pH Meter
pH meter is a device used for potentiometric pH measurements.
pH can be measured using either pH indicators (like phenolophtaleine) - in form of solution or pH strips - or using potentiometric method. Strips are very useful when all you need is 0.5-1 pH unit accuracy. When you need higher precision, pH meter is the only way to go.
In potentiometric methods you measure potential difference between known reference electrode and the measuring pH electrode. The latter depends on the activities of ions you want to measure. This dependence is described by Nernst equation, thus once the potential has been measured you can calculate the activity.
pH meter is nothing else but precise voltmeter, connected to the pH electrode, and scaled in such a way that it displays not the measured potential, but ready pH value.
pH COMBINATION ELECTRODES Vs GLASS pH ELECTRODES
Most often used pH electrodes are glass electrodes. Typical model is made of glass tube ended with small glass bubble. Inside of the electrode is usually filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed. pH of internal solution varies - for example it can be 1.0 (0.1M HCl) or 7.0 (different buffers used by different producers).
Active part of the electrode is the glass bubble. While tube has strong and thick walls, bubble is made to be as thin as possible. Surface of the glass is protonated by both internal and external solution till equilibrium is achieved. Both sides of the glass are charged by the adsorbed protons, this charge is responsible for potential difference. This potential in turn is described by the Nernst equation and is directly proportional to the pH difference between solutions on both sides of the glass.
The majority of pH electrodes available commercially are combination electrodes that have both glass H+ ion sensitive electrode and additional reference electrode conveniently placed in one housing. For some specific applications separate pH electrodes and reference electrodes are still used - they allow higher precision needed sometimes for research purposes. In most cases combination electrodes are precise enough and much more convenient to use.
Construction of combination electrode is in large part defined by the processes that must take place when measuring pH. We need to measure difference of potentials between sides of glass in the glass electrode. To do so we need a closed circuit.
Circuit is closed through the solutions - internal and external - and the pH meter. However, for correct and stable results of measurements reference electrode must be isolated from the solution so that they will not crosscontaminate - and it is not an easy task to connect and isolate two solutions at the same time.
Connection is made through a small hole in the electrode body. This hole is blocked by porous membrane, or ceramic (asbestous in older models) wick. Internal solution flows very slowly through the junction, thus such electrodes are called flowing electrodes. To slow down the leaking, in gel electrodes internal solution is gelled.
pH GEL ELECTRODEIn flowing electrodes internal solution of KCl slowly flows to the outside through the junction - small hole with porous membrane, or ceramic or - in older models - asbestous wick. While such electrode contaminates solution with KCl it does it very slowly and traces of K+ and Cl- ions released during measurements are in most cases just spectators.
As the internal solution is lost from the flowing electrodes it must must be refilled so that its level is always above the level of the external (measured) solution. This way internal solution should never get contaminated. However, refilling of the combination electrodes adds to their maintenance cost and makes them difficult to use in portable pH meters.
To overcome problems with filling internal solution is sometimes gelled. While this helps slow down leak, it doesn't prevent diffusional ion exchange through junction - thus internal solution gets contaminated by the ions diffusing from the external sources, at the same time
loosing its own ions. As the composition of internal solution changes and can't be restored by refilling, gel electrodes have in general shorter life time, although they are easier to use and maintain.
Other method of prolonging the lifetime of the electrode is use of double junction.
Speed of the flow is one of important electrode parameters. It can't be too fast nor to slow. Flow can be too fast in case of broken membrane or lost (loose) wick, it can be too slow if the membrane/wick was clogged by some chemical precipitate - for example AgCl if the electrode was used to measure pH of solution containing Ag+ ions.
SINGLE AND DOUBLE JUNCTION pH ELECTRODES
DOUBLE JUNCTION SINGLE JUNCTIONpH ELECTRODE pH ELECTRODE
In classical combined pH electrode reference electrode is separated from the external solution by the junction through which the electrolyte leaks. Lost electrolyte must be periodically refilled through the filling hole, which makes these elctrodes inconvenient to use, especially in field. Methods of slowing down the leak - like gelling of the electrolyte - have a side effect of shortening the lifetime of the electrode, as it is more prone to the changes in electrolyte composition due to contamination and diffusional leak of the ions. Contaminated gel can not be replaced, thus lifetime of gel electrode is rarely longer then several months.
