pH - MEASUREMENT

pH: -

It is a convenient measure of acidity / alkalinity of a aqueous solution at a specific temperature.

It is measured on a continuous scale from 0 to 14

In a chemical laboratory, pH is commonly measured using an electronic pH meter with a scale 0.01 pH meter

pH - MEASUREMENT

WHY IS pH MEASURED?

To test a sample against a legal requirement

To test a chemical against a specification As a part of analytical method

Process control in chemical industry

Environmental monitoring of waste and effluents

Monitoring and controlling biochemical reactions, many of which only take place in a particular and sometimes narrow pH range.

pH - MEASUREMENT

PRINCIPLES: -

The pH value of a given solution is a measure of the activity of the hydrogen ion (H+) in that solution

The scale is logarithmic In a solution of pH 5, the hydrogen ion activity is 10 times

higher than it is in a solution of pH 6 In aqueous solutions, the actual species is the hydronium ion

H3+, rather than the hydrogen ion H+

pH = log10 1/a H+ = -log10aH+

Activity of a species is a measure of its effective or available concentration rather than its actual or total concentration.aH+ = γ x cH+

γ = activity of coefficient

pH - MEASUREMENT

PRINCIPLES: - In dilute solution (< 0.001 mol) of simple univalent electrolyte γ

approximate to unity and activity and concentration are approximately equal

In other type of aqueous solution, there is no easily determined relationship between activity and concentration

From the theoretical, thermodynamic view point, the activity of single ion species such as H+ is an exact quantity

pH - MEASUREMENT

PRINCIPLES: -

pH value to be regarded as convenient and comparitative measure of acidity

pH is usually determined by electrochemical measurement, in which the potential of a pH electrode immersed in the test solution is measured.

The pH electrode respond quantitatively and specifically to hydrogen ions even in the presence of other positive ions

pH - MEASUREMENT

ELECTROMETRIC DETERMINATION OF pH

The pH measurement is possible because emf of certain chemical cell varies with the hydrogen ion concentration of the solution on the cell.

pH electrode/test solution to be measured// reference electrode symbol//signifies the presence of a liquid junction between test solution and reference electrode

If other variables in the cells are controlled, emf of the cell can be correlated with pH

Pt, H2 (p) \H+ (a) \ reference electrode.- left hand electrode is the hydrogen electrode- right hand electrode whose potential is not affected by pH

pH - MEASUREMENT

ELECTROMETRIC DETERMINATION OF pH

Ecell = EH2, H+ + Eref

Ecell - Eref = EH2, H+

Ecell - Eref = RT/nF x 2.303 log10 aH+

R = gas constant (8.314 joule k-1mol-1)T = Absolute temperature (Kelvin)F = A electrochemical constant (96487 coulomb mol-1)N = 1 for univalent species such as H+

Ecell - Eref = 0.0591 pH

pH - MEASUREMENT

ELECTROMETRIC DETERMINATION OF pH

Theoretical slope of a pH electrode is such that change in pH of 1 unit results in a change in the potential (E) of the electrode of 59.1 mV at 25°C

Mathematically Delta E / Delta pH = 59.1 mV The value of ERef include the following- The standard potential of the pH electrode (i.e. the potential

when a H+ = 1)- The potential of the reference electrode- The liquid junction potential- The asymmetry potential

pH - MEASUREMENT

ELECTROMETRIC DETERMINATION OF pH

The value of Eref cannot be determined in routine measurements mainly due to the indeterminate

- Nature of the liquid junction potential- Asymmetry potential- Value of asymmetry potential tends to drift as the condition of

the electrodes changes- The equation not used in a basic way to determine pH of a

solution- The response of the glass electrode is calibrated using

standard aqueous buffer solutions with known reference pH values. Reading of pH meter adjusted so that they correspond

- to the reference values.

pH - MEASUREMENT

ELECTROMETRIC DETERMINATION OF pH

The pH of a test solution may then be measured pH = -log a H+

pH measurement reflects the activity rather the concentration of hydrogen ion

pH - MEASUREMENT

BUFFERS:-

The resistance of a solution to change in Hydrogen ion concentration upon the addition of small amounts of acid or alkali is termed as buffer action and a solution which posses this property is called buffer solution.

