3 Ph Routine Maintenance Guide En

8/17/2019 3 Ph Routine Maintenance Guide En

1/17

Day-to-Day Routine Maintenance

of pH Meters and Sensors

M a i n t e n a n c e G

u i d e

Measurement Hints

Tips for Care

Calibration

System Check


2/17

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 t ransfer 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.


3/17

3METTLER TOLEDO pH and Routine Maintenance

C o n t e n t Content

1 Care and Measurement Technique 4

2 Calibration 8

3 Perform an Easy System Check 10

4 pH Electrode Troubleshooting 12

5 More Information 16


4/17


C a r e a n d

M e a s u

r e m e n t T

e c h n i q u

e 1. 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. 3

mol/L KCl), or in a pH 4 or pH 7 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 4 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] This InLab® storage solution can be ordered from METTLER TOLEDO (30111142)

[2] This thiourea solution can be ordered from METTLER TOLEDO (51340070)


5/17


C a r e a n d

M e a s u

r e m e n t T

e c h n i q u

e 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 inan 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 electrode in an ultrasonic bath with water

or a 0.1 mol/L HCl solution.

1.4 pH Electrode Regeneration and Lifetime

Even elect rodes 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, do 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.

• 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.

Figure 1: Temperature sensor of

an InLab® electrode

[1] [3] This regeneration solution can be ordered from METTLER TOLEDO (51350104)

[2] This pepsin solution can be ordered from METTLER TOLEDO (51340068)


6/17


C a r e a n d

M e a s u

r e m e n t T

e c h n i q u

e 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 climate-

controlled 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.

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 out-

flow when measuring TRIS-based buffers or proteinaceous samples. 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 buf fers – widely used in biologicalresearch ranging from molecular biology to histology – can damage standard pH equipment.

How does TRIS do 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-basedbuffers, 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

Figure 2: Temperature and

MTC indication on a pH meter


7/17


C a r e a n d

M e a s u

r e m e n t T

e c h n i q u

e Clean with RNase and DNase cleansers and autoclave to eliminate biohazardThe 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 significantly reduced.

Figure 4: pH electrode sterilization by autoclaving


8/17


C

a l i b r a t i o n 2. Calibration

A pH electrode needs to be calibrated regularly. It is recommended that you do 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 0 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.

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 do 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 3 calibration points. Most pH meters can do

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 do not calibrate directly in the original buffer solution bottle.

mV

pH

7

Theoretical behaviour(Slope –59.16 mV/pH, offset: 0 mV)

Offset correction➀

Slope and offset correction ➀+ ➁

▲

➀

➁

▲

▲


9/17


C

a l i b r a t i o n • 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.

Figure 6: Recommended calibration points and range

1312111098654321 7 14

∆pH ≈ 6

3 or more P

recommended e.g. pH 4, 7, 10

∆pH

≈ 1

1P

e.g.pH 7

∆pH ≈ 3

2P

recommended

e.g. pH 4 and 7

measured value withincalibration range

measured value outside ofcalibration range


10/17


P e r f o r m

a n E a s y

S y

s t e

m C

h e c k 3. 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 7 buffer. The reading should be 0 mV ±30 mV with an

Ag/AgCl reference. Next read a pH 4 or pH 10 buffer – the solution should be greater than 150 mV different f rom

the pH 7 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 shor ting clip (standard delivery) or Test Plug Set. If this

plug does not set the potential to 0 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 do 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.


11/17


P e r f o r m

a n E a s y

S y

s t e

m C

h e c k 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 4


12/17


p H E l e c t r o d e T r o u

b l e s h o o t i n g 4. pH Electrode Troubleshooting

The electrode is the key to successful analysis. Since the electrode is the only part of the analytical instrument

that is in direct contact with the sample, its selection and maintenance (and therefore sensitivity) has the

strongest influence on precise and accurate measurements. However, an electrode that has been correctly

selected and that has been working properly may nevertheless suddenly start performing badly. In this article

we will help you to identify possible reasons for this and suggest a number of procedures for restoring the

electrode to its original performance.

4.1 Before beginning the diagnostic procedure

Before testing an electrode, make sure that the electrode cable and the

instrument are working properly. Then examine the sensor closely. Visual

inspection can very often provide important clues about the cause of the

problem, e.g. a clogged diaphragm or an air bubble in the tip of the electrode.