To prolong lifetime of such electrodes double junction is sometimes used. In double junction electrodes additional chamber is introduced between reference electrode and external solution. Before contamination from the external solution can get to the reference electrode it must diffuse through not one junction, but two (hence the name). Additional chamber works as a buffer, slowing down the changes in the composition of the reference electrode electrolyte. Double junction electrodes can work longer, but they are more difficult to make, thus more expensive.
Note, that single or double junction refers only to the way reference electrode is made. While you will often see combination electrodes described as pH double junction, external reference electrode can be made double junction as well.
SOLID STATE pH ELECTRODECommercially available solid state pH electrodes are mainly built around Ion Selective Field Effect Transistors (ISFET).
The basic principle of the ISFET working is the control of the current flowing between two semiconductor elements (drain and souce) by electrostatic field, generated by the protonated oxide gate. Protonation of the gate is in a way identical to the process taking place in glass pH electrode, just the methodology used to measure protonation degree is different. Instead of measuring potential difference on two sides of the glass, we measure the current flowing through the transistor. The lower the pH, the more protonated and charged gate is which changes its electric field - changing in turn current flowing through the transistor. This current is a signal that can be measured to check the pH value.
ISFET electrodes can be very small when compared to the bulky glass bubble of the standard glass electrode. They are also much more sturdy, so they can be easily used in places where fragile glass electrodes will not survive. However, ISFET electrode can't be used with standard pH meters (unless it is connected through special interface) and the pH measurements are generally less precise when compared to glass electrode.
pH ELECTRODE POTENTIALPotential measured by pH meter is a sum of all potentials present in the system. Putting aside junction potentials that can be present in the experimental setup, we are left with three sources of electromotive force. First builds up on the glass electrode, thanks to different activities of the H+ ions on both sides of the glass. Second source is the glass electrode silver wire covered with AgCl and immersed in the solution of chlorides, and third is the reference electrode - silver chloride or calomel, depending on the application.
Thus the real potential measured is sum of three potentials:
where
E glass =
E'o +
0.0591 pH outside
which finally leads us to the (almost - read on) final equation describing measured potential:
=
E''o +
0.0591 pH outside
Where E0'' contains all constants mentioned above and in the Nernst equation section. As you see after taking (almost - read on) all factors into account we can expect perfect linear dependence between measured potential and pH.
One may ask at this point, why do we complicate things adding two additional sources of potential (EAg/AgCl and Eref), instead of measuring just the potential of glass electrode which behaves as the concentration cell? The answer is simple - there is no easy and practical way of measuring the glass electrode potential. We may think of two added reference cells just as of reliable contacts, interfacing metal wires and solution. While they add their own potentials shifting glass electrode potential readouts, it doesn't matter. First of all, we never need absolute values of glass electrode potential, as only difference being proportional to the difference in pH of both sides of the glass counts. Second, even if we will be able to measure absolute potential it will not help us much, as it depends on many additional things - like internal tensions in the glass, or the smoothness of the glass surface. As we already have to compensate for these impredictable factors, additional, constant shift in voltage doesn't change our situation.
Junction potential, that we have ignored in the above equation, in practice can be an important source of error. It was an important issue back in the early eighties of the 20th century. Most modern electrodes are less prone to this effect.
Every electrode has a characteristic pH where its potential is 0 (so called isopotential point). Carefully choosing potentials of both reference electrodes (which can be done with selection of chlorides concentration) it is possible to compensate for all other sources of potential in the electrode so that isopotential point is at pH=7.0. Most modern pH electrodes are made this way.
As it was mentioned above so far we have looked at almost all factors, but some are still left uncommented. Glass electrode potential depends on the presence of other then H+ ions in the solution. While carefull selection of the glass used makes this difference small, it can't be neglected. More on that in electrode selectivity section.
pH ELECTRODE SELECTIVITY
Ideal pH electrode should have potential dependent solely on the activity of the H+. Unfortunately, there is no such thing as ideal pH electrode.
Potential that builds up on the electrode surfaces has its source in the ions attaching themselves to the glass surface. Glass structure is such that only single charged ions are attracted. Depending on the ion this effect can be stronger or weaker, but the result will be always the same - other ions will interfere with the determination of pH.
To describe effect of other ions on the electrode potential we can use slightly simplified version of Nicolsky-Eisenman equation:
E =
Eo +
0.0591 log ([
H+] + k1[
Na+] + k2[
K+] +.....)
where ki are so called selectivity coefficients, determined experimentally.
