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pH Electrode Maintenance

The entire glass 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 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. 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.

Dry storage is recommended if the electrode will not be used for two weeks or more. Before use, the electrode should be soaked well.

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.

In order to establish a stable, hydrated glass membrane, new or dry-stored 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 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 dictate the need for 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. Soaked 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.

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, the electrode should first be rinsed with a solvent which is miscible with both water and the solvent before rinsing with water.

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 and is highly individualistic. 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, whether a monoprobe or a combination electrode, will usually last for 12 - 18 months.

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.

The electrode must always be filled with the salt bridge solution. For most electrodes, saturated potassium chloride, KCl, is normally used. The presence of KCl crystals in the salt bridge assures saturation.

In certain cases, the salt bridge solution may be different than saturated KCl, depending on the special nature of the reference electrode, i.e., the need for a chloride free solution or for non-aqueous solutions. In these instances, potassium sulfate or lithium chloride solutions will be used, respectively. When reference electrodes with a double salt bridge are used , saturated potassium chloride is normally found in the inner salt bridge, while the outer salt bridge contains a suitable salt of high concentration, such as KNO3, NH4NO3, or Li-Acetate.

The reference electrode should preferably be stored in a small beaker containing the salt bridge solution for short term storage. For long term storage, the electrode should be rinsed, dried, and stored with the end cap on and the rubber band covering the filling hole in place.

The solution in the salt bridge should always be on a higher level than the solution to be measured, as infiltration of the sample solution may occur in the salt bridge, that is, the direction of flow should always be from the salt bridge into the sample solution. If this cannot always be achieved, monthly changing the salt bridge solution should be the norm.

Annular ceramic junctions and porous pin junctions can occasionally become blocked due to crystallization of the salt bridge filling solution. Soaking in the salt bridge solution usually remedies the situation, but, on those instance that it doesn't, raising the temperature to the maximum allowable for the reference system will often help. A precipitate of silver chloride or sulfide may clog the porous pin. The gentle use of an abrasive paper, e.g., emery cloth, will remove most blockages. Soaking the porous pin for a few hours in acidic solution of thiourea ( 1 M thiourea in 0.1 M HCl ) will usually do what the abrasive paper sometimes fails to do, that is, chemically clean the blockage.

Trapped air bubbles can also cause malfunctions. These bubbles can be removed by gently tapping the electrode or shaking it downward as one would a clinical thermometer.

The useful lifetime of a reference electrode depends on the maintenance and care given the electrode. The electrode should never be allowed to dry out, the junction should be kept clean, and the salt bridge should always be filled to the level intended by the manufacturer. With proper maintenance, the lifetime for a reference electrode is indefinite, but usually greater than two years.

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.

ISE Electrode Maintenance

Because there are many different types of Ion Selective Electrodes (Glass Bulb, Solid-State, Gas Sensing, etc.), for many different ions, it is recommended to follow the recommendations and procedures included in the user's manual for the specific ISE electrode in question. This section may be updated to include each electrode's maintenance procedure in the future.

Conductivity Electrode Maintenance

Storage: It is best to store cells so that the electrodes are immersed in deionized 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 erronious measurements and sporadic responses.

Electrodes can be cleaned using a mixture of 1 part by volume of iospropyl 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 aplication of aqua regia to the electrodes, or electrolysis in HCl is frequently successful.

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. Other diluted acids (sulfuric, hydrochloric, chromic) may be used for cleaning except for the aqua region. For a stronger cleaning solution, try concentrated hydrochloric acid mixed into 50% isopropanol. Rinse the cell several times with distilled or deionized water and remeasure the cell constant before use.

Electrodes can be replatinized by preparing a solution of 0.025 N HCl with 3% Chloroplatinic Acid (H2PtCl6) and 0.025% lead acetate. Connect the cell to a 3-4 Volt battery to which a variable resistor (varistor) has been connected. Immerse the cell in the chloroplatinic acid solution and electrolyze at 10 mA/cm for 10-15 minutes. Reverse the polarity every 30 seconds until both electrodes are covered with a thin black layer. The deposit should present a black, velvety appearance and should adhere well to the electrode surface. Disconnect the cell and save the platinizing solution. It may be reused many times and should not be discarded as it is expensive to make. Rinse the electrode for 1 to 2 minutes, followed by distilled or deionized water. Store in distilled or deionized water until ready for use.

Dissolved Oxygen Electrode(s) Maintenance

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 deionized water or for longer periods (overnight) in the protective sheath supplied, with a sponge in the bottom of the cap soaked in deionized water.

Before use, or after replacing a membrane, ensure that the probe has been given sufficient time to polarize, refer to the Instruction Manual for the probe for details.

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

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

If after some time the electrode response becomes sluggish or eratic even with a new membrane, it may be necessary to clean the cathode and anode to restore normal operation. To do this, remove the membrane assembly and discard it. Lay a very fine abrasive material (crocus paper) on a perfecly flat and smooth surface, hold the electrode vertically on the abrasive and make small circles to polish the surface of the (gold) cathode. Silver (anode) may be cleaned with a tooth brush dipped in a dilute ammonia solution. Rinse thoroughly with deionized water and reassemble with a new membrane (and electrolyte if required).

 

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