pH measurement – a basic introduction

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pH measurement – a basic introduction

An introduction to the theory behind pH and how to measure it. Measurement made easy

Why does pH need to be measured? pH is widely measured across a range of industrial applications. In basic terms, the measurement of pH gives an indication of the degree of acidity or alkalinity of aqueous solutions. It measures the level of hydrogen ion activity in a solution. The ratio of positively charged hydrogen ions to negatively charged hydroxyl ions decides the pH of a solution over a scale of 0 to 14pH. If the concentration is equal, i.e. it is neither acid nor alkaline, then the pH is said to be neutral, with a pH value of 7. The more positive ions that are present, the more acidic a solution becomes, with pH values less than 7. Conversely, the fewer hydrogen ions, the more alkaline the solution, with pH values greater than 7.

How is pH measured? A rough indication of pH can be obtained using pH papers or liquid indictors, which change colour as the pH varies. These indicators have limited accuracy; discrimination is difficult to interpret in coloured or murky samples. They are also unsuitable for any sort of on-line monitoring of pH or control. An electrochemical approach which has now become established for over fifty years, is based on a sensor known as a glass pH electrode. This electrode is used in conjunction with a second device known as a reference electrode to complete the electrical circuit. The basic pH measuring electrode arrangement is shown in Fig. 1.

pH Meter Glass electrode

Reference electrode Liquid junction tube

Glass electrode stem Internal element

Calomel Internal reference Silver element chloride

Internal filling solution

Internal filling solution

pH sensitive glass membrane

Porous junction

Fig. 1: Basic pH measuring system.

pH measurement – a basic introduction

The glass electrode The basic glass electrode comprises an inert glass stem, sealed to a glass bulb or membrane made from a special glass formulation responsive to hydrogen ions, i.e. pH. A chemical reaction takes place between a sample solution and the membrane surface, generating an electrical potential dependent on the pH of the solution. The mechanism involved is somewhat complex and is outside the scope of this paper, but it is sufficient to say that an ion-exchange process takes place between the hydrogen ions in the solution and the ions at the surface of the glass membrane. This develops a charge on the membrane surface which is then transferred through the membrane where it is picked up on the inner surface. The electrode chamber is filled with an aqueous internal solution (internal filling solution) of a known pH and containing chloride ions. A silver wire coated with silver chloride, called an internal element, is immersed in the internal filling solution. The purpose of the internal filling solution is to provide electrical continuity between the inner surface of the glass membrane and the internal element, thus providing an electrical connection to the ‘pH meter’.

The reference electrode To complete the electrical circuit, the reference electrode is used provide a return path to the sample solution. However, since this contact must be provided by electrochemical means, i.e. a metal immersed in a chemical solution, it is impossible to avoid generating an electrical potential in series with the potential developed by the glass electrode. It is essential that the reference electrode potential is very stable and is not affected by chemical changes in the solution. reference electrodes come in two forms: 1) Silver chloride – this is the most common type of electrode used for pH measurement today. 2) Calomel – this is based on mercury and although it was frequently used as standard in the past, it has now fallen from common use. 1) Silver chloride electrode The silver/silver chloride reference electrode (see Fig. 2) contains a chloridised silver wire immersed in a solution of potassium chloride (KCl), saturated with silver chloride (AgCl). This internal filling solution slowly seeps out of the electrode through a ceramic junction and acts as an electrical connection between the reference element and the sample. Potassium chloride is used because it is inexpensive and does not normally interfere with the measurement. The solution also

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includes silver chloride to prevent dissolution of the coating on the reference element. It is therefore necessary to maintain the level of solution in the electrode.

Filling solution hole

Reference chamber or Liquid junction tube

Silver chloride coated on silver wire

3.5 M KCl Saturated AgCl Liquid junction (porous ceramic plug) Fig. 2: Silver chloride reference electrode

2) Calomel reference electrode As Fig. 3 illustrates, the calomel electrode has a much more complicated construction than the silver chloride although the chemistry behind it is similar. It uses a mercury/mercurous chloride (mercury, mercury(I) chloride) element (known as Calomel) with a KCl internal filling solution. The slight variation in construction compared with the silver chloride electrode is because the mercury is liquid at room temperature and therefore has to be housed in a small glass tube.