The reference pH values for primary pH standards are established by high accuracy potentiometric measurements

Using specially designed electrochemical cells and platinum-hydrogen gas electrode rather than a glass electrode, as the H+ - sensing electrode

Avoids error in potential measurement arising from liquid junction potential and the asymmetry potential

pH - MEASUREMENT

BUFFERS:-

Enables the pH of the standard solution to be determined from the Nernest equation.

Such specialized measurements done at NIST NIST supplies a number of high purity salts as standard

reference materials for pH Each of which has a certificate Detailed instructions of preparation and use of solution The pH values certified to 3 decimal places with typical

uncertainties of ± 0.005 pH unit at a range temperature Enables pH meters to be calibrated in a manner that is

traceable to a services of internationally recognized standards

pH - MEASUREMENT

HIGH PURITY SALTS USED AS PRIMARY pH STANDARD

Note: The uncertainty of the tabulated pH values are estimated to be ± 0.01

High purity salts Concentration/g/L pH at stated temperature

15°C 20°C 25°C 30°C

Potassium tetraoxolate 12.61 1.67 1.68 1.68 1.68

Potassium hydrogen phthalate 10.13 4.00 4.00 4.01 4.02

Potassium di-hydrogen phosphate

Di sodium hydrogen phosphate

3.39

3.53

6.90 6.88 6.87 6.85

Sodium tetra borate decahydrate

3.80 9.28 9.23 9.18 9.14

Sodium hydrogen carbonate

Sodium carbonate

2.09

2.64

10.12 10.06 10.01 9.97

pH - MEASUREMENT

BUFFERS:-

Buffer solutions to be prepared using salts of the highest purity available

Certain of the salts should be dried before use Potassium hydrogen phthalate- Dry at 110°C for one hour Potassium di-hydrogen phosphate - Dry at 110°C for one hour Disodium hydrogen phosphate - Dry at 110°C for one hour Sodium carbonate – Dry at 270°C for one hour Carbon dioxide-free distilled water to be used to prepare the

solutions Important for those buffer solution with a pH > 6

pH - MEASUREMENT

BUFFERS:-

Prepared solutions to be stored in a well stoppered Pyrex or polythene bottles

Normally to be replaced after 2 to 4 weeks and sooner if mould or sediment is observed

pH - MEASUREMENT

GLASS ELECTRODE

A – Glass bulb

B – Tube filled with 0.1 mol HCl

C – Silver-Silver chloride

D – Saturated KCl solution saturated with AgCl

E – Silver-Silver chloride electrode

pH - MEASUREMENT

GLASS ELECTRODE

Potential is developed in an aqueous solution is proportional to hydrogen activity or pH of the solution

Hydrogen ion in the solution forms a dynamic equilibrium with hydrogen ions that are “bound” to the membrane surface in ion exchange type process, thereby establishing a potential across the membrane

Magnitude of potential is proportional to the pH of the solution

pH - MEASUREMENT

CARE OF pH ELECTRODES

Manufacturer’s instructions to be consulted for specific guidance on particular electrodes and situation

For reliable pH measurement, pH electrodes are properly stored and maintained

Response of the pH electrode to hydrogen ion activity depends acutely on such matters as:

- Cleanliness and condition of the glass membrane- Conditions of the reference electrode liquid junction- Conditions of reference electrode and its filling solution- Cleanliness of the electrode body

pH - MEASUREMENT

CAR E OF pH ELECTRODES

On exposure to air and allowing to dry out, glass membrane becomes dehydrated and liquid junction may also deteriorate

pH electrode should be stored in an appropriate storage solution, when not in use

Mixture of different salts of similar composition to the reference electrode filling solution, a buffer to provide suitable pH(e.g.4 to7) and mild cleansing agent

