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Level 1 pH Theory

Level 1 pH Theory. Section 1 . What is pH and How is it Measured? . Why Measure pH?. Final product quality depends on pH. Pharmaceutical Paper Metal plating Drinking water Food and Beverages Alternative fuel Chemical reaction rates are often a function of pH Corrosion Scaling

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Level 1 pH Theory

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  1. Level 1 pH Theory

  2. Section 1 What is pH and How is it Measured?

  3. Why Measure pH? • Final product quality depends on pH. • Pharmaceutical • Paper • Metal plating • Drinking water • Food and Beverages • Alternative fuel • Chemical reaction rates are often a function of pH • Corrosion • Scaling • Precipitation (salts) • Fermentation • Environmental Monitoring

  4. NH4OH NH4+ + OH- H2O H+ + OH- HAc H+ + Ac- HCl H+ + Cl- NaOH Na+ + OH- Acid and Base Basics Strong Acids dissociate completely in water releasing hydrogen ions. Weak Acids only partially dissociate in water releasing hydrogen ions. Strong Bases dissociate completely in water releasing hydroxyl ions. Weak Bases only partially dissociate in water releasing hydroxyl ions. Water itself partially dissociates releasing hydrogen and hydroxyl ions. There is an equilibrium in water between hydrogen and hydroxyl ions.

  5. What is pH? • pH is the “unit of measure” for the acidity of a solution. It is defined as the negative logarithm of the hydrogen ion activity,aH+ • Activity is related to the concentration of Hydrogen Ion, H+ by an activity coefficient. • In general, pH is used as the measure of acidity and rarely used as a direct measurement of concentration by users. pH = - log [aH+] or aH+=10-pH aH+ = g cH+

  6. pH Scale • pH scale is based on dissociation constant of water Kw • Kw = aH+aOH- = 10-7·10-7 = 10-14 mol/liter at 25°C (and ONLY at 25°C)

  7. pH values of everyday solutions • pH 0 Sulfuric Acid (Battery Acid) • pH 1 Gastric Juice, 0.5% Sulfuric Acid • pH 2.3 Lemon Juice • pH 3 Vinegar, Coca Cola • pH 4.3-4 Beer, Sour Milk • pH 4.8 Pure water air equilibrated • pH 6.5 Fresh Milk • pH 7 Pure Water • pH 7.36 Blood • pH 8.3 Sea Water • pH 13 0.4 % NaOH • pH 14 NaOH (Drain Opener)

  8. Key pH Sensor Components • Measuring Electrode • Develops a millivolt potential directly proportional to pH in an aqueous solution • Reference Electrode • Maintains a stable reference potential regardless of changes in solution pH or other ionic activity • Reference Electrode Liquid Junction • Maintains electrical contact between the pH measuring electrode and the reference cell via the process solution • Temperature Compensator • Corrects for changes in the millivolt output of the pH sensor due to process temperature change

  9. pH-Glass Electrode • The purpose of a pH glass electrode is to develop a millivolt potential directly proportional to pH of an aqueous solution Shield GlassBody Ag/AgClInternalWire Buffered FillSolution pH SensitiveGlass

  10. The Reference Electrode The purpose of the reference electrode is to provide stable and reproducible potential to which glass electrode potential may be referenced. It completes the circuit by contacting the sample solution through a liquid junction. The liquid junction allows diffusion of the electrolytes (ions) into and from the process, to maintain electrolytic contact. Most reference electrodes are termed ‘non-flowing’, because contact with the process is by ionic diffusion and not flow of the filling solution. GlassBody KClFill Solution Ag/AgClInternalWire LiquidJunction pH17

  11. Liquid Junctions KCl Diffusion Rate • Liquid Junction • A porous plug that allows liquid contact between the internal KCl solution and outside process solution but restricts the flow. • The larger the porosity is, the lower the electrical resistance is and higher the diffusion rate is. • The smaller the porosity is, the higher the electrical resistance is and the lower the diffusion rate. • There is a tradeoff between high flow with good measurement accuracy, and low flow with longer reference life and less stable junction potentials. • Potassium and Chloride ions diffuse out the reference at the essentially same rate. • Positive and negative Ions in the process can diffuse into the reference at different rates, which leads to a build up of an electric charge across the liquid junction. • This is called Liquid Junction Potential, which normally causes a small error in the pH measurement, which can be calibrated out by standardization 5K ohms 6-8 month life KCl Diffusion Rate 20K ohms 1-2 year life

  12. pH Sensor Construction

  13. Basic circuits for a pH measurement Sensor Potential (E) using a pH sensor with a Solution Ground. E = (EpH - ESG) – (ERef – ESG) A solution ground makes it possible to measure reference electrode impedance and use reference impedance as a diagnostic tool. ESG Temp EpH ERef Sensor Potential (E) using a pH sensor without a Solution Ground. E = EpH – ERef Temp EpH ERef

