temperature. Microprocessor technology has created many new convenience features for pH measurement; auto-buffer recognition, calculated slope and % efficiency, log tables for concentration of ions and more.

pH Buffers

These solutions of known pH value allow the user to adjust the system to read accurate measurements. For best accuracy.

Standardization should be performed with fresh buffer solutions.
Buffer used should frame the range of pH for the samples being tested.
Buffers should be at the same temperature as the samples. (For example: if all your samples are at 50 °C, warm your buffers to 50 °C using a beaker in a warm bath).

pH Electrode Training Guide

SECTION 1: ELECTRODE CONSTRUCTION THE PH SENSITIVE MEMBRANE:

The most common type of sensitive membrane used on a pH electrode is a blown glass bulb or rod. The glass used on Russell Mainstream electrodes is suitable for most applications. If, however, the application involves the constant monitoring of high temperature liquids or high pH values (above pH 13), then an alternative glass type can be specified. A bulb configuration will provide a fast response and accurate results when used in a sample of low ionic strength whereas a rod or bullet shaped membrane is very rugged and will be more resistant to breakage.

THE REFERENCE CELL:

Housed within the outer chamber of the pH electrode is a reference system which is designed to provide a stable reference voltage for the sensor. This reference 'half-cell' will maintain a constant output in all liquids. Reference cells consist of an internal element (usually a Ag/AgCl wire), an electrolyte (usually KCl solution) and a liquid junction. The liquid junction provides a leak path for the internal electrolyte to 'weep' into the sample chamber and provide an electrical contact with the liquid to be measured. If the liquid junction is not efficient then measurements will be inaccurate.

THE CAP/CABLE/CONNECTOR:

Electrodes used in laboratories are usually fitted with 16mm diameter caps to suit cantilever arm electrode stands. The cable used is a high grade, screened coaxial type with low noise characteristics. Because of the high impedance of pH electrodes, typically 100 megohms, connectors should always be kept clean and dry. Detachable cable electrodes should not be used in very humid environments.

SECTION 2: HOW TO SPECIFY AN ELECTRODE:

The following check list, when used with thepH electrode selection chart, will help to identify a suitable electrode for any given application

Sample Type
Temperature
Pressure
Expected pH Range
Viscosity of Sample
Sample Volume
Make and Model of pH Meter (to determine type of connector)
Cable Length Required
Preferred Body Material (Glass or Plastic)

SECTION 3: CALIBRATION OF PH METER AND ELECTRODE:

To achieve accurate, reproducible results a great deal of attention needs to be paid to the calibration method. A decision should be made on the accuracy required for the measurement. This will enable the user to choose the type of calibration required and the appropriate type of equipment to be used. The following recommendations will ensure the best levels of accuracy.

All solutions should be stirred gently to ensure the sensor is measuring a true representation of the beaker contents.
Calibration buffers should be chosen which have pH values either side of the expected sample value, i.e, for a sample which has an expected pH of 5, pH buffers with a value of pH 7 and pH 4 should be used.
Always use a 'control' buffer to keep a check on the drift of the electrode. A method commonly used is to place the electrode into a buffer, which has a value close to the sample pH, between measurements.
Fresh buffer solutions should be used. Changing all solutions daily is a good practice.
All solutions should be maintained at an equal temperature.
Rinse the electrode thoroughly in deionised water between measurements.
When calibrating the electrodes, allow sufficient time to elapse for the reading to stabilise before adjusting the meter. At least one minute, preferably longer.

SECTION 4: PROCEDURE FOR CALIBRATING THE PH METER

1 x high quality pH/mV meter.
1 x 100ml pH 7.00 buffer solution.
1 x 100ml pH 4.00 buffer solution.
1 x 100ml pH 5.00 buffer solution.
1 x calibrated glass thermometer.
1 x temperature controlled water bath (required if the sample value is different to ambient).
1 x combination pH electrode.
4 x 200ml beakers.
3 x Teflon stirrer paddles.
1 x magnetic stirrer.
1 x cantilever arm electrode stand.
1 x fast flow wash bottle containing deionizer water.

METHOD:

Assemble all equipment.
Lower fill hole sleeve on the electrode (if fitted) and thoroughly rinse the electrode tip.
Lower electrode into gently stirred pH 7.00 buffer and allow to stabilise.
Check the temperature of the calibration solutions and adjust the default reading on the pH meter, if applicable.
After 1 - 2 minutes adjust the calibration control on the pH meter to the appropriate pH value.
Raise electrode from beaker and thoroughly rinse with deionised water.
Lower electrode into gently stirred pH 4.00 buffer and allow to stabilise.
After 1 - 2 minutes adjust the slope control on the pH meter to the appropriate temperature corrected value. NoTE: Many modern microprocessor controlled pH meters have automatic buffer recognition.
Please consult the instrument manual for specific adjustment information.
Rinse the electrode and repeat stages c) to h) to confirm‎! calibration.
Rinse the electrode and lower into pH 5.00 buffer.
After stabilising, record the reading in pH 5.00 buffer.
Between measurements in the sample, rinse and lower the electrode into the control buffer (pH 5.00) for comparison with the recorded reading (remember to check temperature pH versus pH values).

SECTION 5: CARE AND MAINTENANCE OF ELECTRODES:

By following this advice, it is possible to significantly increase the expected life of an electrode and also to improve the quality of measurement results.

pH electrodes must always be stored wet. There are many opinions on which storage solution is the best. Russell Mainstream electrodes are all supplied soaked in a saturated KCl solution with the exception of double junction electrodes which are stored in the appropriate refill electrolyte for their application.
For short term storage, soak the electrode in KCl.
For long term storage, fill the soaking boot, fit over the end of the electrode and seal with parafilm.
Electrodes should never be stored in any of the following liquids: Deionised water, sample solutions, solvents, hydrochloric acid, pH buffers containing mercury based preservatives.
Sensing tips should always be rinsed after use.
Reference cells should always be kept regularly topped up with electrolyte.
Connectors must be kept clean and dry.
If the electrode needs to be cleaned physically, always use soft tissue soaked in a mild detergent or propanol.
Regularly inspect the glass pH sensitive membrane for cracks or chips.