Electrode to be immersed to a sufficient depth to cover both the glass membrane and the liquid junction

A pH electrode should not be stored in water Periodically the level of the filling solution in reference electrode

should be checked and topped-up as necessary

pH - MEASUREMENT

CARE OF pH ELECTRODES

When not in use, the filling hole of the reference electrode should be covered with the plug provided for this

If pH electrode has been left in air and dried out, it should be re-hydrated by immersion in the storage solution for at least 12 hours

A pH electrode should be regularly examined for salt crystal build up and membrane and liquid junction deposits

For additional cleaning soak the pH electrode in either 0.1 molar HCl or 0.1 molar Nitric acid

pH - MEASUREMENT

SELECTION OF pH ELECTRODE:

For pH measurements in the range 1 to 10, general purpose glass electrodes suitable

When the concentration of alkali metal ions become large these ions seems to permeate the membrane, resulting in a significant errors in the measured pH

This is referred to as ‘alkaline error’ or a negative error in pH is observed

New glasses rich in lithium have been introduced and gives reliable results

At very low pH values the activity of water is reduced due to salting out type effect arising from very high concentration of Hydrogen ions

Nature of glass membrane is altered and values are higher than true values. This is referred as acidic errors

pH - MEASUREMENT

ACCURACY

Electrodes available that are capable of measuring pH to typically 0.01, 0.02, 0.02 and 0.1 pH unit

Possible to record pH to 0.001 pH unit, only done to study small differences or changes in pH

pH - MEASUREMENT

COMPOSITE LIQUIDS

Composite liquid samples such colloids, suspensions sludge, slurries, emulsions etc presents number of difficulties for pH measurements

The liquid junction easily becomes clogged and contaminated, leading to poor and inaccurate response

Electrodes with modified liquid junctions (e.g. sleeve or double junctions) may over come the problem

Attention to correct depth of immersion of the electrode in the liquid and electrode cleaning after use are important

pH - MEASUREMENT

CALIBRATION

Essential to calibrate the response of a pH electrode by using standard buffer solutions of known pH

At least two buffer solution should be used pH of the test sample is bracketed by two buffers The chosen buffers should not be more than 3 pH unit or no

less than 1 pH unit apart To be free from contamination, sediment and mould Each laboratory to determine the shelf life of their buffer

solutions based on practical experience, frequency of use, storage conditions and the importance of the pH measurements being made.

pH - MEASUREMENT

CALIBRATION

When a buffer solutions requirement for use, a suitable quantity should be transferred to clean dry beaker and close buffer solution bottle quickly, used buffer solution should not be returned to the stock bottle and pH electrode should not be immersed directly into stock bottle

Ensure that buffer solutions and test solutions whose pH values are to be measured are equilibrated to ambient temperature

pH - MEASUREMENT

USE OF BUFFER SOLUTION:-

First buffer solution- Immerse the pH electrode in the first buffer solution ensuring

that the glass membrane and the liquid junction are covered by the solution

- The depth of immersion should be such that the level of the filling liquid in the reference compartment is about 25 mm above the level of the solution being measured

- This will ensure a uniform flow of filling solution through the liquid junction and a stable liquid junction potential

- Solution should be stirred using a magnetic stirrer at a moderate speed taking care not to contact and damage the glass membrane

pH - MEASUREMENT

USE OF BUFFER SOLUTION:-

- When the reading has stabilized to within about ± 0.02 pH unit, adjust the reading using ‘set-buffer’ control so that the displayed reading is equal to the reference pH value of the buffer solution

- Discard the buffer solution and re-fill the beaker with a fresh portion of the same buffer

- Observe the pH reading, if it is not within about ±0.02 pH unit of the reference value, re-adjust the ‘set-buffer’ control so that the displayed reading is equal to the reference pH value of the buffer.