Filling solution hole Reference chamber or Liquid junction tube KCl

Internal wire Glass tube Mercury

Ceramic junction Fig. 3: Calomel reference electrode.

Mercury/Mercury(I) chloride paste

pH measurement – a basic introduction

Electrodes also exist which combine both the glass and reference electrodes into the same assembly, usually by wrapping the reference chamber around the stem of the glass electrode. The performance of these ‘combination’ electrodes is generally the same except that the volume of the reference chamber is somewhat reduced, which in turn reduces the life of the electrode or requires more frequent topping up with filling solution. As well as the different types of reference electrode chemistries, various configurations of reference chamber arrangements also exist:

As shown, the reference electrode is available in many different arrangements and styles which reflects the difficulty of providing a stable return path reliably in most applications. Usually the reference electrode causes more problems than the glass electrode.

Electrode output The output from the pH ‘electrode pair’ is governed by a relationship between temperature and pH known as the Nernst equation. A simplified version is as follows: mV = 0.1985 x (°C + 273) x pH

– Refillable reference – This type of electrode (shown in both Figs. 2 and 3) is the most simple type of arrangement. It requires frequent ‘topping up’ to maintain the level of the internal filling solution. This electrode is mainly used in the laboratory, on a few industrial electrodes and in on-line ionselective monitors. – R eservoir fed reference – As the name suggests, a container filled with internal filling solution acts as a reservoir to bleed solution into a reference chamber. This greatly reduces the ‘topping up’ frequency. Furthermore, because it is possible to place the reservoir much higher than the sample (providing a large head pressure) the flow of solution through the junction may be increased. An increased reference solution flowrate helps to prevent blockage of the junction and prevent a contaminated sample from entering the reference chamber. – Sealed reference – This type of electrode has a sealed reference chamber, with a ‘liquid’ internal filling solution, or a slurry. The slurry slowly dissolves, maintaining the concentration of the filling solution, which is lost into the process via the junction. This electrode is more convenient because there is less maintenance, i.e. ‘topping up’, but once the solution is consumed or contaminated the electrode has to be thrown away. Some of these electrodes can be dismantled and refilled with fresh solution or slurry. However, due to the lack of flow of filling solution through the junction, the junction is more prone to contamination or blockage. – Double junction – These electrodes usually consist of a silver chloride sealed electrode with its own junction, fitted into a second chamber with a second junction in contact with the sample. The main advantage of this electrode is that the reference solution in the second chamber, usually just potassium chloride, can be chosen to be compatible with both the ‘inner electrode’ solution and the sample. This electrode can have a slurry-filled sealed outer chamber or a reservoir fed arrangement to suit the application.

This equates to the following electrode output for 1pH change in the sample at various temperatures: C

0

25

50

75

100

mV

54.20

59.15

64.12

69.08

74.04

o

As the pH scale covers a range of 0 to 14pH, at 25°C the full mV span is 14 x 59.15 = 818.1mV. In addition to the potential developed by the pH of the sample (VpH), there are other fixed internal potentials generated by both the glass (Vglass) and reference (Vref) electrode themselves, in series with the circuit. These are represented in the circuit shown in Fig 4. pH Meter

V

glass

V

pH

V

ref

Fig. 4: Potentials generated in the pH electrode pair.

pH measurement – a basic introduction | White paper

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pH measurement – a basic introduction

The three main sources of these potentials are: 1) B etween the glass electrode element and the internal solution. 2) O n the inside surface glass electrode membrane due to the pH of the internal solution. 3) B etween the reference element and the internal filling solution. These potentials are ‘manipulated’ by adjusting the composition of the internal filling solution of the glass electrode so that the output of the electrode pair is around 0mV at 7pH. Therefore, in practice, the mV output at 25°C will be as follows:

If the temperature variations are large, unacceptable errors are produced on the instrument display in the order indicated in Fig. 6. Electrode error, pH Units 0.4 0.3 0.2

15 10

0.1

5

0.0

0

–0.1

–5

–0.2

–10 –15

–0.3 –0.4 0

0pH

7pH

Temperature change from 25oC

1

2

3

4

14pH

5

6 7 8 pH Value

9 10 11 12 13 14

Fig. 6: Errors caused by change in the sample temperature.

