Maintenance Guide Measurement Hints Tips for Care Calibration System Check Day-to-Day Routine Maintenance of pH Meters and Sensors Editorial Dear Reader, The determination of pH value, conductivity and related parameters such as ion concentration, resistivity, and salinity, are frequent and ordinary tasks in many labs Samples may originate from many different areas Sample composition, i.e solvent and major components, can differ considerably and cover a wide concentration range The most common solvent of pH and conductivity samples is water However, other solvents are used as well In addition, user needs in the lab reach from simple, manual determination to fully automated analysis systems including data gathering via software and other features To meet this array of requirements, a big number of methods exist and a wide variety of instrument solutions have been developed by meter and sensor manufacturers Standard methods are in use for numerous applications For special needs many dedicated solutions exist as well However, the vast number of possibilities can make selecting the right instrument and/or sensor cumbersome This guide provides some insights into meter and sensor characteristics and performance, enabling users to make better decisions and find the right instrument and electrode Tips and hints for sensor maintenance and care help to exhaust their usable life and achieve reliable results This wealth of information helps finding the most suitable instrument solution but as well measuring successfully each time METTLER TOLEDO Disclaimer This guide represents selected, possible application examples Examples have been tested with all possible care in our lab with the analytical instrument mentioned in the applications The experiments were conducted and the resulting data evaluated based on our current state of knowledge However, this guide does not absolve you from personally testing its suitability for your intended methods, instruments and purposes As the use and transfer of an application example are beyond our control, we cannot accept responsibility When chemicals and solvents are used, the general safety rules and the directions of the producer must be observed Content Content Care and Measurement Technique Calibration Perform an Easy System Check 10 pH Electrode Troubleshooting 12 More Information 16 METTLER TOLEDO pH and Routine Maintenance Care and Measurement Technique Care and Measurement Technique This section provides an overview of how to properly care for pH and conductivity sensors and some hints regarding measurement techniques In addition, the advantages of Intelligent Sensor Management (ISM®) and the testing of ultra-pure water are explained 1.1 pH Electrode Maintenance Regular maintenance is very important for prolonging the lifetime of any pH electrode Electrodes with liquid electrolyte need the electrolyte to be topped-up when the level threatens to become lower than the level of the sample solution This way a reflux of the sample into the electrode is avoided The complete reference electrolyte should also be changed regularly, e.g once a month This ensures that the electrolyte is fresh and that no crystallization occurs despite evaporation from the open filling port during measurement Be careful not to get any bubbles on the inside of the electrode, especially near the junction If this happens the measurements will be unstable To get rid of any bubbles, gently shake the electrode in the vertical motion like with a fever thermometer 1.2 pH Electrode Storage Electrodes should always be stored in aqueous and ion-rich solutions This ensures that the pH-sensitive gel layer which forms on the pH glass membrane remains hydrated and ion rich This is necessary for the pH membrane to react in a reliable way with respect to the pH value of a sample Short term storage In between measurements or when the electrode is not being used for brief periods of time, it is best to keep the electrode in a holder containing the special InLab® storage solution[1], its inner electrolyte solution (e.g mol/L KCl), or in a pH or pH buffer Ensure that the level of solution in the beaker is below that of the filling solution in the electrode Long term storage For long term storage, keep the electrode wetting cap filled with the InLab® storage solution[1] or, alternatively, with the inner electrolyte solution, pH buffer or 0.1 mol/L HCl Make sure that the filling port for reference and combination electrodes is closed so as to avoid loss of the electrolyte solution through evaporation, which can cause the formation of crystals within the electrode and junction Never store the electrode dry or in distilled water as this will affect the pH-sensitive glass membrane and thus shorten the lifetime of the electrode Although an electrode that has been incorrectly stored can be restored by regeneration procedures, following the above mentioned recommendations will ensure that your electrode is always ready to use Temperature sensors Rinse the temperature sensors after use and store dry in the packing box to prevent damage 1.3 pH Electrode Cleaning To clean the