- Repeat this procedure until two successive readings agree to within about ±0.02 pH unit

- Record details of buffer solution and relevant results obtained

pH - MEASUREMENT

SECOND BUFFER SOLUTION

- Rinse the beaker with water and discard rinsing- Dry the interior of the beaker with a tissue and add the required

amount of the second buffer solution- Rinse the electrode with water and then the buffer solution and

dry by touching with a tissue- Immerse the electrode in the second buffer solution, allow the

reading to stabilize to within about ±0.02 pH unit and note the pH reading

- If the electrode is giving a Nernstian response, the displayed pH reading will not differ significantly (i.e. by more than about ±0.04) from the reference value of 2nd buffer

- Adjust the displayed reading if necessary to equal the reference pH values of the second buffer

pH - MEASUREMENT

SECOND BUFFER SOLUTION

- Make this adjustment using slope control- The adjust required should not be unduly large, typically no

more than ±0.3 pH unit- If the difference between the displayed readings and reference

value for the second buffer solution exceeds +0.3 unit, it indicates electrode response could be markedly non-Nernstanian

- That is electrode slope value is significantly different from the theoretical value of 59.1mV per ph unit

- Manufacturer of electrodes often specify the slope value for an electrode in good conditions

pH - MEASUREMENT

SECOND BUFFER SOLUTION

- Specified slope values are typically in the range of 90 – 105%

- Discard buffer solution, replace it with fresh portion of the second buffer, immerse the electrode and observe the displayed pH value. It should be within ±0.02 pH unit of the reference value

pH - MEASUREMENT

THE COMPOSITION OF THE AQUEOUS SOLUTION: -

Aqueous solution with a relatively high ionic strength is to be measured, a special pH electrode with a modified reference electrode, to counter act effects due to liquid junction potential

Water or aqueous solution of very low ionic strength are to be measured, response of the glass electrode is slow, noisy, subject to drift and inaccurate

These samples have low buffering capacity and absorption of carbon-dioxide from the atmosphere can cause unsteady reading

Use of additive to increase ionic strength of the sample Sample contains components that react with silver, the use of a

pH electrode with a silver/silver chloride reference electrode is prohibited.

pH - MEASUREMENT

THE COMPOSITION OF THE AQUEOUS SOLUTION: -

Reaction products deposited in the liquid junction causing a slow response or no response

Sample causing such problems include waste waters and lea hates containing sulfur compounds

These samples have low buffering capacity and absorption of carbon-dioxide from the atmosphere can cause unsteady reading

Use of additive to increase ionic strength of the sample Sample contains components that react with silver, the use of a

pH electrode with a silver/silver chloride reference electrode is prohibited.

pH - MEASUREMENT

MEASUREMENT OF TEST SOLUTION

- Rinse the beaker with water and discard rinsing- Rinse it with test solution and add required amount of the test

solution whose pH is to be determined- Rinse the electrode with water and then with test solution and dry

by touching with a tissue- Immerse the electrode in the test solution and observe the pH

reading- Discard the test solution and replace it with fresh portion of the

test solution and re-measure the pH- For a well buffered test solution duplicate pH result should

normally agree within about ± 0.02 pH unit- For slightly buffered test solution, replicate results may only

agree within about ±0.1 pH unit

pH - MEASUREMENT

MEASUREMENT OF TEST SOLUTION

- Record pH values obtained

- It lengthy series of measurements is made, re-measure the buffer solutions at regular intervals e.g. every 10 test solutions

pH - MEASUREMENT

UNCERTAINTY OF PH MEASUREMENTSUsing a glass electrode

Below are some possible source of uncertainty in pH measurements along with the estimate of their magnitude

Source of uncertainty Standard Uncertainty

pH value of the standard buffer solutions ±0.01

The repeatability of pH measurements ±0.01

Drift in the electrode response between the calibration

±0.01

The liquid junction potential ±0.015

Temperature (where known to ±1°C) 0.005

pH - MEASUREMENT

THE STANDARD UNCERTAINTIES IN THE TABLE ARE IN THE FORM OF STANDARD DEVIATION

Estimates of the contribution that each of these sources of uncertainty makes to the overall error of a pH measurement are some what subjective

