 |
|
|
FAQ on pH Electrodes
Calibration
Recalibrate meter at least once a week or daily if in doubt of results being accurate or reproducible. Depending on user's measurement frequency, sample types and operating environment, the more regularly he calibrates the better is its meter's highest accuracy being maintained. A good pratise is at least calibrate meter before each use and condition electrode first if not used for sometime.
If meter offers more than 3-point calibration, it is recomended to calibrate at least 2 or more points for better meter accuracy. Always start calibration with pH 7 being the first-point (offset), followed by next available pH buffers eg. pH 4 or/and pH 10 and so forth. If measurement tends to be in acidic range (below pH 7), user should calibrate at pH 7 firstly, then pH 4 or/and pH 1.68. Similarly for alkaline range (above pH 7), start calibrating with pH 7, then pH 10 or/and pH 12.45. If only 1-point calibration is available, choose buffer cloest to the expected measurement value for good accuracy.
Buffers
Buffers are calibration solutions for pH meter system and their respective values remain stable or unchanged if unopened throughout shelf-life provided they are stored in dry, cool indoor environment. DO NOT use expired buffer or any buffer solution left uncapped for sometime, especially above pH 9 like pH 10.01 or pH 12.45 buffer as it gets oxidised by air and actual buffer value differs. Always pour out from bottle into dry clean container and NEVER pour back used buffer into bottle after calibration. Discard used buffer appropriately. Typically buffer solutions can be used for a year after opened and capped tightly while those opened yet uncapped may last few weeks. For freshness and adherence to strict laboratory regulations, air-tight calibration sachets or pouches are preferred over bottle types as they are highly accurate and used as singles. These hassle-free pouches also serve as containers for user to dip electrode directly while calibrating his meter. Other available buffers come in tablet or powder form but they have to be pre-mixed fully with desired volume of distilled water before using.
Rinsing
Always rinse electrode with de-ionised, ditilled water or clean tap water before and after each measurement. Blot dry electrode housing and gently shake electrode to dislodge any droplets on its glass bulb. This ensures electrode is not adhered to any known solution that may cross-contaminate the sample before dipping the electrode and taking measured sample value. Similarly rinse electrode in-between each calibration point performed. Also rinse electrode before storing or keeping in storage bottle.
Measurement
For refillable electrodes, always expose or open the refill opening found on top of the electrode housing while measuring. Close it when idle or not in use. Keep electrode vertically or upright by resting it onto electrode holder stand or swivel arm of pH meter if available. Ensure the reference fill solution or electrolyte level is at least 2/3 electrode stem length or below the refill opening. Drain out old fill solution or suspension residue inside using disposable dropper and refill with fresh reference fill solution. If there are trapped air bubbles found in refillable electrode, invert electrode (without storage bottle) a few times to dislodge. For gel-filled sealed electrode, hold tightly onto electrode's cable (without storage bottle) at half length and swing around (away from benctop or object) a few times to remove trapped air bubbles.
Before use, uncap from storage bottle first before pulling electrode out of cap opening. This reduces build-up pressure and allows electrode to be removed more easily. Similarly to put electrode back into storage solution after use, uncap first before inserting electrode gently into cap opening and then fasten the cap into position. DO NOT attempt to vigorously push electrode into opening when it is capped as trapped air inside bottle would pressurise storage solution to ingress back through electrode junction leading to poisoning of electrode reference over time, causing electrode to produce erratic readings.
Electrode Condition
Typically a good electrode has acceptable electrode slope of 90 to 105% and offset error not more than +/- 30 mV. Offset error is based on the asymmetry potential of pH 7 buffer at 25 °C. Electrode slope (expressed in percentage) can be derived by dividing the electrode span being (mathematical difference of mV readings for pH 7 and 4 buffer) by theoretical electrode output value of 177.5 mV and multiply by 100. Slope below 95% indicates that the electrode may require cleaning or if cleaning does not help, the electrode should be replaced. Slope above 105% indicates that the pH buffers are contaminated. For example,
Reading of electrode at pH 7 buffer at 25 °C = -5.6 mV (offset error)
Reading of electrode at pH 4 buffer at 25 °C = +168.4 mV
Electrode span = +168.4 -(-5.6) mV = 174.0 mV
Electrode slope = 174/177.5 mV x 100 = 98 %
Most electrodes have useful lifespan and user needs to check the electrode condition from time to time in ensuring electrode performance is not compromised.
Storage And Conditioning Electrode
It is best to store them in the storage bottle filled with electrode storage solution, 4M KCl, pH 4 buffer and pH 7 buffer in order of preference. This hydrates glass bulb so that electrode is ready to use. If kept dry, condition electrode by soakting into buffer solution or tap water for at least half an hour before using. Replenish or replace with fresh storage solution at least fortnightly or 2 weeks once. NEVER store electrode in de-ionised or distilled water as it is meant for rinising only. Prolong storage in de-ionised water would cause electrode performance to be sluggish and electrode become unuseable.
Eletrode Selection
Fundamentally, user needs to know the following criteria before deciding which suitable electrode to be used for specific measurement :-
- Intended Use : Quick test, Surface measurement, Submersible or immersion use, Continuous inline monitoring, Autoclavable, Sterilisable
- Expected Measured Range : Neutral (around pH 7), Alkaline (above pH 7), Acidic (below pH 7), High pH (pH 12-14), Low pH (pH 0 to 2) = select suitable glass sensor type
- Work Environments : Field, Indoor Plant, Laboratory, General Purpose for outdoor or indoor use = determine electrode housing material i.e. glass or plastic-body
- Sample or Media Types : Aqueous or Organic samples; Contain any protein, sulfides, specific ions, heavy metals present; Low in ionic strength like pure, de-ionised or distilled water; Viscosity nature - slurry, gel or semi-solids; Small sample volumes or flat surface media - agar, skin, micro culture samples = determine type of electrode construction, shape of glass bulb, reference junction and refillable electrolyte.