Every glass electrode potential depends not only on pH but on concentrations of all other single charged ions present. Carefull selection of the glass composition is crucial, as glass is solely responsible for the selectivity coefficients values. These can take values from the 10-1 - 10-15 range. The smaller the value the better. Importance of the small selectivity coefficient can be shown with simple example. Let's assume selectivity coefficient H+/Na+ of 10-8 and 0.1M Na+ solution:
Real pH Measured pH
1.00 1.00
7.00 7.00
8.00 7.96
9.00 8.70
10.00 8.96
Measurement will never show pH above 9.00 in this case. This effect is called alkaline error or sodium error, but not only sodium can interfere with pH measurements. Other single charged cations interfere as well. It is especially important in the case of buffers (for example TRIS based) where the concentration of interfering ion can be relatively high. Most commercial pH electrodes have selectivity ceofficients high enough to not allow such situations. Detailed information about selectivity should be available from electrode manufacturer.
It is worth of noting here, that using proper glass one can make glass electrode that can be used for determination of other single charged ions - like Ag+, Na+, K+ and so on.
HOW TO CHOOSE A pH ELECTRODE
There are many types of pH glass electrodes. In some specific applications you should be very carefull when selecting one, but in most cases the selection is easy. Look for other users working in similar environment and ask them about their experience with different types and makes of electrodes, that way you should be able to find the best offer pretty fast.
If you are working with aqueous solutions containing at least 5% water and your solutions doesn't contain any substances reacting with silver, look for general purpose electrodes.
If you work with solutions containing organic material, proteins, TRIS buffers, heavy metals, or with very low ionic strength solutions, look for calomel electrode. Listed substances can react with silver and clog the junction.
Instead of using calomel electrode you may look for double junction electrode, as it will have similar properties.
If you are working with solutions that can clog normal electrode junction (like oils, foods or paints) look for teflon junction electrode. Junction in these electrodes is made of porous teflon, making it resistant to impurities.
Don't forget to check electrode pH range - some electrodes can't work in high pH, and electrode temperature range - especially if you are going to measure pH in solutions above 60°C.
In most cases manufacturers sites contain a wealth of information about available pH electrode
models and their applications.
pH ELECTRODE MAINTENANCEHandle electrode with care - it is fragile!
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.
Fill electrode (the flowing type) with correct filling solution (as recommended by manufacturer - usually KCl solution, 3M to saturated) to not let it dry internally.
If the electrode will be not used for a long period of time, you may store it dry to prevent aging (aging takes place only when the electrode is wet). Don't try it with gel electrodes - these have to be stored in concentrated solution of KCl only.
If dried incidentally, or after storing - soak for at least 24 hours before using.
If you are using the electrode in solution containing substances able to clog the junction or stick to the glass bubble, clean the electrode as soon as possible after use.
Don't put electrode in solutions that can dissolve glass - hydrofluroic acid (or acidified fluroide solution), concentrated alkalies.
Don't put electrode into dehydrating solution such as ethanol, sulfuric acid, etc.
Don't rub or wipe electrode bulb, to reduce chance of error due to polarization.
Don't use organic solvents for cleaning of the electrode with epoxy body.
HOW TO STORE pH ELECTRODES
Electrodes with liquid electrolytes (not gel types) may be stored either wet or dry.
A wet stored electrode allows an immediate use and a short response time, which is not true for dry stored ones. Unfortunately, the wet stored electrode is aging faster, because the process of aging (changing of the structure in the membrane) proceeds also in the case of non-use.
Keeping electrodes wet should preferably be made in KCl solution (3M-4M). Most electrodes have a protective cap that can be filled with storage solution before placing.
To store pH electrode dry you must first remove internal solution, rinse the electrode in DI/RO water, and let it dry.
Note that you can't store dry combination electrodes and gel electrodes. In fact electrodes that can be stored dry are getting more and more rare.
If electrode is stored wet, don't forget to cover fill hole to prohibit evaporation of reference fill solution.
Gel type electrodes can be stored only wet, soaked in the KCl solution (3M-4M). Never store them in DI/RO water.
Check your electrode owners manual for details, as these may depend on the electrode make.
HOW TO CALIBRATE pH ELECTRODES
Before measuring 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 the 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. Note that this setting changes only slope of the calibration curve and doesn't take into account fact, that buffer pH changes with temperature.
Next step is to put the electrode into pH 7.00 buffer. Rinse the electrode with distilled water from a wash bottle into an empty beaker before immersing it into new solution. You should do it every time electrode is moved from one solution to other to minimise contamination. Check if the working part of the electrode is completely immersed in the buffer. Take care to not hit bottom of the baker with the electrode. Wait for the reading to stabilize (it takes seconds usually, up to a minute sometimes).
