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FAQ
I need to measure pH in difficult samples such as foodstuffs, mining and industrial wastes, untreated waste. Which pH probe would you recommend in these applications?
The IJ-44 and IJ-40 are suitable for nearly all sample types. The intermediate junction and the replaceable sleeve electrolyte assures accurate results and long life. For more information on applications, please download our pH electrode selection guide.
What is automatic temperature compensation?
pH measurements need to be corrected for temperature when calibrating, and this can be done manually or with an ATC probe. This is the Nernstian slope correction. At 25oC, this value is 59.2mV/pH unit, however at 10oC the value is 56.2 mV/pH unit. Regardless of whether you use an ATC or not, it is NOT possible to compensate the pH of a sample at one temperature to another temperature, as the coefficients are unknown. The correct procedure is to calibrate and measure at the same temperature (ideally within 5oC), and report the pH at the measured temperature.
I have an ATC (automatic temperature compensation probe). Does this mean I can calibrate at room temperature and measure my samples straight from the fridge?
No, the ATC only compensates for the Nernst slope. in practice, you should measure within 5°C of you calibration temperature. If you wish to perform measurements at 4°C, you should cool your buffers down to 4°C for calibration. Similarly to measure at 50°C, you will need to calibrate at 50°C.
What happens to the resistance of the glass pH membrane as the temperature decreases?
As the temperature decreases the resistance (impedance) increases. An increase in the response time will also be observed, and measurements below 5oC can end up being noisy.
What is Sodium Error?
At very high pH values, artificially high pH values can be reported at high sodium levels. To minimise this effect, you should use a pH electrode with a low sodium error if you are measuring above pH 12. The glass formulations in the IJ44C and the PBFB have a very low sodium error.
If I purchase a pH meter, what accessories do I need to use with it?
You need a pH electrode and at least two pH buffers, one at pH 7 and the other at either pH 4 or 10. Electrodes, meters and buffers can be purchased from www.selectscientific.com.au
I want to measure pH with a multimeter. Is this possible?
No, because the impedance needed is in the order of 1012 Ohms, which is substantially higher than in multimeters. Using a pH electrode on a multimeter would cause large measurement errors and would irreversibly damage the probe (polarization).
I am having problems calibrating my pH electrode. Is there a simple checking procedure?
With your meter in mV mode, check the potentials of the pH 7 and 4 buffers respectively. The 7 buffer should read 0+/- 30mV, and the 4 buffer should read at least 170 mV higher than the 7 buffer (@25°C). If the difference is less than 170mV, you may need to replace the buffers or the electrode.
What is the recommended slope range for a pH electrode?
For laboratory and field measurements, a slope range of 95-102% is suitable. A low slope is normally indicative of ageing or mechanical damage to the glass membrane.
I am following a procedure which refers to the use of a calomel reference electrode. Can you advise if there is a substitute?
Calomel electrodes are often referred to in old Chemistry papers, Corrosion papers, and have also been recommended for measurements with Tris buffers. However, in most circumstances, the IJ-40 will give similar performance. Simply add 46mV to the calomel potential to obtain the equivalent saturated Ag/AgCl potential. Another point to bear in mind: Calomel electrodes contain Mercury.
What is the recommended storage solution for combined pH electrodes?
We normally recommend using a 20% w/w solution of potassium chloride in distilled water. We can provide this in either 200ml or 1L bottles on request.
How often should I replace the reference electrolyte in the IJ-44 pH electrode?
Reference electrolyte replacement will be more frequent in contaminated samples. In clean waters, replacement may only be required every few months, but it could be even daily in extremely polluted samples.
What is the normal operational life of an Ionode electrode?
As a general rule, electrode life is dependent on the sample it is being used in. In clean and routine analysis, well in excess of one year would be expected and in many cases, beyond five years of serviceable life has been reported. In difficult or abrasive samples, sensor life naturally can be greatly reduced.
What is the normal shelf life of pH buffers after opening?
As a general rule the pH4 and 7 buffers should last at least 3 months after opening, whereas the lifetime for buffers such as 9.19 and 10.01 may only be 1-2 months. The lifetimes will be longer if you minimise exposure to the atmosphere, and it is best to store the buffers away from direct sunlight.
How do I get reliable and accurate measurements in distilled water and other samples with low ionic strength?
Do not use a sealed gel electrode, but instead use either the IJ-40, PBFC or similar. Also try to minimise contact of the sample with the atmosphere and allow sufficient time for the measurement to achieve stability.
How do I clean the glass pH membrane?
This depends on the type of sample you have been measuring. For inorganic deposits, use dilute acid or base; use ethanol or similar to remove fatty deposits, and use either Jif or Pepsin/HCl to remove protein deposits.
I want to perform pH measurements but I do not want the sample contaminated by chloride ions. How do I do this?
Use a reference electrolyte such as 1M Potassium Nitrate in the sleeve junction of the IJ-44.
What is the recommended reference electrolyte for measurement or titration in non-aqueous samples?
This depends on the sample, but in many instances, use an electrolyte such as 1M Lithium Chloride in Isopropanol.
What is the correct type of pH electrode for use in Tris buffers?
The IJ-40 and IJ-44 Intermediate Junction pH electrodes are suitable for use in Tris buffers.
How do I store my pH electrode?
Proper electrode storage maximizes electrode performance and life. For shert term storage of up to 2 weeks store in pH storage solution KS-250.
This will extend the life of the reference and prevent dehydration of the membrane and the junction. Short term storage allows for quicker start-up when the electrode is used frequently.
DO NOT USE DISTILLED WATER.
For long term storage of a refillable electrode it is preferable to remove the sleeve, remove the electrolyte, clean the membrane and stem, replace sleeve without adding electrolyte and store safely. For sealed gel type simply fit the supplied rubber wetting cap and store safely.
Following long term storage, electrodes will require soaking in storage solution for at least one hour to hydrate the membrane and calibration to ensure accuracy.
What is pH?
The pH is a measurement for the acidity or alkalinity of a solution. In pure water the hydrogen ion (H+) and hydroxyl ion (OH-) concentrations are equal at 10-7 M (25°C). To provide a convenient and effective means of defining acidity and alkalinity, the pH is defined as the negative logarithm of hydrogen activity:
pH = -log [H+]
The heart of a pH measuring system is a membrane made from special pH-selective glass on which a very thin layer of hydrogen ions is formed when dipped in water. At high pH values, this layer will have a low hydrogen concentration. However, at low pH values a large number of H+ ions diffuse in the layer. By measuring the generated electrical potential (E) in the layer the corresponding pH can be computed.
Solution-1: sample to be measured
Solution-2 : known buffer solution (7 pH) Reference-1: silver wire in a salt-bridge (KCI) Reference-2: silver wire in a salt-bridge (KCI) Membrane layer-1: H+ ions generated by the sample Membrane layer-2: H+ ions generated by the buffer
The potential (E) between both wires will vary with the pH difference between sample and known buffer according the Nernst-equation (-59.2 mV/pH at 25°C). A salt-bridge around each wire prevents direct metal contact with the solutions by using a wet junction for a stable electrical behaviour.