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Basics of pH
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- 2. www.hannainst.in What is pH? pHis defined as the negative logarithm of the Hydrogen ion concentration. It is the measure of Acidity and Alkalinity, measured commonly from 0 to 14. 2 Number of Hydrogen ions (H+) present determine if a solution is Acidic or Alkaline Logarithm of H+ Concentration in pH14 • Log(0.00000000000001) = Log(10-14 ) = -14 Logarithm of H+ Concentration in pH 7 • Log(0.0000001) = Log(10-7 ) = -7 Logarithm of H+ Concentration in pH1 • Log(0.1) = Log(10-1 ) = -1
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- 4. pH vs Acidityand Alkalinity 4 pH • Indicates if a solution is an acid or a base (alkaline). Acidity • Indicates how much base a solution can absorb without changing the pH. Alkalinity • Indicates how much acid a solution can absorb without changing pH.
- 5. Measurement Methods 5 Chemical testKit Photometer pH meter
- 6. Elements of aPotentiometric cell? 6 pH Indicator electrode • To sense Hydrogen Ion activity Reference electrode • To provide stable reference voltage Sample Solution • Sample of which pH is to measured Volta meter • To measure generated voltage
- 7. The Sensing Electrode •Electrode-generated potentials – pH sensitive glass bulb – Buffered KCl Solution (typically 3.5 M) – Metal - Metal Salt internal element • The essential element of the glass electrode is a pH sensitive glass membrane. • An electrical potential develops at glass liquid interfaces . Ag / AgCl Internal Wire AgCl + e- Ag + Cl- Metal Buffered KCl Solution pH Sensitive Glass 7
- 8. Reference Electrode Two functions: 1.Supply a stable reference potential against which any change in the indicator electrode is measured. 2. Provide electrical continuity with the solution being analyzed. The primary requirement of a reference electrode is... STABILITY! 8
- 9. Types of pHelectrode • Depending on the constructing criteria pH electrodes are: • Single (half cell) • Combined • Single junction electrodes • Double junction electrodes 9
- 10. Types of pHelectrode 10
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- 12. Characteristics of Junctions TypeCharacteristics Flow Rate Single ceramic Single flow junction for standard water testing applications. 15 μl/hr Triple ceramic Moderate flow rate system for pure water applications, emulsions, and high viscosity samples. 45 μl/hr Cloth Higher flow rate for standard to applications with contaminants and high viscosity. 50 μl/hr Teflon Higher flow rate for chemically aggressive applications at moderate to high temperatures. 50 μl/hr Open Extremely high flow rate for surface testing and applications with low hydration 500 μl/hr Sleeve Clogging prevention system (CPSTM), PTFE sleeve junction which controls a steady predictable flow 12
- 13. Types of Junctions PTFEJunction Sleeve Junction Ceramic Junction Open Junction 13
- 14. 14 E1 E2 E3 E4 E5 E6 E7 E8 E9 Complete CellPotentials E1: Sensing wire E2: Sensing buffer solution E3: Inner hydrated layer E4: Glass layer E5: Outer hydrated layer E6: Solution E7: Reference junction E8: Reference fill solution E9: Reference wire
- 15. 15 How does themeasurement take place? - in acidic solution the H+ diffuse in, and a positive charge is established on the outer side of gel layer - in alkaline solution the H+ diffuse out, and a negative charge is established on the outer side of gel layer The total membrane potential is a result of the difference between the inner and outer charge.
- 16. 16 Limitation of pHelectrodes
- 17. Acid Error andAlkali Error • Gel layer absorbs acid molecules – Increases the layer’s H+ ion activity because it has become dehydrated. – Creates a higher (more basic) reading than the actual value. – Decreased efficiency. • Only noticeable at very low pH values • H+ ions in the gel layer are partially or completely replaced by alkali Ions, particularly sodium ions. • Low H+ ion activity of the gel layer results in lower (more acidic) pH than actual values. 17
- 18. 18 Elements that competein pH measurement Temperature effect • pH value changes with temperature • The slope of the electrode changes with the temperature • High temperature shortens the life span of the electrode Interference Effect • Interference due to Sodium Ions Conductivity Effect • Low conductivity effect stability.
