pH

1. pH is a number which represents conventionally the hydrogen ion concentration of an aqueous solution.

2. The pH of a solution can be measured by potentiometric instrument (pH meter) capable of reproducing pH value to 0.02 pH units.

3. The potentiometric determination of pH is made by measuring the potential difference between two appropriate electrodes immersed in the solution to be examined.

4. One of these electrode is sensitive to hydrogen ions (usually glass electrode) and other is reference electrode (for example a saturated calomel electrode)

5. The pH of a solution to be examined is related to that of a reference solution (pHs) by following Nernst equation.

                     E - Es

pH= pHs - ---------------

                       K

Where

E = Potential in volts of cell containing the solution to be examined.

Es= Potential in volts of cell containing the known pH (pHs)

K = Change in potential per unit change in pH expressed in volts.

Theoretically K= (0.05916+.000198(t-25) volts at any temperature t.

Measurement of pH:

pH is defined as -Log [H⁺]. It is important to be able to measure pH accurately. And also it is important to know how the instrument measure pH because several factors can cause the observed value to differ from the actual pH.

A pH meter measures the voltage between two electrodes placed in the solution. The important part of the system is an electrode whose potential is pH dependent. The most commonly used pH dependent unit is glass electrode. The action of this electrode is based upon the fact that the certain types of borosilicate glass are permeable to H⁺ ions but not other cat ions or anions. Therefore, if thin glass layer of such glass is interposed between two solutions of different H⁺ ion concentration H⁺ ions will moves across the glass from the solution of high to that of low H⁺ concentration. Because passage of H⁺ ions through the glass adds a positive ion to the solution of low H⁺ concentration and leaves behind (high H⁺ concentration) a negative ion an electric potential develops across the glass.

If inner H+ moves to outer, a negative ion at inner wall and a positive ion at outer wall developed. If outer H+ moves to inner, a negative ion at outer wall and a negative ion at outer wall developed

The magnitude of this potential is given by the equation        

Potential=E= {2.303 RT/F}{Log [H⁺]1/ [H⁺]2}

R=Gas constant

T=Absolute temperature

F=Faraday constant

[H⁺]1 & [H⁺]2 = The H⁺ concentration of inside and outside of the glass respectively.

Clearly if H⁺ concentration of one of the solution is fixed, the potential will be proportional to the pH of the other solution. So the second part of the pH meter is a reference electrode whose concentration is fixed. (most commonly contains Hg-HgCl2 paste in saturated KCl. If high temperature is required Ag-Agcl2 is required instead of Hg-HgCl2). This is called a calomel electrode. KCl serves to make contact between Hg-HgCl2 or Ag-Agcl2 unit and the solution being measured. The calomel electrode tube is made of glass that is impermeable to H⁺ ions (so that potential is pH independent

So the pH measured by such system is primarily the difference between the potentials of that of the glass and reference electrodes. And then the potential is converted to pH.

1 pH=59.12 mv

However there are three other potentials present in the circuit.

1.The liquid junction potential of the reference electrode resulting from the fact that K+ & Cl- do not diffuse at the same rate so that a charge is generated at the interface between the KCl solution in the reference cell and the sample.

2. Asymmetry potential which develop across glass even when the pH on both sides is the same.

3. Potential of Ag-Agcl in glass electrode unit which is itself an electrode because of its contact with the CL- of the HCl.

              These three potentials and that the reference electrodes itself are relatively independent of pH and of ionic strength in the range normally encountered. Hence voltage V, measured with the total system may be expressed as the difference between the fixed potentials and that of the glass electrode. Therefore the voltage generated is linearly related to the ph of the solution.

pH electrode standardization – Calibration:

Before measuring the pH you have to calibrate (standardize) electrode. To calibrate the electrode you need at least two solutions of known pH. Most commonly used commercially available calibration buffers have pH of 4.01, 7.00 and 10.00.

Details of calibration procedure depend on the pH meter model.

 First step is usually related to temperature correction. Some models will measure temperature by itself, others need external temperature probe, or you will have to enter temperature measured by others means using dials or buttons. Buffer pH changes with temperature.

Check the pH of buffer 4.01, 7.00 and 10.00 at 25. If they are not in 0.02 ranges adjust with up and down arrows. (If desirable check the buffers of pH 1.00 and 12.45 at 20°)

 Then observe the slope value which displays automatically whether is in specified limit of 80-120 or not.

pH electrode test procedure or Slope calculation:

         Mathematical difference between two buffer mill volts (Electrode span)

Slope = -------------------------------------------------------------------- X100

                              Theoretical span (Theoretical span)

Electrode span= 7 buffer millivolts reading – 4 buffer millivolts reading (at 25°)

Theoretical span= 176.9

Cleaning of pH electrode:

·      General

·         Soak in 0.1M HCl for half an hour.