Modern pH meter models working in calibration mode often recognize the buffer automatically and take necessary action by themselves. In case of older pH meters you will probably have to turn one of calibration knobs so that the pH meter shows 7.00.
Sometimes pH readings will oscillate. If the oscillations are small try to find out the best position of the knob so that 7.00 is a mean displayed value. If the oscillations are large and erratic, they may be caused by faulty junction (check all), faulty cable (check them), faulty electrode (try other electrode) or faulty pH meter. Sometimes also static electricity can be a reason of erratic readings - consider changing clothes, grounding yourself or shielding pH meter, cables and electrode. If you are using magnetic stirrer check if switching it off doesn't stop oscillations.
Next steps will depend on the solution you want to measure pH of. If you plan to measure pH in acidic solutions, use pH=4.01 buffer. If you plan to measure high pH use pH=10.00 buffer. If you want to be able to measure pH in the wider range, you may want to proceed with three point calibration and you will need both buffers. Remember that high pH buffers tend to absorb atmospheric CO2 thus they should be used as fresh as possible - don't left the bottle open and do the calibration immediately after filling the beaker with the buffer.
Rinse the electrode and move it to the second buffer. Once again pH meter will either act on itself, or you will have to use a knob (probably different from the one used in the previous step). Repeat the action for the third buffer if needed (using third knob - if present).
After that you are ready to take measurements.
Please remember, that above outline is very general. Different pH meters may require slightly different operating procedures. You should consult your manual to be sure how to proceed and how to maintain the electrode.
STANDARD CALIBRATION BUFFERS FOR pH ELECTRODES
In general you will probably use commercially available calibration buffers, sold either as ready solutions or as tablets to dissolve in deionized water. However, it may be interesting to look at the table of standard solutions that can be used for the electrode calibration. pH given is for 25°C:
Standard calibration buffers
_________________________________________________substance(s) concentration pH
______________________________________________________
hydrochloric acidHCl 0.1000M 1.094
potassium trihydrogen
oxalateKH3C4O8 0.05000m 1.679
potassium hydrogen
phthalateKHC8H4O4 0.05000m 4.005
potassium hydrogen
tartrateKHC4H4O6 saturated in 25°C 3.557
disodium hydrogen
phosphateNa2HPO4
potassium dihydrogen
phosphateKH2PO4 0.02500m
0.02500m 6.865
disodium hydrogen
phosphateNa2HPO4
potassium dihydrogen
phosphateKH2PO4 0.03043m
0.008695m 7.413
disodium tetraborate
Na2B4O7 0.01000m 9.180
sodium hydrogen
carbonateNaHCO3
sodium carbonateNa2CO3 0.02500m
0.02500m 10.012
calcium hydroxide
Ca(OH)2 saturated in 25°C 12.45
TESTING PARAMETERS OF pH ELECTRODE
To ensure your electrode works correctly you may want to measure its parameters.
example correct pH electrode parameters
Property Value
Isopotential point ± 15 mV original value
Slope 55 - 61 mV/pH unit
Glass membrane resistance 20-100 MΩ
Electrolyte leak rate 0.2 to 1.5 mL/24 hours
Note that values presented can be wrong in case of specific electrode (for example electrolyte leak rate in electrodes with sleeve junction can be much faster). Consult owners manual if in doubt.
To test isopotential point and slope, switch your pH meter to display results in mV, not as pH units. If not possible, you have to use other pH meter or laboratory voltmeter able to work with pH electrodes.
Isopotential point of most general use electrodes is set at pH=7.00. To measure it put the electrode into pH 7 buffer and measure the electrode potential. Remember to take the measurements in the temperature electrode was originally calibrated in.
To check the slope move electrode to the pH 4 buffer. When potential stabilizes, read the value.
The difference between previous reading and current reading should be in the 166-184 mV range.
Measurements of the electrode resistance and electrolyte leak rate are more difficult. Glass membrane resistance measurements require electrode test stand. To check electrolyte leak rate fill the electrode wil filling solution. Use a waterproof pen to mark the initiall fill level. Suspend the electrode in a beaker of pH 7 or pH 4 buffer so that the lower electrode plug is level with the water. Wait for 24 hours. Refill the electrode using a serological pipet, noting the volume of filling solution required. Amount of solution added divided by 24 is a flow rate.
