file name: ph.pptfeb 2001 ph theory applied to proper hardware selection

File Name: pH.pptFeb 2001

pH Theory

Applied to Proper Hardware Selection


pH Theory

pH is a Unit of Measurement• pH = 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 Solution• Practical pH Scale for Industrial Instrumentation

• 0 - 14 pH


Acids and Bases

Acid dissolves in water to furnish H+ ions• HCl H+ +

Cl-

• HNO3 H+ +

NO3-

• HF H+ + F-

Base dissolves in water to furnish OH- ions• NaOH Na+ +

OH-

• KOH K+ + OH-

• NH4OH NH4

+ +

OH-


Ion Concentration (mol/l)

pH Hydrogen Ion (H+) Hydroxyl Ion (OH-)Acid 0 1 0.00000000000001

1 0.1 0.00000000000012 0.01 0.0000000000013 0.001 0.000000000014 0.0001 0.00000000015 0.00001 0.0000000016 0.000001 0.00000001

Neutral 7 0.0000001 0.00000018 0.00000001 0.0000019 0.000000001 0.00001

10 0.0000000001 0.000111 0.00000000001 0.00112 0.000000000001 0.0113 0.0000000000001 0.1

Alkaline 14 0.00000000000001 1


Typical pH Values

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0

4.9 % H2SO4

Lemons

Oranges

Tomatoes

Beer

Milk

Pure Water

Blood

Crackers

4.0% NaOH

pH


pH is a Potentiometric Measurement

The Measuring System consists of a pH Measuring Electrode and Reference Electrode

The Potential Difference Between the Two Electrodes is a Function of the pH Value

of the Measured Solution The Solution Must Be

Conductive and is Part of the Electrical Circuit

pH Measuring Electrode Reference Electrode


pH Sensor Components

pH Measuring Electrode• Purpose is to Develop a Millivolt

Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution

Reference Cell• Purpose is to Maintain a

Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution

Reference Cell Liquid Junction• Purpose is to Maintain Electrical

Contact Between the Reference Electrode and the Measuring Electrode by way of the Solution

Ag/AgCl Element

pH Measuring Electrode Reference Cell

pH Sensitive Glass Liquid Junction

KCl Gel

KCl Buffered to 7 pH


Process

Internal Fill Solution

How the pH Sensitive Glass Works

Lithium Ions in the pH sensitive glass act as current carriers

Positive Charged Free Hydrogen Ions (H+)Develop Positive mV Potential Relative to Internal Buffer• Acidic Solutions

Fewer Hydrogen Ions Relative to Internal Buffer Produce a Negative mV Potential• Alkaline Solutions

H+H+H+H+ H+ H+

pH GlassExternal Gel Layer

Internal Gel Layer

H+

H+ H+ H+ H+ H+

Li Li

Li

Li Li

Li


pH Temperature Slope

-500

-400

-300

-200

-100

0

100

200

300

400

500

0 2 4 6 8 10 12 14

pH

mV

0C 25C 50C

Acids = Positive mV Signal Base = Negative mV Signal 7.0 pH = 0 mV Output Sensor Output Changes with

Temperature• 0o C ~ 54.2 mV/pH

• 25oC ~ 59.2 mV/pH

• 50oC ~ 64.1 mV/pH

Sensor Output is Corrected to 25oC with Automatic Temperature Compensation


Temperature Error in pH Units

pH Electrode Output

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14pH

Err

or

0C 10oC 20C 25C 30C 40C 50C


pH Measuring Electrode

Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution

• Process Effects• High Temperature

• Faster Response / Lower Impedance• Accelerates Aging, Lithium Ions Leached from Membrane

• Short Span

• Low Temperature• Slower Response / Higher Impedance

• Measurement > 10.0 pH• Alkaline / Sodium Ion Error

• Coatings• Slower Response• Increase Zero Offset

• < 50% Water• Dehydration

• Steam Sterilization• Dehydration• Ag/AgCl Dissolves from Silver Reference Element


OHM’s Law Applied to pH

The pH Measuring Electrode Develops 59.2 mV per pH Unit

pH Membrane Impedance is Approximately 100 Meg Ohms @ 25oC

Recommendations• Shielded Cable is Required

to Transmit a Reliable Signal

• Use Self-Powered Unity Gain Preamplifier

• Mount the Transmitter 15 feet or Less from Sensor

E

I R

.059 V/pH

100,000,000 Ohms10-9 Nano Amps


Temperature Affects pH Electrode Response Time

1

10

100

1,000

10,000

-5C 3C 11C 17C 25C 33C 41C 49C 57C 65C 73C

Megohms

pH Glass Electrode Impedance is approximately 100 MegOhms @ 25oC

For 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 Inoperable

• Recommendations• Hold Distance Between

Transmitter and Sensor to a Minimum

• Use Low Temperature pH Membrane


pH Electrode Life is Temperature Dependent

0

2

4

6

8

10

12

14

25C 50C 75C 100C 125C

Temperature

Months

Typical pH Electrode Life is 12 - 18 Months

Life is Reduced Approximately 50% for Every 25oC Increase in Operating Temperature

Recommendations• Sample Cooling

• Intermittent vs Continuous Measurement


pH Glass Electrode Options

Continuous Operation > 11.0 pH• HPH Option (High Alkaline)

Continuous Operation > 176oF (80oC)• HT Option (High Temperature)

Continuous Operation < 41oF (5oC)• LT Option (Low Temperature to -13oF (-25oC)

Hydrofluoric Acid• HF 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 = OK• 10,000 ppm HF @ 20oC for 100 hrs = Fail• Total time 340 hours or 14 days


pH Reference Cell

Purpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution• Process Effects

• Dilution of the Electrolyte• Air Entrapment • Normal Process Temperature and Pressure Fluctuations• Sugar, Organics such as Methylene Chloride, Chlorine Gas