+414.05mV

0mV

-414.05mV

The point at which the output of the electrode pair goes through zero is known as the Check Reading. This electrode parameter is determined during calibration by the instrument software (to be covered later). Tracking this value provides a useful indication of any zero drift, implying changes in the internal reference potential (Vref). As the internal chamber of the glass electrode is sealed, the value of Vglass is unlikely to change.

Temperature compensation As already indicated, the output of the electrode pair is affected by the temperature of the sample solution. Basically, as the temperature is increased, the output or slope of the electrode pair, i.e. mV/pH, also increases, see Fig. 5. Increase in slope with increase in temperature

Check reading

+ve mV

–ve mV

0pH

7pH

Fig. 5: Effects of temperature on the glass electrode slope.

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pH measurement – a basic introduction | White paper

14pH

To overcome these errors, pH meters have software designed to apply automatic temperature compensation, using a compensator electrode inserted into the sample.

Isopotential The graph shown in Fig. 5 assumes that changes in the sample temperature affects the slope (shown as a broken line) of the glass electrode only. However, there can be a shift in the reference electrode potential – see Fig 7. The intersection of the new line (shown as a dotted line ) and the original line (shown as a solid line), is known as the Isopotential Point. For the pH meter to compensate for the zero shift for a particular reference electrode, the Isopotential Point value needs to be entered into the instrument software so the necessary correction can be made. Generally, with the current trend to use silver chloride reference electrodes and where the element is close to the solution temperature, the Isopotential Point is fixed at 7pH. This removes the need to have adjustable values in the software scrolls thus simplifying the setting up of the instrument. However, some older electrode configurations, particularly Calomel reference electrdoes, require an instrument where the setting can be entered correctly. The setting will be given in the appropriate manual for the electrode pair, but generally for a calomel electrode where the element is positioned close to the sample temperature, the setting is 5.5. For the same type of element where it is remote from the solution, i.e. fitted into the reservoir, the value is 7.5.

pH measurement – a basic introduction

Buffer descriptions Isopotential point Zero drift

‘pH1’ DIN 19267 0.1M hydrochloric acid. ‘pH4’ BS 1647 0.05M potassium hydrogen phthalate. ‘pH7’  Mixture of disodium hydrogen phosphate and monopotassium dihydrogen phosphate. ‘pH9’ BS 1647 0.05M disodium tetraborate (borax). ‘pH10’ BS 1647 0.025M sodium hydrogen carbonate + 0.025M sodium carbonate.

Check reading

+ve mV –ve mV

0pH

7pH

14pH

Fig. 7: Effects of temperature on the reference electrode potential.

Electrode calibration The output of a pH ‘electrode pair’ is dependent on the individual ‘electrode pair’ and is subject to change over time, depending on the application. This necessitates the need for electrode calibration which is carried out with calibration solutions normally referred to as ‘buffer solutions’. A buffer solution is a chemical term for a pH solution that maintains a nearly constant pH despite the addition (possibly due to contamination) of small amounts of acid or alkali. Most stable solutions of known pH value can be used, but standard buffer solutions are given in Table 1 for different temperatures. C

‘pH1’

‘pH4’

‘pH7’

‘pH9’

‘pH10’

0

1.08

4.000

7.11

9.475

10.270

10

1.09

3.997

7.06

9.347

10.154

20

1.09

4.000

7.01

9.233

10.045

25

1.09

4.005

7.00

9.182

9.995

30

1.10

4.011

6.98

9.134

9.984

40

1.10

4.027

6.97

9.051

9.866

50

1.11

4.050

6.97

8.983

9.800

60

1.11

4.080

6.97

8.982

9.753

70

1.11

4.116

6.99

8.898

9.728

80

1.12

4.159

7.03

8.880

9.725

90

1.13

4.208

7.08

8.840

9.750

o

Conventionally two buffer solutions are chosen, spanning the pH range of interest, the most commonly used being the ‘4pH’ and ‘9pH’ solutions. Changes in the electrode output affect the slope, the zero (or Check Reading) or both, so regular calibration needs to be carried out throughout the life of the electrode. The frequency of calibration depends on the application but can be between several times each day in extreme or very critical applications, to once every several weeks where the application is ‘kind’ to the electrodes or where accuracy is not too important. It is also important to remember that an ‘electrode pair’ may drift or fail without warning. Calibration, at an appropriate frequency, is essential to maintain good performance and integrity of the measurement. Changes to the electrode response or performance can be due to many causes, but the more common are as follows: Low slope a) Coated glass membrane b) Aged glass electrode membrane High or low Check Reading a) Contaminated internal reference filling solution b) Contaminated porous reference junction Drifting readings a) High resistance or open circuit reference electrode b) Aged glass electrode membrane The offending electrode may require maintenance or replacement.