- 19. Temperature and thepH Electrode 19
- 20. Bulb Shapes 20 Spherical tip •is recommended for general use in aqueous or liquid solutions and provides a wide area of contact with the solution Conical tip • is recommended for semi-solid products like emulsions, cheese, meat and food, in general. Flat tip • is recommended for direct surface measurement on skin, leather paper, etc…
- 21. The Nernst Equation •Electrode cell potentials and hydrogen ion activity are related by the Nernst Equation, which states: 𝑬 𝒐𝒃𝒔 = 𝑬 𝒄 + ln(𝟏𝟎)(𝑹𝑻 / 𝒏𝑭) log(𝒂𝑯 + ) – Eobs = Observed potential (sum of reference and liquid junction potentials) – Ec = Reference potential including other stable and fixed potentials – R = Gas Constant (8.31432J / KMol) – T = Temperature in Kelvin (C° + 273.15) – n = Valency of the ion measured (1) – F =Faraday’s constant (9.64845 x 104) – aH+= The Hydrogen ion activity 21
- 22. Slope and Offset •Offset is the mV generated at pH 7 • Theoretically offset should be 0 mV • Slope is the change in mV potential per pH unit • Theoretical slope is + 59.16 mV (@25 oC) • Relative to pH 7.0 – Acids will generate an increasingly positive voltage. – Bases will generate an increasingly negative voltage. Slope -450 -350 -250 -150 -50 50 150 250 350 450 0 2 4 6 8 10 12 14 mV pH 22 𝑺𝒍𝒐𝒑𝒆% = 𝒅𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒄𝒆 𝒃𝒆𝒕𝒘𝒆𝒆𝒏 𝒎𝑽 𝒈𝒆𝒏𝒆𝒓𝒂𝒕𝒆𝒅 𝒂𝒕 𝒕𝒘𝒐 𝒑𝒐𝒊𝒏𝒕𝒔 𝒅𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒄𝒆 𝒃𝒆𝒕𝒘𝒆𝒆𝒏 𝒑𝑯 𝒖𝒏𝒊𝒕𝒔 ∗ 𝟓𝟗. 𝟏𝟔 × 𝟏𝟎𝟎
- 23. Acceptable Slope/Offset at25 oC Theoretical User Offset (pH 7.0 value) 0 mV ± 15 mV Slope (mV/pH ) 100% (59.16 mV) 95 to 105% (56.2 to 62.1 mV) 23
- 24. 24 Calibration WHY...? pH cellsare not perfect. Therefore they must be calibrated. Calibration tries to compensate any deviation from ideal behavior Calibration is made with solutions having exactly the known pH. The calibration process is generally performed by measuring in two different buffer solutions.
- 25. 25 Steps for theCalibration Process Rinsing solutions 1 2 3 4 5 6 Measuring solutions
- 26. Electrode Behavior • pHelectrodes differ from ideal behavior due to: – Electrode condition and cleaning – Electrode aging – Manufacturing process • Calibration tries to compensate for all the above effects without knowing which effect is causing the deviation from ideal behavior. 26
- 27. 27 Possible errors duringpH measurement Offset Error Error in mV of slope points Use of contaminated buffers Clogged junction. Calibrating dirty electrode Storing pH electrode dry or in DI water Wiping the sensing glass of pH electrode Calibrating/ measuring with closed filling cap Low electrolyte fill level
- 28. 28 Precautions to betaken while measuring pH
- 29. Use application specificelectrode • Make sure that you use electrode that is suitable to your application • Flowrate of electrolyte is enough to get fast and stable readings • Junction and membrane suits the sample and is in contact with sample simultaneously. 29
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