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pH & its measurement

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pH & itsMeasurement 
Dr Prasenjit Mitra 
Department of Biochemistry 
Sikkim ManipalInstitute of Medical Sciences

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Origin of pH 
SørenP. L. Sörenson 
(1868-1939) 
H+ 
H+ 
H+ 
H+ 
Number of Hydrogen ions (H+) determine acidity or alkalini...

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Determination of pH 
•Litmus paper 
•pH paper 
Indicators 
Colorimeter 
pH meters

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pH & its measurement

Slides giving an overview on pH and its measurement.
Contains information about pH meters, its calibration, maintenance , types of ph electrode and modern definition of pH

Slides giving an overview on pH and its measurement.
Contains information about pH meters, its calibration, maintenance , types of ph electrode and modern definition of pH

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pH & its measurement

  1. 1. pH & itsMeasurement Dr Prasenjit Mitra Department of Biochemistry Sikkim ManipalInstitute of Medical Sciences
  2. 2. Origin of pH SørenP. L. Sörenson (1868-1939) H+ H+ H+ H+ Number of Hydrogen ions (H+) determine acidity or alkalinity •Number of Hydrogen ions (H+) in water = 0.0000001 mol/L •Logarithm of H+Concentration •Log (0.0000001) = log (10-7) = -7 •Negative Logarithm of H+Concentration •-[Log (0.0000001)] = -log (10-7) = -(-7) = 7 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Scale Alkaline Acidic Power of Hydrogen p H
  3. 3. Determination of pH •Litmus paper •pH paper Indicators Colorimeter pH meters
  4. 4. pH meters -History Arnold Orville Beckman (1900-2004)
  5. 5. pH meters -Types Handheld pH meter Bench top pH meter Continuous in line pH meter
  6. 6. pH meters –pH Electrode Glass electrode Reference electrode Combination electrode 3 in 1 electrode
  7. 7. pH meter --Glass Electrode Glass Electrode Membrane Inner Buffer Lead-off Element Shield Fig Measuring (Glass) electrode
  8. 8. pH Reference Electrode Porous ceramic junction Electrolyte Reference Element Refill opening Reference Electrode Reference electrolyte •Inert •High ion concentration Low electrical resistance •Contact with measuring solution Popular Reference Systems •Mercury/calomel •Silver/Silver chloride Fig Reference electrode
  9. 9. pH meter –Combination Electrode Membrane Inner Buffer Combination Electrode Lead-off Element Reference Element Reference Electrolyte Fig Combination electrode
  10. 10. pH meter-Working principle Glass Electrode Reference Electrode The potential of glass electrode is measured against that of reference electrode 퐸=퐸0+ 2.303푅푇 푛퐹 log aH+ Standard potential when aH+ = 1mol/L Nernst potential (EN)/Slope factor Change in potential per pH unit. Depends on absolute temperature Fig Closed circuit of pH meter
  11. 11. pH Electrode –Working principle Glass Membrane Internal Buffer Acidic Solution Alkaline Solution Eel= E0–S(pHa-pHi) H+ H+ Fig Magnified view of membrane
  12. 12. Inner Buffer H+= Constant Li+ SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 Li+ Li+ Li+ Li+ Li+ Li+ Li+ Li+ Li+ SiO2 pH meter –working principle Inner gel layer Outer gel layer Measuring Solution H+ H+ Fig Magnified view of membrane with ion model
  13. 13. Calibration of pH meter The measuring electrode and reference electrode, when put in a zero solution (7.0 pH buffer) provides a zero mV output. Factors causing differences or changes in potential •Contamination of the reference electrolyte solution. •Electrolyte evaporation/depletion •Chemical attack of the silver/silver chloride wire. •Junction potential. •Aging of the measuring electrode.
  14. 14. Calibration of pH meter +mv pH -mv 0 Before calibration 7 After calibration 14 +mv pH -mv 0 56.0 Before calibration 7 After calibration 14 59.2
  15. 15. Calibration of pH meter 2 point calibration Multi point calibration Fig pH meter with calibrators
  16. 16. Errors in determination of pH Alkaline error Acidic error Due to reactivity of reference electrolyte Error due to temperature variation
  17. 17. Temperature Compensation Typeof Solution pH value at 200C 300C 0.001Mol/L HCl 3.00 3.00 0.001 Mol/L NaOH 11.17 10.83 PhosphateBuffer 7.43 7.40 TrisBuffer 7.84 7.56 Table –Changes in pH with change in temperature The linear function for temperature versus pH change 0.003 pH error/pH unit/°C
  18. 18. Automated temperature compensation (ATC) +mv pH -mv 0 Real isothermal intersection point Theoretical isothermal intersection point Error Eis T2 T1
  19. 19. Maintenance & Storage of pH electrode Dehydration Dehydration of glass electrode Dehydration of reference electrode Factors detrimental to electrode life Chemical attack Stripping of gel layer Transport Avoidance of freezing, extreme heat, mechanical shock and vibration Storage At ambient temperatures (10- 30 0C) Capped Ideal storage solution 3 -3.5 M KCl solution
  20. 20. Definition of pH –a misnomer •Concentration versus activity •Activity depends on ionic strength of a solution •푝퐻=−log퐻 + ×푓푤ℎ푒푟푒푓푖푠푎푐푡푖푣푖푡푦푐표−푒푓푓푖푐푖푒푛푡 •Activity co-efficient depends on total molality of a solution Molality 0.001 0.005 0.01 0.05 0.1 Activity co-efficient 0.964 0.935 0.915 0.857 0.829 pH of 0.01 M HCl = -log (0.01 x 0.915) =2.04 pH of 0.01 M HClwith 0.09 M KCl = -log (0.01 x 0.829) =2.08 pH is negative logarithm of hydrogen ion activity in a solution
  21. 21. References Principles & Techniques of Biochemistry and Molecular Biology –Wilson & Walker A guide to pH Measurement –Internet pH and its measurement --Frederick J. Kohlmann http://chemwiki.ucdavis.edu/ www.hach.com http://chemed.chem.purdue.edu/genchem/lab/equipment/phmeter/use.html www.hannainst.com www.mrclab.com
  22. 22. Thank you ….

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