Table 1: Standard buffer solutions.

pH measurement – a basic introduction | White paper

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pH measurement – a basic introduction

Glass electrode impedance As already stated, the Glass Electrode produces the potential on the outer surface of the pH responsive membrane. The potential has to be transferred through the wall of the glass to the inner surface of the membrane which is in contact with the internal filling solution. Therefore, the glass wall is in series with the electrical circuit. Glass is usually considered as a near perfect insulator, so the formulation used for membrane glass is designed to have a resistance as low as possible whilst combining a good pH response, durability and mechanical strength. This cannot be achieved with one type of glass membrane formulation for all applications. So out of many possibilities, three different formulations are produced. The resistance of these membrane glasses at different temperatures is shown in Fig. 8.

Hi

4

gh

10

tem

pe

3

10

St

2

Resistance MΩ 10 (100mm Glass Membrane) 10

Maximum Impedance tolerated by the pH meter.

Lo

w

an

0.1 0

ure

res

rd e

an

ce

ele

ctr od

da

ist

1

rat

es

lec

tro

de s ctr od es

ele

10 20 30 40

Glass membrane resistance up to 109Ω

Voltage produced by glass electrode

R in pH meter input amplifier Fig. 9: Potential divider produced by the glass membrane resistance and the input impedance of the pH meter.

In practice the input impedance of the pH meter is usually 1013 to 1014Ω. By any standard, this is a massive input impedance for any amplifier and it is this that differentiates a normal digital mV meter (often having an input impedance of 10MΩ or in some cases 100MΩ) from a pH meter. The main implication of this very large source impedance of the electrodes, is designing a very special pH meter input circuit and maintaining the high insulation in all electrode terminations. This insulation has to be maintained not only between the glass and reference terminations but also between glass and earth (ground) as the solution is often at earth potential. The full electrical requirements are shown in Fig.10.

50 60 70 80 90 100

Temperature (oC) Fig. 8: Glass membrane resistance verses temperature.

The ‘standard’ membrane glass is used for the majority of purposes; it has a good response over 0 to 14pH and can be used between 0 and 100°C. The ‘low resistance’ membrane glass has a much faster pH response by a factor of 10 (because of its lower resistance) but it can only be used over a pH range of 0 to 10 and up to temperatures of 60°C. Finally, as the name suggests, the ‘high temperature’ membrane glass is used at temperatures between 50 and 130°C. As shown, for most purposes, the resistance of membrane glasses lies between 1 and 1000MΩ. One of the fundamental problems of pH measurement is the very high internal, or source impedance, which is in series with the input to the pH meter. If we set a maximum impedance of 1000MΩ (109Ω), to avoid any voltage drop across this impedance, almost all the potential is developed across the input impedance of the pH meter. Thus the pH meter input impedance needs to be at least 1000 times higher, that is 1,000,000MΩ (1012Ω) – see Fig. 9.

9

Max value of R glass = 1000MΩ (10 Ω) 4

Max value of R ref = 10kΩ (10 Ω) 13 R G-R +R G-E + R in > 10,000,000MΩ (10 Ω ) 8 R R-E > 100MΩ (10 Ω ) Resistance MΩ (100mm glass membrane)

Glass electrode connection

R glass VpH Earthed solution

Vref R ref

pH amplifier RG-R

R in

leakage

Reference RG-E leakage electrode connection

RR-E

leakage

Earth Fig. 10: Equivalent electrical circuit for a pH electrode pair.