·         Drain and refill the reference solution.

·         Soak the electrode in filling solution for one hour.

·      Inorganic Contaminants

·         Soak in 0.1M tetra sodium EDTA solution for 15 min.

·         Drain and refill the reference solution.

·         Soak the electrode in filling solution for one hour.

·      Protein Contaminants

·         Soak in 1% pepsin/0.1M HCl for 15 min.

·         Drain and refill the reference solution.

·         Soak the electrode in filling solution for one hour.

·      Grease and Oil

·         Rinse with detergent or ethanol solution.

·         Drain and refill the reference solution.

·      Soak the electrode in filling solution for one hour. Electrode response may be enhanced by substituting a mixture of 1:1 pH 4 buffer and filling solution for the soaking solution.

Cleaning of clogged junction

·         Pollution by sulfides

      ·                     Use a solution of 8% thiocarbamide in 1mol/L HCl.

      ·                      Keep the electrode in the above solution till junction color turns pale.

·         Pollution by Silver chloride

·                     Use concentrated ammonia solution.

·                     Keep the electrode above solution for about 12 hours.

·                     Rinse and put into pH 4 buffer for at least 1 hour.

Other contaminants have to be removed by cleaning with distilled water, alcohol or mixture of acids .If nothing else help you may consider use of ultrasonic cleaners as last resort.

Regenerating pH electrode:

Following procedure is the last resort. They may work, they may won’t .You may try them before throwing electrode away.

First of all clean the electrode as described in electrode cleaning section then

·         Soak the electrode for 4-8 hours in 1M HCl solution.

·         Rinse it and move to pH 7 buffer for an hour.

If electrode is still not working

·         Fill the electrode with filling solution.

·         Move to the fume hood

·         Place the electrode in the 10% nitric acid solution on a hotplate. Heat to boiling and keep it in the solution for 10 min.

·         Place 50ml of filling solution in a second clean beaker. Heat although boiling is not necessary.

·         While the electrode is still hot, transfer it to the beaker of heated filling solution, set aside to cool.

·         Some manufacturers suggest the electrode may be reactivated by treating with diluted solution of hydrofluoric acid followed by subsequent conditioning in filling solution. (Before going to HF, remember it is highly dangerous and dissolves glass).

pH electrode maintenance:

·           Handle electrode with care-it is fragile (easily broken or damaged).

·          Keep electrode always immersed. Use the solution recommended by manufacturer or neutral solution of KCl (3M-4M).

·         Remember to always keep internal level of filling solution above the level of measured solution.

·         Don’t put electrode in a solution that can dissolve glass-HF (or acidified fluoride solution), concentrated alkalies.

·         Don’t put electrode into dehydrating solution such as ethanol, sulphuric acid, etc.

·         Don’t rub or wipe electrode bulb, to reduce chance of error due to polarization.

·           Fill the electrode with correct filling solution to not let it dry internally.

·         If you are using electrode in a solution containing substances able to clog the junction or stick to the glass bubble clean the electrode as soon as possible after use.

·         If electrode will not be used for long time, you may store it dry to prevent aging (aging take place only when electrode is wet).

·          If dried incidentally, or after storing-soak for at least 24 hrs before using.

pH electrode storing

·      A wet stored electrode allows an immediate use and a short response time, which is not true for dry storage ones. Unfortunately, the wet stored electrode is aging faster, because the process of aging (changing the structure in the membrane) proceeds also in the case of non-use. Keeping the electrodes wet should preferably be made in KCl solution. Most electrodes have preventive cap that can be filled with storage solution before placing.

·          To store electrode dry you must first remove internal solution, rinse the electrode in DI/RO water, and let it dry. (in fact electrodes that can be stored dry are getting more and more rare)

·          Always check electrode owner manual for details, as these may depend on the electrode make.

USP Criteria for pH measurement:

·           Measure at Temperature 25 +2^o.

·           Reproducibility 0.02.

·           To standardize the p H meter, select two buffers whose difference does not exceed 4 units and such that the expected pH of the test solution should falls in them.

·           Check the first buffer and set the value to std and check for second buffer, if more than +0.07 units examine the electrode.

·           If electrode is good, adjust slope or electrolyte changing etc.

·           Repeat the buffers checking until the pH values within+0.02