- Operating Temperature : Ambient room or Varying temperature conditions; Glass or Epoxy housing electrode; Necessary to have built-in ATC temperature sensor or separate temperature probe.
- Meter System : Accuracy of measurement; Support electrode connector type eg. BNC, banand plug, DIN type, US Standard, etc; Any propriety built-in pre-amplified input (which may not be compatible with other electrode maker); ATC temperature probe element used on meter - 10 KΩ, 30KΩ, Pt100, Pt1000 or non-standard NTC; Meter has MTC operaton mode by adjusting to desired set value for better meter accuracy in the event of a faulty temperature sensor or there is no temperature probe available.
Electrode Types
Combination Electrode vs Sensing Electrode With A Reference Cell
A combination electrode is more convenient and requires a smaller container and volume. The sensing electrode with reference cell combination allows user to select the reference cell most compatible with test solution. Combination electrode is much preferred and better option for most applications. However separate ones allow user to replace each cell individually.
Shapes Of Glass Bulb
A spherical shaped (round) bulb will provide 95% response in less than one second. It has low impedance and fast response but is relatively fragile. pH electrodes with spherical shaped surfaces are designed so that the bulb is recessed inside the electrode body. Such designs protect the glass bulb against breakage.
A hemispherical shaped (semi-round) bulb is a stronger shape mechanically and, as a result, it has a higher impedance and slightly slower response. These shapes are often used in a fully exposed manner for fast response.
A flat measuring surface is the most durable of all the shapes. It makes good sample contact, is easily cleaned, is very strong mechanically but has the highest impedance and the slowest speed of response at 95% in less than 5 seconds.
Glass Sensor Type
GX-1 is especially suited for low temperature, nonaqueous solutions, and pH measurements under 12 pH.
GX-2 is best suited for most pH measurements where minimal or no Na+ is present. It is a low-resistance glass with a very fast and stable response and is designed for pH ranges of 0 to 13 and temperatures of 0 to 100 °C.
GX-3 is especially formulated for continuous, long-term use at high temperatures, particularly in strong alkaline solutions above pH 11. It experiences negligible Na+ ion error above 13 pH. The impedance of the glass is much higher than other glasses, and a slower response will be experienced at room temperatures and below.
GX-V is specially designed for low ionic strength and high pH measurements, and offers superior accuracy and repeatability with range of 0 to 14 pH and temperature up to 135 °C.
Housing Construction
Glass-body pH electrodes may be used in most sample types, suited for most routine pH measurements for accuracy, high temperature, and ease of cleaning. However due care must be taken when handling as it is fragile and not suitable for outdoor or used by inexperienced users e.g. students.
Epoxy-body pH electrodes are designed for rugged environments, field and plant applications where rough handling and breakage of glass-body electrodes are a major problem. However epoxy-body pH electrodes should not be used in organic solvents.
For situations containing proteins, sulfide, and TRIS buffers, use speciality pH electrodes that are designed to handle well.
For viscous or dirty samples, use flushable junction or free-flow junction electrodes for best results that do not clog reference junction and offer easy cleaning.
Electrode Reference Junction
Reference junctions are the porous material that contacts the sample. The common types available are: ceramic, open junction, flushable junction, fibre-glass, and porous plugs;
Refillable vs Sealed Reference
Refillable reference is selected for high accuracy, stability, and longer electrode life. Refillable types sacrifice convenience and ease of maintenance. Sealed gel-filled reference electrodes are designed for convenience where minimal maintenance is required. However slightly lower accuracy and shorter useful life must be taken in account.
Single vs Double Junction Reference
For many applications, a single junction reference electrode is satisfactory. However, if samples contain proteins, sulfides, heavy metals or any other material which interacts with silver ions, unwanted side reactions may occur. These reactions can lead to erroneous reference signals or to precipitation at the reference junction leading to a short lifespan. A double junction reference design affords a barrier of protection to combat the above interactions. When in doubt about using single or double junction designs, the safest approach is to use the double junction; they can be used anywhere a single junction design can be used. Conversely, single junction designs should not be used where double junction designs are needed. In most process applications, it is recommended to use double junction electrodes.
Always keep a spare electrode in the event an existng one needs to be replaced as its useful lifespan has ended or damaged incidentally.
Measurement On pH Of Gas
The only way to measure the pH of a gas is to dissolve it into a distilled water and measure the mixture instantly. Technically speaking, the pH of the distilled water/gas mixture will be that of the gas.
Measurement On pH of Soil
Use soil pH electrode. Prepare the test sample by combining same ratio of sample with distilled water in a clean container, say 10 g soil sample with 10 ml distilled water. Mix them thoroughly, and allowing the mixture to settle for 5-10 minutes. Insert electrode fully into container such that the glass bulb is in the soil part and the junction is in the supernatant. Allow reading to stabilise before taking measured value.
Measurement On pH Of Meat or Semi-Solid Food
To measure the pH of meat (poultry, fish or meat) or semi-solid foods (cheese to hard jelly or media) it is necessary to determine whether sample surface is soft or hard for typical glass spear tip or sharp tip pH electrode to be inserted directly into it. If sample surface is too hard, the feasible way to puncture its surface with a sharp object to create a cavity so that user could insert electrode gently and push further into it. Allow reading to stabilise before taking measured value. Rinse electrode thoroughly after each measurement and occassionally use protein removal solution or electrode cleaning solution to clean electrode for better response.
|
|
|
|
|
|