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pH measurement – a basic introduction | White paper

pH measurement – a basic introduction

Low insulation in the electrode terminations is a very common problem, especially in damp conditions. However, it is vital that this problem is resolved if the system is to operate satisfactorily. It is interesting to note that the materials of construction that are use in many plugs and sockets and termination blocks are totally inadequate for pH use due to their ‘low’ insulation. Components of this type must be carefully chosen to obtain a high integrity of the measurement. The insulation of the electrode terminations cannot be measured directly due to the lack of commercially available insulation testers. However, the presence of low insulation can be determined by using a pH electrode simulator. This device is basically an mV source with the added feature of being able to switch in a 1000MΩ resistor into the glass electrode output lead, to simulate the impedance of the electrode. A pH simulator is therefore a vital piece of test equipment for any fault finding investigations on pH systems.

Solution temperature compensation As already described, the pH meter uses a sensor to measure the temperature and apply the necessary compensation for changes in the output of the electrode pair, both in terms of slope and zero offset. Most solutions change their pH value with temperature variation which may introduce discrepancies between the on-line and laboratory measurement, see Fig. 11. This should always be remembered where such disputes exist. 9.0 8.0

NaOH

7.0 6.0

H2O NaCl H2O DIST.

pH 5.0

3.0

HCl H2SO4 HCL

2.0

HCl

4.0

20 40 60 80 100 120 140 160 Temperature (oC)

For some applications, it is possible to refer the value to a reference temperature, such as 25°C, so removing the effect on the measurement of temperature change. This is only possible where the sample is of a fixed composition so the temperature coefficient is constant. One typical application is the measurement of boiler water in steam generation plant. Here the feed water is dosed with ammonia to control the pH, see Fig. 12 (where sodium hydroxide is used the coefficient is almost the same). 0.8 mgl –1 Ammonia solution 0.2 mgl –1 (-0.035pH/oC) 10

9 pH Buffer solution (0.05M borax) (-0.008pH/oC)

9.5

9.34pH 9.19pH 8.9pH

9 pH 8.5 8 7.5 0

10

20

30

40

50

Temperature (oC) Fig. 12: Effect of temperature on the pH of ammonia solutions.

This type of compensation has been incorporated into the design of several industrial pH meters for many years. If this option is required, it is necessary to select this in the software and enter the sample coefficient value, within the range of 0 to ±0.04pH/°C; the reference temperature is usually fixed at 25°C. One small complication, is that although the online pH system may incorporate sample compensation, very few laboratory instruments have this facility. It is therefore important that the laboratory test is made at either 25°C or the relevant correction needs to be applied.

Antimony pH electrodes There are some very aggressive applications where a glass pH electrode is attacked by the process and its use precluded. A common example is effluent containing hydrofluoric acid (HF), commonly encountered in the glass and semiconductor industries. HF etches the surface of glass membranes, eventually dissolving them away completely.

Fig. 11: Effect of Temperature on the Sample pH.

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pH measurement – a basic introduction

An alternative pH sensor can by used under these circumstances based on antimony. This electrode consists of a plastic body with an antimony billet fitted into the end. This is used with a conventional reference electrode. The surface of anitimony oxidises when first immersed in the solution, and it is this oxide layer that is responsive to pH. Athough this electrode can be useful in these difficult applications, there are a number of disadvantages which should be considered before being recommended: – The response to pH is poor and non-linear. – T he surface must be renewed by periodic scraping to remove the oxide layer. Type

Basically, an antimony electrode is a very poor pH sensor, so it should be considered as a last resort. It is worth noting that a solution of HF above 4pH does not have such a detrimental effect on glass electrodes. Under these circumstances it may be better to use a glass electrode which could offer a superior performance despite a relatively short life.

Description

Range

Operating temperature °C

°F

Impedance

Flat glass

Suitable for high density applications or processes where heavy fouling is expected

0 to 14pH

10 to 100

60 to 212

650MΩ

General purpose glass

For light to medium duty and lower temperature applications. Not for high pH

0 to 12pH

0 to 100

32 to 212

200MΩ at 25°C (77°F)

Ruggedized glass

Thicker glass in bulb area for rough handling and immersion in streams carrying abrasives. Not recommended for high pH applications

0 to 12pH

0 to 100

32 to 212

200MΩ at 25°C (77°F)

High temperature glass

Very versatile. Suitable for both high and low pH measurements, strong chemicals, and high purity water

0 to 14 pH

10 to 140

50 to 284

300 MW at 25°C (77°F)

Redox (ORP)

Platinum (Pt) electrode as active element

0 to ±2000 mV

0 to 140

32 to 284