In general, three procedures can be followed to restore an electrode to its

normal working state. First of all, the glass membrane can be regenerated,

secondly, the reference diaphragm may have to be cleaned 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

a) Diagnosis

One symptom may very often have various causes. The following table will

help you to find out what caused your electrode to fail with regard to the pH-

sensitive glass membrane:Figure 7: What can be fixed?


13/17



b l e s h o o t i n g

b) Regeneration procedure A reduced calibration slope as a result of changes in the gel-layer on the glass membrane can very of ten be

observed with older electrodes or electrodes that have been stored dry. Similar effects are noticeable when an

electrode is used for non-aqueous applications because the gel-layer is dehydrated. The pHsensitive glass

membrane is reactivated using a regeneration solution. This solution is a mixture of hydrochloric (HCl) and

hydrofluoric acids (HF).

Since these acids are extremely aggressive, make sure that you observe all the necessary safety

precautions, e.g. wear protective goggles, a laboratory coat and chemical-resistant gloves!

Cause

Symptom

Ageing ofmembraneglass

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?)

Reducedslope(>80% -


14/17


15/17



b l e s h o o t i n g

Table 2: Clogged reference diaphragm: Causes and cleaning procedures

Despite all the corrective procedures mentioned above, it is not always possible to repair a faulty electrode.

Damage that has occurred to the wiring or casing of an electrode during use, is irreparable. In such cases the

electrode must be replaced.

Type of

contamination

Cleaning agent Reaction time Remarks

Silver sulfide Thiourea 5 –60 mins. Leave until discolorationdisappears.

All possible types ofcontamination. Firstrecommendation for

removing unknownsubstances.

HCl 0.1 mol/L Approx. 12 hrs. Can also be used forinternal cleaning.

All possible types ofcontamination.Secondrecommendation forremoving unknownsubstances.

Chromic-sulfuric acidmixture

Approx. 30 mins. Also cleans deposits onthe membrane very well.Sensor must beregenerated after thisprocedure.

Proteins Pepsin / HCl cleaningsolution

> 1 hr. Can also be used forinternal cleaning.

Proteins NaOH 2% Approx. 20 mins.

Lipophilicsubstances

Ethanol, acetone Approx. 30 mins. Highly suitable for edibleoils. Possibly with support

of a soft brush.Calcium, scale Acetic acid Approx. 30 mins. Sensor must be

regenerated after thisprocedure.

Soaps, tensides Hot water (80° C) Approx. 12 hrs. Rinse sensor well with hot water. Afterwards, immersein hot water and leave tocool, approximately 12hours. Only use tap water,not distilled or de-ionised water.


16/17


M o r e I n f o r m a t i o n 5. More Information

5.1 Guides

A Guide to pH Measurement, Mettler-Toledo AG, 51300057, (2013)

Guide pour les mesures de pH, Mettler-Toledo AG, 51300185, (2013)

Anleitung zur Messung von pH, Mettler-Toledo AG, 51300058, (2013)

A Guide to Conductivity Measurement, Mett ler-Toledo AG, 30099121, (2013)

Guide des mesures de conductivité, Mettler-Toledo AG, 30099123, (2013)

Ein Leit faden für Leitfähigkeitsmessungen, Met tler-Toledo AG, 30099122, (2013)

Guía para la medición de la conductividad, Mettler-Toledo AG, 30099124, (2013)

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

We provide web-based seminars (webinars) on different topics. You can participate in on-demand webinars

at any convenient time and place.

Live webinars offer the added benefit of allowing you to ask questions and discuss points of interest with

METTLER TOLEDO specialists and other participants.

www.mt.com/webinars


17/17

For more information

Mettler-Toledo International Inc

Laboratory Division

CH-8606 Greifensee, Switzerland

Subject to technical changes

© 06/2015 Mettler-Toledo AG

Global MarCom Switzerland / MC

Learn more about Good Electrochemistry Practices program

www.mt.com/GEP

www.mt.com

Good Measuring Practices

Five Steps to Improved Measuring Results

The five steps of all Good Measuring Practices guidelines start with

an evaluation of the measuring needs of your processes and their

associated risks.

Using this information, Good Measuring Practices provide straight

forward recommendations for selecting, installing, calibrating and

operating laboratory equipment and devices.

• Preservation of the accuracy and precision of results

• Compliance with regulations, secure audits

• Increased productivity, reduced costs

• Professional qualification and training

Good Electrochemistry Practice™

Reliable pH measurements – thanks to GEP™

Documents

3 Ph Routine Maintenance Guide En