electrode, rinse it with deionized water after each measurement but never wipe it clean with a tissue The rough surface of the paper tissue will scratch and damage the pH-sensitive glass membrane removing the gel-layer and creating an electrostatic charge on the electrode This electrostatic charge causes the measured signal to become very unstable Special cleaning procedures may be necessary after contamination with certain samples These are described in greater detail below [1] [2] This InLab® storage solution can be ordered from METTLER TOLEDO (30111142) This thiourea solution can be ordered from METTLER TOLEDO (51340070) METTLER TOLEDO pH and Routine Maintenance Care and Measurement Technique Blockage with silver sulfide (Ag2S) If the reference electrolyte contains silver ions and the sample being measured contains sulfides, the junction will get contaminated with a silver sulfide precipitate To clear the junction of this contamination, clean it with 8% thiourea in 0.1 mol/L HCl solution.[1] Blockage with silver chloride (AgCl) The silver ions from the reference electrolyte can also react with samples that contain chloride ions, resulting in an AgCl precipitate This precipitate can be removed by soaking the electrode in a concentrated ammonia solution Blockage with proteins Junctions contaminated with proteins can often be cleaned by i­mmersing the electrode into a pepsin/HCI (5% pepsin in 0.1 mol/L HCl) solution for several hours.[2] Other junction blockages If the junction is blocked with other contaminations, try cleaning the e­ lectrode in an ultrasonic bath with water or a 0.1 mol/L HCl solution 1.4 pH Electrode Regeneration and Lifetime Even electrodes that have been well maintained and properly stored may start performing poorly after some time In such cases it may be possible to regenerate the pH-sensitive glass membrane and restore the electrode to its previous level of performance using an ammonium bifluoride regeneration solution[3] This regeneration solution is based on a highly diluted solution of hydrofluoric acid which etches away a very thin layer of the glass membrane, exposing a fresh ­surface area When using the regeneration mixture, not to leave the electrode in the solution for longer than 1–2 minutes or the whole pH-sensitive membrane will be corroded away and the electrode rendered useless The expected lifetime of a correctly used and maintained pH electrode is around one to three years Factors that contribute to a reduction of the lifetime of an electrode include high temperatures and measuring at ­ extreme pH values 1.5 Measuring pH – Temperature is a Critical Component pH results are only correct if the sample temperature is taken into account With these simple but effective rules for avoiding negative temperature effects, it’s easy to obtain accurate, reproducible results Automatic Temperature Compensation (ATC) ATC works best with normal-size samples • Use a sensor with integrated temperature probe and wait for a stable signal The meter automatically corrects the pH signal ATC works best in samples larger than 10 mL Figure 1: Temperature sensor of an InLab® electrode • Any “Pro” type InLab® sensor – InLab® Micro Pro, Science Pro, Expert Pro – has integrated temperature probes, eliminating worries over wrong temperature settings or not capturing temperature • For sensors without an integrated temperature probe, using a separate temperature probe is recommended [1] [3] This regeneration solution can be ordered from METTLER TOLEDO (51350104) [2] This pepsin solution can be ordered from METTLER TOLEDO (51340068) METTLER TOLEDO pH and Routine Maintenance Care and Measurement Technique Manual Temperature Compensation (MTC) MTC is extremely accurate, but can be time-consuming • If the temperature of your sample is known (you are working in a climatecontrolled room or the samples just came out of the refrigerator) enter this known temperature in the measuring settings of your instrument to correct the pH (or conductivity) signal • When measuring samples with different temperatures, MTC can be time consuming, because the setting must be changed with every temperature change Figure 2: Temperature and MTC indication on a pH meter Measure the sample, not your sensor With very small samples, the sensor can take so long to reach equilibrium that the sensor temperature is wrongly interpreted as the sample temperature The sample mass is negligible compared with the sensor mass, so take the time necessary to ensure that you actually measure the sample temperature Best practice is to keep the sensor with the sample Make sure temperatures match by storing the sensor with samples in the refrigerator or incubator, or at room temperature This guarantees the highest accuracy because the pH membrane, reference system and sample are at the same temperature 1.6 Contamination Control of pH Electrodes When measuring samples there is always the risk of contamination, either by sample carry-over or by microbiological or genetic contamination Conventional pH electrodes can also be damaged by electrolyte outflow when measuring TRIS-based buffers or proteinaceous