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pH measurement equipment is used for a host ofapplications across a variety of industries. Getting thebest from this equipment requires consideration of arange of factors to achieve optimum efficiency and costeffectiveness. Mike Modla, Product Manager, ABBLimited, offers a selection of tips on how to get thebest performance from your pH equipment.

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The measurement and control of pH – the degree ofalkalinity or acidity of a liquid or solution – is instrumentalin many processes throughout industry. It covers adiverse range of applications, from checking andmaintaining product quality in the chemical,pharmaceutical, food and beverage industries throughto helping water industry users ensure they meetregulatory limits for acidity and alkalinity whendischarging water.

In basic terms, pH is a measurement of the relativeamount of hydrogen and hydroxide ions in an aqueoussolution. It is an electrochemical measurement usingmeasuring and reference electrodes and an analysisand display unit for calculating and displaying pHreadings. These systems may be standalone or formpart of a more sophisticated control system to ensurethat pH is maintained at a certain level.

The aggressive nature of many pH measurementapplications means that periodic maintenance andchecking are required as a matter of good practice toensure continued accuracy. This should be understoodwhen specifying a pH system.

Keeping a pH system in good working order requiresadditional expenditure, for example on buffer solutionsto help recalibrate the sensor electrodes.

The following is a collection of hints that can help youto optimise the performance of your pH monitoringsystems.

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There’s no such thing as a ‘universal’ sensor suitablefor measuring everything, be it pressure, flow, humidityor pH. Instead, where pH is concerned, a range ofversions is available, which varies according to theapplications concerned. Some examples of the typicalvariations on offer include:

� High temperature glass

High temperature applications can degrade generalpurpose pH sensors. In particular, premature ageing ofthe sensor glass can reduce both the accuracy of thesensor and its overall service life. The solution is to usesensors made from special ly formulated hightemperature glass. These sensors are ideal where theprocess temperature is 90˚C or higher, making themsuitable for heavy process applications in the pulp andpaper, pharmaceutical and chemical industries.

� Low temperature glass

Sensors made from low temperature glass provide thebest speed of response for measuring pH in applicationswith temperatures from 15˚C down to below zero. Theyare ideal for use in municipal and industrial wastewaterapplications, particularly in cold climates.

� Flat profile glass

Flat profile glass sensors offer a self-cleansing solutionfor applications such as in the pulp and paper industrywhere high levels of particles are present which couldfoul the sensor. However, they are only able to self-cleanse if mounted in line at an angle of 90˚ to a uni-directional fast flow, making them unsuitable for dip-type measurement applications with varying,multidirectional flow.

� Bulb glass

Bulb glass sensors are the prime choice for anyapplication up to 140˚C and 10 bar g. Their robustconstruction makes them suitable for in-line, dip andretractor type installations in a variety of industries, frommunicipal through to heavy duty chemical processing.

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� Reference type

Solid reference

While for many applications a simple gelled referenceis adequate, solid reference electrodes provideadditional protection. These types of sensors offerexcellent low maintenance by preventing the ingressof ‘poisons’ in the sample process liquid that couldattack and destroy the sensor reference electrode. Asolid gel or potassium chloride (KCl) impregnated woodprovides a barrier preventing contact between thesample liquid and the reference electrode, reducing therisk of contamination and greatly prolonging sensor life.

Solid electrode sensors are ideal for most industrial andmunicipal waste water applications, where high levelsof sulphides are present that could contaminate thereference electrode of a standard pH sensor. They arealso ideal for use in pressurised environments such astanks and pipelines. Their one main drawback, however,is their limited life in pure waters. For that a flowingreference electrode is recommended.

Flowing reference

Flowing reference electrodes are the best choicewherever pH monitoring is required for high purity waterapplications, such as steam-raising for power plantsor for use in semiconductor manufacture. The inherentlyaggressive nature of high purity water applications withtheir low ion concentration can quickly leach away thepotassium chloride filling solution in solid electrodesensors, rendering them ineffective.

Flowing reference sensors overcome this problem byusing a liquid filling which flows to the areas depletedby attack. A separate liquid-filled reservoir also enablesthe sensor to self-fill. Provided that this reservoir isperiodically topped up, a flowing reference sensor cancontinue to operate indefinitely.

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Installing your pH sensor where it can be easilyaccessed will reduce the effort required whenevercalibration, checking or occasional replacement isneeded.

pH sensors can be installed and operated in severalways, each offering their own set of advantages anddisadvantages.

For an immersion-type installation, keeping the dip-tubeshorter than two metres will make calibration andreplacement a lot easier. A flow cell in a bypass line,where the sample is diverted from the main line, offersmany advantages. If mounted at ground level, thebypass provides easy access to the sensor, as well ashelping to minimise cable lengths. Constructing abypass can, however, add to the cost of installation.

A final alternative is to use a ‘hot-tap retractor’, mounteddirectly into the process line. As well as enablingmeasurements to be performed virtually anywhere, thismethod also allows self-cleaning flat glass sensors tobe used to best effect, greatly reducing fouling even inhigh consistency pulp & paper lines.

For any method of installation, locating the transmitterand sensors close to each other will make it easier tocheck and calibrate the system.

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Exposure to air can dry out pH glass and form crystallinedeposits at the reference junction, dramatically reducingthe sensor’s service life. For this reason, sensors shouldnever be installed at the top of a pipe, as a half-emptypipe will not permit direct contact with the process. Toavoid the sensor drying out, it should always bemounted where it is constantly wetted. A good idea isto install the sensor in a u-bend, which will ensure thata sample is always captured even if the line goes dry.