• Heavy Metal Poisoning from Lead, Mercury, Silver• Drift

• Sulfide (H2S) Poisoning• Plugs Liquid Junction• Loss of Signal

• High Purity Water < 100 uS/cm Conductivity• Flow Sensitivity

• Low pH Values (<1.0 pH) or High pH Values (>13.0 pH)• Junction Potential Offsets


Reference Cell Contamination

Silver Reacts with Sulfides Forming a Non-Conductive Precipitant at the Liquid Junction

Result is an Open Electrical Circuit Between the Measuring and Reference

Other 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-)


pH Control

Start, Stop, Slow Down or Speed Up a Chemical Reaction


Weight in Kilograms (100% active) required Volume in liters required to Acid Solution to neutralize 1000 liters of acid neutralize 1000 liters of acid

Anhydrous Caustic Soda Quick Lime Hydrated Lime Soda Ash 20% Solution 10% slurry 10% SolutionMW Norm. pH Density Ammonia NaOH CaO Ca(OH)2 Na2CO3 NaOH Ca(OH)2 Na2CO3

.1% H2SO4 98 0.02 1.8 1 0.347 0.816 0.572 0.755 2.161 0.335 0.675 1.976Sulfuric 1% H2SO4 98 0.205 0.9 1.0051 3.49 8.2 5.75 7.59 21.72 3.36 6.78 19.86Sulfuric 10% H2SO4 98 2.17 0 1.0661 37 87 61 80.5 230.4 35.7 71.9 210.6Sulfuric 93% H2SO4 98 34.66 -1.1 1.8279 590.3 1386.9 972 1284.3 3674.1 568.6 1146.7 3358.4Sulfuric .1% HCL 36 0.027 1.6 1 0.467 1.097 0.769 1.016 2.906 0.45 0.907 2.657Hydrochloric1% HCL 36 0.275 0.7 1.0031 4.68 11 7.71 10.19 29.15 4.51 9.1 26.65Hydrochloric10% HCL 36 2.87 -0.4 1.0476 48.9 114.9 80.5 106.4 304.5 47.1 95 278.3Hydrochloric37% HCL 36 12.02 -1 1.185 204.7 481 337.1 445.4 1274.2 197.2 397.7 1164.7Hydrochloric1% HNO3 63 0.159 0.9 1.0037 2.71 6.37 4.47 5.9 16.88 2.61 5.27 15.43Nitric70% HNO3 63 15.7 1.4134 267.4 628.1 440.2 581.6 1664 257.5 519.3 1521Nitric1% H3PO4 98 0.307 1 1.0038 5.23 12.29 8.62 11.38 32.57 5.04 10.17 29.77Phosphoric85% H3PO4 98 43.84 1.685 746.7 1754.2 1229.4 1624.3 4647.1 719.2 1450.4 4247.8Phosphoric1% Acetic 60 0.166 2.8 0.9996 2.83 6.66 4.67 6.17 17.64 2.73 5.51 16.13CH3COOH100% Acetic 60 17.45 1.2 1.0477 297.1 698.1 489.2 646.4 1849.2 286.2 577.2 1690.3CH3COOH

Acid Equivalents Chart


Weight in Kilograms (100% active) required Volume in liters required to Base Solution to neutralize 1000 liters of alkali neutralize 1000 liters of alkali

Sulfuric Acid Hydrochloric Acid Carbon Dioxide Sulfur Dioxide 93% Sulfuric 37% HydrochloricMW Norm. pH Density H2SO4 HCl CO2 SO2 H2SO4 HCl

0.1% NaOH 40 0.025 12 1 1.23 0.91 0.55 0.8 0.72 2.07Sodium Hydroxide1% NaOH 40 0.252 13 1.0095 12.37 9.2 5.55 8.07 7.28 20.9Sodium Hydroxide10% NaOH 40 2.772 14 1.1089 135.9 101.1 61 88.7 80 229.6Sodium Hydroxide.1% NH3 17 0.059 11 1 2.88 2.14 1.29 1.88 1.69 4.86Ammonia1% NH3 17 0.581 11 0.99 28.5 21.2 12.8 18.6 16.8 48.2Ammonia10% NH3 17 5.622 12 0.9575 275.7 205.1 123.7 179.9 162.2 465.7Ammonia.1% CaO 56 0.036 13 1 1.75 1.3 1.58 1.14 1.03 2.96Lime1% CaO 56 0.361 13 1.01 17.7 13.2 15.8 11.5 10.4 29.9Lime100 ppm Alkalinity 100 0.001 9.4 1 0.049 0.0364 0.022 0.032 0.0288 0.0827 (as CaCO3)1000 ppm Alkalinity 100 0.01 9.4 1.001 0.49 0.365 0.22 0.32 0.288 0.828 (as CaCO3)10,000 ppm Alkalinity 100 0.101 9.4 1.01 4.95 3.68 2.22 3.23 2.91 8.36 (as CaCO3)

Base Equivalents Chart


Titration CurveStrong Acid with Strong Base

01234567

89

1011121314

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1

Base Addition

pH


Two Position On - Off Control

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

Measured variable will cycle around control point (7.0 pH) Guidelines for control

• Tank Retention Time > 5 minutes• Good Chemical Mixing, Tank turnover every two minutes• Use alarm dead band and/or delay time on/off to

compensate for minor variations in control


Proportional + Integral (PI) Control

0.00

1.00

2.00

3.004.00

5.00

6.00

7.00

8.00

Tank or Vessel retention time < 5 minutes In-line mixing prior to sensor important Adjust to achieve 4:1 decay ratio after process upset

file name: ph.pptfeb 2001 ph theory applied to proper hardware selection

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