Vary with the precise conditions of the measurement and equipment used

Using above data, the total standard uncertainty in a pH measurement is estimated to be about ±0.025 pH unit

When temperature difference is ± 5°C, the total standard uncertainty is about ±0.035 pH units

For very dilute or concentrated test solutions higher uncertainty may apply

pH - MEASUREMENT

THE ANALYST MUST KNOW:

The meaning of pH and the pH scale Acid/base reactions, the relationship or pH to acidity and

basicity, the Nernest equation The different techniques for measuring pH (indicators, pH

papers and pH meters) How to operate a pH meter – read the appropriate instruction

manuals How to follow written instructions and obtain satisfactory results

– standard test procedures How to check that the equipment is working correctly and what

to do if there is a problem

pH - MEASUREMENT

THE ANALYST MUST KNOW:

The effect of temperature on pH- Information issued with buffer solutions states the temperature

at which the pH should be determined- Samples should be allowed to equilibrate to the same

temperature as the buffer solutions before measurement- Some pH meters have a thermometer on the probe, these will

correct for temperature differences

The importance of a homogeneous solution

That the solutions used for calibration must be relevant to the pH range of the sample

pH - MEASUREMENT

PRACTICAL TECHNIQUE : ESSENTIAL KNOWLEDGE SHOULD INCLUDE:

Knowing how to stir solutions without damaging probe

Knowing when to decide when a reading is stable

Knowing how to achieve acceptable precision when measuring the pH of samples

Knowing how to clean and maintain a pH electrode- General cleaning - Wash with de-ionised water before and after each measurement- Removal of deposits- Storage requirements, electrodes should be stored in a solution

appropriate to the filling medium

pH - MEASUREMENT

ANALYSTS SHOULD BE AWARE OF:

The limitations of the equipment in terms of accuracy

Interferences:- Potential problems measuring solutions with very high or very

low pH values - Measurement of pH in non-aqueous ·solutions- Matrix effects of the samples under test, e.g., samples with high

solids or high organic solvent content

Troubleshooting- Drifting readings - stability of the reading should be obtained - within 2 minutes - Slow response- Erratic readings

pH - MEASUREMENT

ANALYSTS SHOULD BE AWARE OF:

Recording results and logbook entries - accurate transcription

How to set up control charts

The level of precision possible to achieve with the available instrument

pH - MEASUREMENT

CALIBRATION :

Buffer standards

- Know how they work - Know different types of buffer and how to use them- Know how to make up buffer solutions for calibration- Know how to store buffer standard solutions correctly- Know that it is necessary to use more than one buffer standard

(one either side of the expected pH range of samples to be tested)

pH - MEASUREMENT

CALIBRATION :

Calibrate the instrument according to the operation manual or an STP

- Check electrode slope is within manufacturers' tolerance - Know when to change the electrode, e.g., when the slope

reading is not within the tolerance limits - Know how often the pH meter needs calibrating - Check that the pH meter has maintained its performance in the

event of a power cut

pH - MEASUREMENT OBSERVATION SIGNS WHICH INDICATE THAT ALL IS NOT WELL

Messy bench area where the analyst has been working

Liquid spilled on the work area

Tops left off reagent bottles - bottles not returned to the shelf

Solutions labeled inadequately, etc. - also labeled correctly but unreadable due to damaged label

Tops left off buffer solutions or solutions inappropriately stored

Evidence of incorrect care of pH electrode - build up of deposits on surface etc

Dirty glassware left lying about rather than being cleaned and put away

pH - MEASUREMENT

OBSERVATION SIGNS WHICH INDICATE THAT ALL IS NOT WELL

Lack of notes in work book relating to calibration of equipment or temperature of measurement if appropriate crossings out in workbook, indicating that the analyst was having trouble deciding when a stable reading had been reached

Only one value recorded - no evidence that a stable reading had been achieved.

pH - MEASUREMENT

PRACTICAL TESTS

Check that all buffer solutions are labeled with the following data:

- The analysts initials- Date of preparation- Identity and review date

Store all buffer solutions in an appropriate manner