s­ amples This is not the case when working with InLab® electrodes Avoid sensor contamination with TRIS buffers Accurate pH measurement is a key factor in buffer quality TRIS-based buffers – widely used in biological research ranging from molecular biology to histology – can damage standard pH equipment How does TRIS its damage? When measuring pH during TRIS buffer preparation, the reference junction on conventional pH electrodes can clog when TRIS reacts with silver ions in the fill solution This reaction can also occur with protein in the buffer, such as BSA (bovine serum albumin) The eventual result is slow or fluctuating readings, or even entirely wrong results InLab® electrodes by METTLER TOLEDO are specifically designed for compatibility with TRIS-based buffers, assuring reliable results and accurate buffer values The electrolyte in InLab® electrodes is guaranteed to be free of silver ions, eliminating the possibility of contamination Figure 3: SevenExcellence meter and InLab® electrode during calibration METTLER TOLEDO pH and Routine Maintenance Care and Measurement Technique Clean with RNase and DNase cleansers and autoclave to eliminate biohazard The pH electrode models InLab® Power, Power Pro, Viscous and Viscous Pro can be sterilized by autoclaving By cleaning the sensors with RNase and DNase decontamination solutions first, the potential for biological contamination is s­ ignificantly reduced Figure 4: pH electrode sterilization by autoclaving METTLER TOLEDO pH and Routine Maintenance A pH electrode needs to be calibrated regularly It is recommended that you this at least once a day before you start measuring In a calibration the slope and offset of an electrode are determined The theoretical slope and offset are given by the Nernst equation: E = E0 + 2.3RT / nF * log [H3O+] = E0 + 2.3RT / nF * pH Slope = 2.3RT / nF Offset = Should be mV at pH 7.00 The calibration is necessary to adjust the slope and offset of an electrode to their true values for the measuring system in question The calibration curve is then used to correlate the measured mV values of the electrode to the pH value of the solution measured mV ▲ Theoretical behaviour (Slope –59.16 mV /pH, offset: mV) Offset correction ➀ Slope and offset correction ➀ + ➁ ➁ ➀ ▲ pH ▲ Calibration Calibration Figure 5: Correlation between mV value measured by pH electrode and pH value in sample Curves shown are for the theoretical behavior, for offset compensated behavior and slope & offset compensated behavior Since an electrode is characterized by both its zero point and its slope, it is advisable to a minimum of a two point calibration for reliable measurements and better precision When measurements are performed over a large range of pH values it is recommended that one takes at least calibration points Most pH meters can 3–5 point calibrations It is important to note that one should only measure samples within the chosen region of calibration When calibrating an electrode, most pH meters request that you input the type of buffers which will be used There are several manufacturers of buffer solutions and the specifications of the most commonly used brands normally already come programmed as tables in the pH meters These tables cover groups of buffers for a range of temperatures In this way a whole group can be chosen at once allowing the temperature dependence of the individual buffers used for calibration, to be taken into account If no internal or external temperature sensor is used, ensure that you calibrate and measure at the same temperature In this case remember to manually input the temperature to allow the meter to perform the buffer temperature correction The buffers which are used for the calibration are very accurate solutions with a guaranteed value and precision To keep the buffer solutions suitable for calibrations for as long as possible after opening it is advisable that you follow these guidelines: • Mark the date of first use on the bottle of the buffer solution • Keep the buffer solution bottles tightly sealed at all times and use the decanted buffer immediately • Never return used buffer back into the original bottle or mix calibration standards from different manufacturers • Ensure that no contaminants enter the buffer solution bottle and always keep the bottle sealed • Store the calibration standard at ambient temperature • Do not store the bottles of buffer solution in direct sunlight • Clean the electrodes before calibration and not calibrate directly in the original buffer solution bottle METTLER TOLEDO pH and Routine Maintenance Calibration • Never use a calibration standard with an expired use by date or that you suspect is contaminated • Replace the buffer solution with a new bottle after it has reached its expiry date Always repeat the calibration after cleaning your electrode, after electrode maintenance, regeneration or long term storage of an electrode, as all these factors have an influence on the pH electrode potential 10 11 12 14 13 ∆pH ≈ or more P recommended e.g pH 4, 7, 10 ∆pH ≈ 2P recommended e.g pH and ∆pH ≈1 1P measured value within calibration