pH - MEASUREMENT

1. 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

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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

11. 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

12. 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

13. pH - MEASUREMENT • HIGH PURITY SALTS USED AS PRIMARY pH STANDARD Note: The uncertainty of the tabulated pH values are estimated to be ± 0.01

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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.

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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.

32. 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.

33. 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

34. 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

35. pH - MEASUREMENT UNCERTAINTY OF PH MEASUREMENTS Using a glass electrode • Below are some possible source of uncertainty in pH measurements along with the estimate of their magnitude

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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)

43. 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

44. 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

45. 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.

46. 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