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The Care and Maintenance of ETI Analytical Instruments

Always read the manufacturer’s Operating Instructions included with the instrument.

Do not subject the instrument to:
• Physical damage
• Heat or Cold ie Strong sunlight, freezing conditions
• Solvents
• Vibration
• Total immersion (unless indicated as IP 67)

Various Calibration/Buffer Solutions are available for the regular calibration of instruments


pH Electrode Maintenance

ETI’s range of pH instruments includes fixed sensor, pocket sized instruments such as the pH PAL tester and the Senz pH Pro tester, and the WalkLAB pH meter with replaceable probes.

The pH probe consists of two parts: a glass pH electrode and a reference electrode.  The pH electrode is a hydrogen ion sensitive glass bulb, with an output voltage that varies with the changes in the relative hydrogen ion concentration inside and outside the bulb. The Ag/AgCl reference electrode is the liquid junction to complete the measuring circuit with the pH glass bulb. It comprises a silver wire coated with silver chloride, a filling solution and a permeable junction through which the filling solution escapes to form the liquid bridge

It is essential that the entire glass bulb/membrane must always be clean. Rinsing the membrane with distilled water will often suffice for aqueous solutions. Rinsing the electrode with a mild detergent solution followed by thorough rinsing with deionised water once a week will be beneficial. An alkaline hypochlorite solution can be used to clean electrode membranes subjected to solutions containing fat or proteins.

Between measurements, store the glass electrode in a pH buffer with pH < 7, or de-ionised water. High temperature measurements, compounded by constant use in strong alkaline solutions or weak solutions of hydrofluoric acid will drastically reduce the lifetime of the electrode, since the glass membrane will slowly dissolve.

With replaceable electrodes with the WalkLAB, trapped air bubbles around the inner reference electrode will produce an unstable reading. Swing the electrode in an arc or tap it gently to remove the bubbles. The electrode may have to be heated gently to approximately 60° C in a water bath if the air bubbles are trapped by KCl crystals.

If the electrode has been allowed to dry out or has become slow to respond or cannot be calibrated, it may be rejuvenated. In order to establish a stable, hydrated glass membrane, electrodes should be soaked overnight in 0.1 M HCl. After overnight soaking, rinsing, soaking in a buffer of pH = 4, and again rinsing, the electrode should then be ready for use. If a shorter soaking time is necessary, the electrode should be calibrated frequently to adjust for drifting potentials.

A sluggish response for a glass electrode, even after proper maintenance has been performed, may require a slight etching of the outer glass layer of the membrane. The following treatment is only recommended after all other measures have been used to improve response and have failed. Soak the glass membrane portion of the glass electrode in a 20% ammonium bifluoride solution for one minute, followed by 15 seconds in 6 M hydrochloric acid. (Since hydrofluoric acid can be formed during this procedure, be careful if choosing this method of electrode rejuvenation.) Rinse the electrode well and soak for 24 hours in a pH buffer with pH < 7.

Please note the dangers of using alkaline or acidic rejuvenating solutions. 

The proper functioning of the glass electrode depends on the hydration of the glass layer that takes place on the surface of the pH sensitive glass membrane during soaking and measurement in aqueous solutions. As long as the electrode is frequently rehydrated, accurate measurements in non-aqueous or partly aqueous solutions are also possible. This can be accomplished by soaking in a slightly acidic buffer. In non-aqueous solvents completely immiscible with water and before soaking, it is important that the electrode should first be rinsed with a solvent which is miscible with both water and the solvent before rinsing with water.

With the WalkLAB, the electrode cable and the electrode plug must be kept clean and dry if reliable measurements are to be obtained because of the very small electrode currents which pass through the glass electrode.

A number of factors dictate the useful lifetime of the glass electrode membrane. High temperatures, frequent measurements in alkaline solutions, repeated etchings, and improper maintenance will reduce the electrode's lifetime, whereas proper maintenance will prolong the useful lifetime. The glass membrane will, however, deteriorate gradually even when stored dry. A standard glass electrode, will usually last for 12 months or approximately 365 uses.

Reference Electrode Care
The reference electrode must also be kept clean. In general, most electrode problems can be traced back to the reference electrode. Cleaning and rinse solutions are the same as for the glass electrode.

By providing good maintenance care to the electrodes, proper calibration should be able to be performed easily. If there is a continued problem, the electrodes should be replaced or examined again.

When performing a calibration with two buffers, stability should occur within approximately one minute in each case. The zero point and sensitivity should be written down after each calibration since a large deviation from one calibration to the next indicates a problem.

Conductivity Electrode Maintenance
The ETI range of Conductivity Meters include TDS Testers, Nutrient Tester, Salt Concentration Testers and WalkLAB Pro Conductivity Meters.

Various Calibration Solutions are available for the regular calibration of instruments
Storage: It is best to store cells so that the electrodes are immersed in deionised water. Any cell that has been stored dry should be soaked in distilled water for 5 to 10 minutes before use to assure complete wetting of the electrodes.

The single most important requirement of accurate and reproducible results in conductivity measurement is a clean cell. A dirty cell will contaminate the solution and cause the conductivity to change. Grease, oil, fingerprints, and other contaminants on the sensing elements can cause erroneous measurements and sporadic responses.

Electrodes can be cleaned using a mixture of 1 part by volume of isopropyl alcohol, 1 part of ethyl ether, and 1 part of HCl (1+1). After cleaning, thoroughly flush the cell with water. If the old platinum black coating to be removed, judicious application of aqua regia to the electrodes, or electrolysis in HCl is frequently successful. This should only be attempted by trained operatives.

Electrodes can also be cleaned with detergent and/or nitric acid (1%) by dipping or filling the cell with cleaning solution and agitating for two or three minutes. Rinse the cell several times with distilled or deionised water and remeasure the cell constant before use.

Dissolved Oxygen Electrode(s) Maintenance
ETI instruments include the WalkLAB Dissolved Oxygen meter and the Senz Redox (ORP) tester.

Always rinse the electrode in distilled water between each test.
The membrane must be kept wet at all times. When not in use the probe should be stored with the tip in a beaker of deionised water or for longer periods (overnight) in the protective sheath supplied, with a sponge in the bottom of the cap soaked in deionised water.

Before use, or after replacing a membrane (WalkLAB), ensure that the probe has been given sufficient time to polarise. Refer to the Instruction Manual for details.

Membrane replacement and electrolyte replenishment/replacement is recommended if readings become sluggish or the electrode does not respond.

Cleaning: In normal use it should only be necessary to rinse the probe in deionised water between each determination, however when using the electrode in liquors, sludge, or polymers, etc., a coating may be deposited on the membrane causing slow response or drifting. This can be reduced by rinsing in deionised water. In such samples more frequent membrane replacement may be required.

When replacing membranes ensure that no air bubbles are trapped in the KCl (electrolyte) fill solution, and check visually to see that the membrane is not damaged or creased when screwed on.