File Name: pH.pptFeb 2001pH TheoryApplied to Proper Hardware Selection

File Name: pH.ppt

File Name: pH.pptFeb 2001pH TheorypH is a Unit of MeasurementpH = Power of Hydrogen (H+)Defined as the Negative Logarithm of Hydrogen Ion Activity pH = log (1/H+)Used for Determining the Acidity or Alkalinity of an Aqueous SolutionPractical pH Scale for Industrial Instrumentation 0 - 14 pH

File Name: pH.ppt

File Name: pH.pptFeb 2001Acids and BasesAcid dissolves in water to furnish H+ ionsHCl H+ + Cl-HNO3 H+ + NO3-HF H+ + F-

Base dissolves in water to furnish OH- ionsNaOH Na+ + OH-KOHK+ + OH-NH4OHNH4+ + OH-

File Name: pH.ppt

File Name: pH.pptFeb 2001Ion Concentration (mol/l)

File Name: pH.ppt

File Name: pH.pptFeb 2001Typical pH Values

File Name: pH.ppt

File Name: pH.pptFeb 2001pH is a Potentiometric MeasurementThe Measuring System consists of a pH Measuring Electrode and Reference ElectrodeThe Potential Difference Between the Two Electrodes is a Function of the pH Value of the Measured SolutionThe Solution Must Be Conductive and is Part of the Electrical CircuitpH Measuring ElectrodeReference Electrode

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Sensor ComponentspH Measuring ElectrodePurpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous SolutionReference CellPurpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the SolutionReference Cell Liquid JunctionPurpose is to Maintain Electrical Contact Between the Reference Electrode and the Measuring Electrode by way of the SolutionAg/AgCl ElementpH Measuring ElectrodeReference CellpH Sensitive GlassLiquid JunctionKCl GelKCl Buffered to 7 pH

File Name: pH.ppt

File Name: pH.pptFeb 2001ProcessInternal Fill SolutionHow the pH Sensitive Glass WorksLithium Ions in the pH sensitive glass act as current carriersPositive Charged Free Hydrogen Ions (H+)Develop Positive mV Potential Relative to Internal BufferAcidic SolutionsFewer Hydrogen Ions Relative to Internal Buffer Produce a Negative mV PotentialAlkaline SolutionspH GlassExternal Gel LayerInternal Gel Layer

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Temperature SlopeAcids = Positive mV SignalBase = Negative mV Signal7.0 pH = 0 mV OutputSensor Output Changes with Temperature0o C ~ 54.2 mV/pH25oC ~ 59.2 mV/pH50oC ~ 64.1 mV/pHSensor Output is Corrected to 25oC with Automatic Temperature Compensation

File Name: pH.ppt

File Name: pH.pptFeb 2001Temperature Error in pH Units

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Measuring ElectrodePurpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous SolutionProcess EffectsHigh TemperatureFaster Response / Lower ImpedanceAccelerates Aging, Lithium Ions Leached from MembraneShort SpanLow TemperatureSlower Response / Higher ImpedanceMeasurement > 10.0 pHAlkaline / Sodium Ion ErrorCoatingsSlower ResponseIncrease Zero Offset< 50% WaterDehydrationSteam SterilizationDehydrationAg/AgCl Dissolves from Silver Reference Element

File Name: pH.ppt

File Name: pH.pptFeb 2001OHMs Law Applied to pHThe pH Measuring Electrode Develops 59.2 mV per pH UnitpH Membrane Impedance is Approximately 100 Meg Ohms @ 25oCRecommendationsShielded Cable is Required to Transmit a Reliable Signal Use Self-Powered Unity Gain PreamplifierMount the Transmitter 15 feet or Less from Sensor

EIR.059 V/pH100,000,000 Ohms10-9 Nano Amps

File Name: pH.ppt

File Name: pH.pptFeb 2001Temperature Affects pH Electrode Response TimepH Glass Electrode Impedance is approximately 100 MegOhms @ 25oCFor approximately every 8oC Step Change from 25oC the pH Glass Impedance Doubles or Halves> 25oC Faster Response< 25oC Slower Response>1000 MegOhms pH Generally Becomes InoperableRecommendationsHold Distance Between Transmitter and Sensor to a MinimumUse Low Temperature pH Membrane

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Electrode Life is Temperature DependentTypical pH Electrode Life is 12 - 18 MonthsLife is Reduced Approximately 50% for Every 25oC Increase in Operating TemperatureRecommendationsSample CoolingIntermittent vs Continuous Measurement

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Glass Electrode OptionsContinuous Operation > 11.0 pHHPH Option (High Alkaline)Continuous Operation > 176oF (80oC)HT Option (High Temperature)Continuous Operation < 41oF (5oC)LT Option (Low Temperature to -13oF (-25oC)Hydrofluoric AcidHF glass in 1N HCl (0.1pH) Test Protocol (Same Electrodes)1000 ppm HF @ 20oC for 140 hrs = OK 1000 ppm HF @ 50oC for 100 hrs = OK10,000 ppm HF @ 20oC for 100 hrs = FailTotal time 340 hours or 14 days

File Name: pH.ppt

File Name: pH.pptFeb 2001pH Reference CellPurpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the SolutionProcess EffectsDilution of the ElectrolyteAir Entrapment Normal Process Temperature and Pressure FluctuationsSugar, Organics such as Methylene Chloride, Chlorine GasHeavy Metal Poisoning from Lead, Mercury, SilverDriftSulfide (H2S) PoisoningPlugs Liquid JunctionLoss of SignalHigh Purity Water < 100 uS/cm ConductivityFlow SensitivityLow pH Values (13.0 pH)Junction Potential Offsets

File Name: pH.ppt

File Name: pH.pptFeb 2001Reference Cell Contamination Silver Reacts with Sulfides Forming a Non-Conductive Precipitant at the Liquid JunctionResult is an Open Electrical Circuit Between the Measuring and ReferenceOther Undesirable Reactions that Change the Chemistry of the Reference Half Cell are Caused by:Silver (Ag)Lead (Pb)Mercury (Hg)Copper (Cu)Bromide (Br-)Iodide (I-)Cyanide (Cn-)

File Name: pH.ppt

File Name: pH.pptFeb 2001pH ControlStart, Stop, Slow Down or Speed Up a Chemical Reaction

File Name: pH.ppt

File Name: pH.pptFeb 2001Acid Equivalents Chart

File Name: pH.ppt

File Name: pH.pptFeb 2001Base Equivalents Chart

File Name: pH.ppt

File Name: pH.pptFeb 2001Titration CurveStrong Acid with Strong Base

File Name: pH.ppt

File Name: pH.pptFeb 2001Two Position On - Off ControlMeasured variable will cycle around control point (7.0 pH)Guidelines for controlTank Retention Time > 5 minutesGood Chemical Mixing, Tank turnover every two minutesUse alarm dead band and/or delay time on/off to compensate for minor variations in control

File Name: pH.ppt

File Name: pH.pptFeb 2001Proportional + Integral (PI) Control Tank or Vessel retention time < 5 minutesIn-line mixing prior to sensor importantAdjust to achieve 4:1 decay ratio after process upset

File Name: pH.ppt

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