  14. Preamplification • pH sensor outputs (mV) are high impedance signals and can be susceptible to noise and interference. • To lower the impedance and amplify the signal, a preamplifier is used: • In the pH sensor • In a remote junction box • In the transmitter • Certain (rare) pH sensors use a 2nd glass electrode as a reference. • Most pH transmitters can be configured to accommodate the high reference impedance of these sensors. Preamp Preamp Preamp

  15. Sensor potential (mV) Isopotential pH Zero offset (mV) Temperature (K) Slope (mV/pH K) Temperature Compensation andthe Calculation of pH The effect of temperature on the pH sensor must be taken into account when determining pH to avoid large errors. How pH is Calculated with Temperature Compensation:

  16. pH Changes with Temperature Changein Strong Acid and Base Solutions The pH of certain solutions can themselves change with temperature. The degree to which this happens is a function of the composition of the solution. In application where this is an issue, a second, solution temperature compensation is provided to correct the measured pH to 25 C.

  17. 9.00 SOLUTION pH CHANGE OF A DETERGENT WITH TEMPERATURE 8.90 8.80 pH Solution Temp.Coefficient o 8.70 o 8.60 o Z= -.0242 pH/°C 8.50 o 8.40 o 8.30 8.20 o 8.10 o 8.00 20 25 30 35 40 45 50 55 SolutionpH Change with Temperature

  18. pH Transmitter ConfigurationAll the Configuration Needed for pH Measurements Location of Preamp Impedance of Reference Electrode (can choose High for special electrodes) Temp Comp On/Off Temp Unit Manual Temp Value Solution Temp Type Coefficient for Linear Solution Temp Comp Isopotential pH (can be changed for special electrodes)

  19. Mounting pH Sensors • pH Sensors should be mounted at least 10 degrees above horizontal. • Otherwise, the air bubble in the glass electrode can cover the inner surface of the glass bulb.

  20. Section 1 Test

  21. Question 1 • pH is proportional to: • The concentration of hydrogen ion • The concentration of hydroxyl ion • The logarithm of the concentration of hydrogen ion • The negative logarithm of the concentration of hydrogen ion

  22. Question 2 • A neutral solution has an equal concentration of hydrogen and hydroxyl ions and its pH: • is always 7.00 pH • is only 7.00 pH at 25 deg C • depends on the temperature of the solution • b and c

  23. Question 3 • The millivolt output of a pH sensor: • changes with temperature • remains constant with temperature changes • can be easily compensated for by measuring temperature • a and c

  24. Question 4 • The actual pH of a solution can: • change with temperature • change with temperature and does not require special temperature compensation • can be compensated by using special temperature compensation • a and c

  25. Question 5 • pH sensors must be mounted at least 10 degrees above horizontal. • True or False?

  26. Answers to Section 1 Test • 1 – d: pH is proportional to the negative logarithm of hydrogen ion concentration • 2 – b and c: The pH of a neutral solution changes with temperature and is only 7.00 pH at 25C • 3 – a and c: The millivolt output of a pH sensor changes with temperature and can easily be compensated for by measuring temperature • 4 – a and c: The actual pH of a solution can change and can be compensated by using special temperature compensation • 5 – True: pH sensor need to be mounted 10 degrees above horizontal

  27. Section 2 When to Use pH and Special pH Applications

  28. When to Use pH -- Acidic Solutions At pH values below 1.0 pH, a bad pH application becomes a good Conductivity application.

  29. When to use pH -- Basic Solutions At pH values above 13.0 pH, a bad pH application becomes a good Conductivity application.

  30. Special pH Applications

  31. Special pH Applications • Most pH applications involve weak solutions or simply water at near room temperature. • A general purpose pH sensor can be used • The sensor should have a long, worry free life • In some pH applications, the temperature, pressure, and composition of the process can create issues with the pH measurement • pH sensors with special features need to be chosen to best deal with these issues

  32. Pure Water (Conductivity < 5 mS/cm) • Liquid Junction Potential (LJP) • In normal applications, a potential at the liquid junction of the reference electrode is set up by the unequal diffusion of ions from the process into liquid junction. • This is usually no more than 15 to 20 mV (~ + 0.3 pH) and can be calibrated out by standardization. • In high purity water application this effect can become large and unstable resulting in major errors and drifting. • This can be made worse by fluctuations in flow and static buildup on the pH sensor due to the low conductivity of the water. • Pure water pH applications need a special sensor designed to take these effects into account.

  33. High Purity Water pH System Components 500 ml Electrolyte Reservoir Vent Tube Reservoir Filter Reference Tubing Air Bleed Screw Combination Electrode Sample OUT Diffuser (inside flow cell) Sample IN Flow Cell

  34. High Temperature Applications • High Temperature • Accelerates the ageing of pH sensor materials. • Increases the impedance of the glass electrode. • Results in a slower response time. • Can quickly destroy a pH sensor not designed for it. • A pH Sensor designed for High Temperature must be used. Traditional pH Sensor Lifetime versus Temperature Current High Temperature pH Sensor Ratings • Sensor 1: Up to 155°C at 400 psig

  35. Process Effects on Glass Electrodes • Chemical Erosion and Attack • Hydrofluoric Acid (HF) • Dissolves glass • If fluoride is present you need to know its concentration and the pH range. • A special sensor may be needed or the application may not be possible with pH. • Sodium and Potassium Hydroxide (NaOH and KOH) • > 4 % (14 pH) will dissolve glass within 8 hours at elevated temperature – there’s no remedy -- go with conductivity • Solutions containing Abrasives • To prevent electrode damage or breakage, protect the electrode from the direct impact of the process flow. • Glass electrodes crack or break in these cases.