range e.g pH measured value outside of calibration range Figure 6: Recommended calibration points and range METTLER TOLEDO pH and Routine Maintenance Perform an Easy System Check Perform an Easy System Check Locating the problem of a pH measuring system that has suddenly started performing badly is the first step to restoring it to its original level of performance 3.1 Where could the problem lie? With the meter set to read mV, dip the electrode into pH buffer The reading should be mV ±30 mV with an Ag/AgCl reference Next read a pH or pH 10 buffer – the solution should be greater than 150 mV different from the pH potential If not then test the following… 3.2 Application Are you using the right electrode for your application? There are different types of pH electrodes for special applications: non-aqueous, low conductivity, TRIS etc To make sure that you are using the right electrode visit the METTLER TOLEDO Sensor ProductGuide at: www.electrodes.net 3.3 Operator It is sometimes worthwhile to check the obvious: • Is the unit properly grounded or plugged into the wall outlet? • Are the electrodes plugged into proper terminals and seated firmly? • Is the meter properly calibrated with the correct buffers? Before taking a measurement, check that the wetting cap has been removed and that the side filling aperture is open Remember to rinse the electrodes before measuring a different buffer or sample 3.4 pH Meter Test the pH meter with the shorting clip (standard delivery) or Test Plug Set If this plug does not set the potential to mV, the meter may be the problem In this case call METTLER TOLEDO Service 3.5 Buffers Ensure that you are using the correct buffers in the correct sequence Always use fresh buffers Check expiry date 3.6 Cable and Connector Test your detachable cable by replacing it with an identical one If you not have a spare cable or are using a hard wired electrode, then check to see whether there is a change in the signal on the instrument when you bend the cable Inspect and clean all connectors including the meter socket If you are using an electrode with a MultiPin™ or S7 connector, make sure that they are free from KCl crystals or other deposits Dirty or corroded connectors lead to erroneous readings METTLER TOLEDO pH and Routine Maintenance 10 Perform an Easy System Check 3.7 Electrodes Visual inspection of the electrode can often provide important clues about the cause of the problem: Filling solution • Ensure that the electrolyte level is above the internal elements • Empty, rinse and refill the electrode reference chamber • Ensure that you are using the correct electrolyte as written on the electrode shaft or in the operating instructions, and that the electrolyte fill port is open Air bubbles • Check for air bubbles inside the electrode If some are present remove them by gently shaking the electrode downward or in the case of electrodes with gel electrolyte placing the electrode upright in warm water Blocked junction • see next chapter METTLER TOLEDO pH and Routine Maintenance 11 White pH Electrode Troubleshooting pH Electrode Troubleshooting The electrode the key toanalysis successful Since the only part The electrode is the key is to successful Since analysis the electrode is thethe onlyelectrode part of the is analytical instrument of the analytical instrument that is in direct contact with the sample, its selection that is in direct contact with the sample, its selection and maintenance (and therefore sensitivity) has the and maintenance (and therefore sensitivity) has the strongest influence on precise strongest on precise and accurate measurements However,that an electrode thatcorrectly has beenselected correctly andinfluence accurate measurements However, an electrode has been selectedand andthat that has been working properly may nevertheless suddenly start performing badly In this article has been working properly may nevertheless suddenly start performing article wereasons will help identify possible reasonsforfor this and we will badly help youInto this identify possible for thisyou and to suggest a number of procedures restoring the suggest a number of procedures for restoring the electrode to its original electrode to its original performance performance 4.1 Before beginning the diagnostic procedure Before testing an electrode, make sure the electrodeprocedure cable and the Before beginning thethat diagnostic instrument are working properly Then examine the sensor closely Visual inspection cantesting very often provide make important cluestheabout the cause of the Before an electrode, sure that electrode cable and the instrument are working properly Then examine the sensor closely problem, e.g a clogged diaphragm or an air bubble in the tip of the electrode Visual inspection can can verybeoften provide important about the In general, three procedures followed to restore an clues electrode to its cause of the problem, e.g a clogged diaphragm or an air bubble in the normaltip working state First of all, the glass membrane can be regenerated, of the electrode secondly, the reference diaphragm may have to be cleaned and thirdly, In