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The frequency of calibration really depends on whetheryou think there is any need for adjustments. In manycases, adjustments are unnecessary if there is adifference of less than 0.2 pH between a samplemeasurement and the process pH meter.

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All pH systems should always be calibrated before use.This requires the pH measurement cell to be calibratedwith a solution with a known pH value. However,calibration does have its own peculiarities, beingaffected by a range of different factors, of whichtemperature is the most important. Just because thebuffer bottle says 9.18 pH doesn’t mean it actually is!Remember, unless the buffer is maintained at anambient temperature of 25°C, its pH will vary. At 0°C,for example, its pH will rise to 9.46.

To compensate, make sure you’ve set the instrumentto the buffers you’re actually going to use. Most modernpH meters will have built-in buffer and temperaturetables and will be able to automatically compensatefor temperature variations. To ensure an identicalmeasurement standard, these tables are based onvalues developed by national standards laboratoriessuch as BSI (British Standards Institute), DIN (DeutscheInstitute für Normung) or NIST (National Institute ofStandards and Technology).

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Beware of variations in laboratory samples whencomparing with the process. Neutral or mild alkali, high-purity waters, for instance, will dissolve CO2 from theair on the way to the lab, resulting in a drop in pH.Ideally, these types of sample should be transported ina sealed polyethylene container. Better still, themeasurement should be made as near as possible tothe process.

The pH of laboratory grab samples can also be affectedby variations in temperature caused by the samplecooling on the way to the laboratory.

Beware also of taking pH measurements fromprocesses where chemical reactions are taking place.In a scrubber using lime for pH control, for example, ifa sample is taken early in the process its pH could differfrom the value of an in-line sample taken later on. Thisoccurs because the measurements have been madeat different stages in the reaction process.

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In-line sensors measure at up to 140°C so may needtime to cool to calibration temperature. This could takequite a while unless using a fast acting temperaturesensor with balanced pH and reference electrodesoffering similar temperature responses, such as ABB’snew AP120 sensor. If you’re unsure, it is alwaysadvisable to wait before attempting a calibration.

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Calibration is achieved by using two different pHstandards or buffers or by comparison to a grabsample. In most applications, calibrating using the firstmethod is fine and will present no difficulties. However,in some processes, relying solely on buffers can resultin incorrect readings.

Consequently, buffer calibration should only be astarting point, followed by one-point grab samplecalibrations. In this type of calibration, the sensor isallowed to acclimatise to the process and a sample ismeasured with a high quality lab sensor, with theresulting value being used to calibrate the process pHmeter.

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Over recent years, changes have occurred in chemicalusage. Factors such as the introduction of new processtechniques, environmental legislation and a generaltrend towards increased process temperatures haveresulted in some users seeing discrepancies in pHvalues from the real process compared to laboratoryand historic sample values. An example is laboratorysamples from pulp & paper mills, which are often basedon temperatures some 5 to 50°C lower than the actualprocess temperature. One method of tracing the causeof such variations is to make a note of the sample andprocess temperatures, as illustrated in the below table.

In the example shown, logging the change withtemperature reveals a correction factor of –0.029 pHper °C, which needs to be entered into the meter’ssolution temperature compensation facility.

etaD emiT HpelpmaS noitcerroCrotcaF

detcerroCHpelpmas

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Hp Cº Cº/Hp920.0– Hp Hp Cº

8–2 00:80 53.11 Cº25 76.0– 86.01 76.01 Cº57

00:01 49.01 Cº26 92.0– 56.01 36.01 Cº27

00:21 66.11 Cº64 09.0– 67.01 37.01 Cº77

00:41 32.11 Cº65 25.0– 17.01 96.01 Cº47

00:61 44.11 Cº15 37.0– 27.01 17.01 Cº67

rorrerosnestnerappAHp66.0~

rorrerosneslautcAHp20.0~

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Up to half of industrial pH applications benefit from somesort of cleaning regime. The simplest way to ensurereduced contamination is to use a flat glass sensor, thebenefits of which were outlined earlier.

This type of sensor needs cleaning much less often. Inpulp stock applications, for instance, changing from abulb to a flat glass sensor could extend periods fromevery three days to every third week.

The requirement for manual cleaning can be furtherreduced by using sensors with an automatic cleaningcapability. These sensors use a jet wash systemcomprised of a cleaning solution, which is controlledby the pH transmitter. The type of cleaning solution useddepends on the conditions of the application. In manycases, ordinary water will be sufficient. For crystallinedeposits, carbonates, metal hydroxides, cyanides andheavy biological coatings, a mild acid may be required,whereas an alkaline detergent or a water soluble solvent,such as alcohol, would be sufficient for grease and oils.

Failure to regularly clean a sensor can result in excessivefouling, reduced accuracy and a shortened service life.If a chalky film is seen on the sensor glass, the sensorshould be wiped down with a clean cloth and somedistilled water. If the film remains, a more astringentcleaning solution, such as isopropyl alcohol, should beused.

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These easy to follow guidelines should help youmeasure pH accurately and keep your sensors in goodworking order, thereby reducing costs while increasingyields, maintaining product quality and reducingemissions. Although pH sensors and monitoringsystems themselves are not complex, their successfuluse requires their performance to be monitored, as wellas a commitment to proper and regular maintenance.

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ABB LimitedOldends Lane, StonehouseGloucestershireGL10 3TAUKTel: +44 (0)1453 826661Fax: +44 (0)1453 820671

ABB has Sales & Customer Supportexpertise in over 100 countries worldwide

www.abb.com

The Company’s policy is one of continuous productimprovement and the right is reserved to modify the

information contained herein without notice.

Printed in UK (06.04)

© ABB 2004