  36. Sensor Coating • A sample velocity > 5 ft/sec will help minimize coating • Cleaning Solutions for: • Alkaline or Scale • 5 % HCl or vinegar • Acidic Coatings • Weak caustic < 4 % • Oil, grease, or organic compounds • Detergent / sensor friendly solvents • Use a pH Sensor designed to resist coating • In line Cleaning can be done using a jet spray cleaner. • Coating increases sensor response time and can cause instability in pH control applications. • Severe coating shuts down the pH measurement altogether.

  37. Reference Electrode Contamination • Plugging • Precipitation of silver in the reference fill solution by ions in process. • Typical villains are sulfide, bromide, and iodide ions • Use a triple junction electrode with a potassium nitrate outer fill. • Plugging causes the pH measurement to drift. • Poisoning • Depletion of the silver in the reference solution by precipitation or complexation of free silver ion. • Precipitation: sulfide, bromide, and iodide • Complexation: ammonia; cyanide is deadly to references • Check the concentration of poisoning ions and use a triple junction electrode, or in extreme cases, a special electrode will be needed. • Poisoning causes a large zero offset (> 60 mV).

  38. What Happens During Reference Cell Poisoning Reference Poisoning can be a slow process. Nothing happens until the silver ion concentration in the reference is irreversibly depleted and the potential changes. Time Reference 3: OK Reference Potential, mV Reference 1: Poisoned Reference 2: Poisoned

  39. Triple Junction Reference Electrode • Reference technology • Single, Double, Triple Junction are used. • Triple junctions slow the diffusion of harmful ions into the innermost portion of the reference. • Reference electrode material • Silver-silver chloride wire in potassium chloride solution • Standard concentrations of silver and chloride ions maintain a standard potential • Poisoning ions disrupt these concentrations • Junction Materials • Ceramic, Teflon, quartz fibers • Electrolyte Fill Solutions • Gelled fill solution – Resists the transport of harmful ions by thermal convection Liquid Junctions

  40. Applying pH • You need Information: • Process Pressure and Temperature • Don’t forget to include transients—a short expose to high temperature or pressure can kill a sensor not designed for it. • Process Composition • Not such a concern in common, well-known applications. • In certain applications, knowing the process composition is essential. • Choose the Sensor for Application • In benign applications go with a sensor easy to mount and maintain • Specially Designed pH Sensors are needed for: • High Purity Water • High Temperature Processes • Processes that Coat • Process with components that Poison

  41. Section 2 Test

  42. Question 1 • Conductivity is a better measurement than pH for concentrated acids and bases. • True or False?

  43. Question 2 • Measuring pH of a solution with a conductivity less than 5mS/cm can requires a special pH sensor. • True or False?

  44. Question 3 • When choosing a pH Sensor the following should be considered: • Process temperature and pressure • The conductivity of the process • The composition of the process • All of the above

  45. Question 4 • A process flow velocity greater than 5 ft/second will help minimize sensor coating. • True or False?

  46. Question 5 • pH measurements in processes with high temperature and pressure require a special pH sensor. • True or False?

  47. Answers to Section 2 Test • 1 – True: Conductivity is a better measurement of concentrated acids and bases, due to problems with pH in these solutions. • 2 – True: Measuring the pH low conductivity solutions requires a special pH sensor. • 3 – d: The temperature, pressure, and composition of a process, including transients needs to be considered when applying pH. • 4 – True: a high sample velocity does help prevent sensor coating. • 5 – True: High sample temperatures and pressures require a special pH sensors; standard pH sensors have a short life in these applications.

  48. pH Calibration and Diagnostics

  49. pH Buffer Solutions • Solutions of known pH that can withstand moderate contamination or dilution without significant pH variation. The more concentrated a buffer solution is the more resistant it is to dilution and acid or base contamination. • Buffers 4 and 7 or 10 are usually used – a difference of 3 pH between buffer values is recommended for (two point) buffer calibrations • Rules of buffering • Use fresh buffer • Rinse between buffers • Allow reading to stabilize • The of a buffer solution can and does change with temperature • This needs to be taken into account during a buffer calibration

  50. pH Buffer Calibration • Two Point Calibration • Verifies Sensor Response to pH Change • Determines Slope and Zero Offset • Transmitter Automatic Buffer Calibration Features • Identifies the Buffer Value • Compensates for Changes in Buffer pH with Temperature • Accepts Calibration only upon Stabilization of the Millivolt Signal Buffer 2 Zero Buffer 1

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