general, three be followed restore an will electrode it may be necessary to procedures replace thecan electrolyte Thistolatter point not to its normal working state First of all, the glass membrane can be be discussed in this article because it forms part of the normal electrode regenerated, secondly, the reference diaphragm may have to be maintenance cleanedprocedure and thirdly, it may be necessary to replace the electrolyte This latter point will not be discussed in this article because it forms part of the normal electrode maintenance procedure 4.2 Glass membrane Glass membrane a) Diagnosis One symptom may very often have various causes The following table will a) Diagnosis help you to find out what caused your electrode to fail with regard to the pHOne symptom may very often have various causes The following table sensitive glass membrane: will help you to find out what caused your electrode to fail with regard to the pH-sensitive glass membrane: Figure What can Fig 17:What canbe befixed? fixed? METTLER TOLEDO pH and Routine Maintenance 12 pH Electrode Troubleshooting Cause Ageing of membrane glass Scratches on membrane Broken membrane or shaft Gel layer destroyed or dehydrated Dry storage of electrode Calcium on glass membrane (whitish film) Oil, fat or tar residues (visible?) Deposits of unknown substances (visible?) Due to High temperature, age of sensor Abrasion, solid particles, incorrect cleaning Mechanical or temperature shock Ion deficient media, nonaqueous applications Wrong storage Measurement media Measurement media Measurement media, no cleaning Procedures for restoring the electrode Regeneration (see next page) Sensor cannot be repaired Sensor cannot be repaired Rehydration with electrolyte or tap water Rehydration with electrolyte or tap water Dip sensor in conc acetic acid until deposits are dissolved Afterwards regeneration Clean sensor with degreasing agent, then rinse with water If necessary, regeneration Clean sensor with suitable cleaning agent If necessary, regeneration Symptom Reduced slope (>80% [...]... METTLER TOLEDO pH and Routine Maintenance 15 More Information 5 More Information 5.1 Guides A Guide to pH Measurement, Mettler-Toledo AG, 5 130 0057, (20 13) Guide pour les mesures de pH, Mettler-Toledo AG, 5 130 0185, (20 13) Anleitung zur Messung von pH, Mettler-Toledo AG, 5 130 0058, (20 13) A Guide to Conductivity Measurement, Mettler-Toledo AG, 30 099121, (20 13) Guide des mesures de conductivité, Mettler-Toledo AG, 30 0991 23, ... to that sensor 3 Fig 2 Regeneration Procedure Figure 8: Regeneration Procedure Reference diaphragm Diagnosis 4 .3 a)Reference diaphragm Faulty electrodes often have a clogged diaphragm In most cases, the diaphragm is visibly discolored a) Diagnosis Some are black, others are gray or just an off-white color Sometimes the discoloration is so faint that it is only visible when ceramic tube is diaphragm viewed... mesures de conductivité, Mettler-Toledo AG, 30 0991 23, (20 13) Ein Leitfaden für Leitfähigkeitsmessungen, Mettler-Toledo AG, 30 099122, (20 13) Guía para la medición de la conductividad, Mettler-Toledo AG, 30 099124, (20 13) A Practical Guide for Life Scientists – pH and Conductivity, Mettler-Toledo AG, (2014) Selected Water Analysis Methods, Application Brochure 37 , Mettler-Toledo AG, 51725072 (2007) 5.2 Webinars... regard to the pHOne symptom may very often have various causes The following table sensitive glass membrane: will help you to find out what caused your electrode to fail with regard to the pH- sensitive glass membrane: Figure What can Fig 17:What canbe befixed? fixed? METTLER TOLEDO pH and Routine Maintenance 12 pH Electrode Troubleshooting Cause Ageing of membrane glass Scratches on membrane Broken membrane... analytical instrument of the analytical instrument that is in direct contact with the sample, its selection that is in direct contact with the sample, its selection and maintenance (and therefore sensitivity) has the and maintenance (and therefore sensitivity) has the strongest influence on precise strongest on precise and accurate measurements However,that an electrode thatcorrectly has beenselected correctly... Rehydration with electrolyte or tap water Rehydration with electrolyte or tap water Dip sensor in conc acetic acid until deposits are dissolved Afterwards regeneration Clean sensor with degreasing agent, then rinse with water If necessary, regeneration Clean sensor with suitable cleaning agent If necessary, regeneration Symptom Reduced slope (>80% ... METTLER TOLEDO pH and Routine Maintenance 15 More Information More Information 5.1 Guides A Guide to pH Measurement, Mettler-Toledo AG, 5 130 0057, (20 13) Guide pour les mesures de pH, Mettler-Toledo AG, 5 130 0185,... specific to that sensor Fig Regeneration Procedure Figure 8: Regeneration Procedure Reference diaphragm Diagnosis 4 .3 a)Reference diaphragm Faulty electrodes often have a clogged diaphragm In most... electrode maintenance, regeneration or long term storage of an electrode, as all these factors have an influence on the pH electrode potential 10 11 12 14 13 pH ≈ or more P recommended e.g pH 4,