Transcript pH Basics & Metering Pump Control

pH Installation &
Maintenance
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Agenda
 What is pH
 pH Electrode Construction
 Installation Tips
 Sensor Storage & Maintenance
 pH Calibration
 pH System Troubleshooting
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What is pH?
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What is pH?
Expressing Concentration in Terms of pH
pH is a Unit of Measurement
 Defined as the Negative Logarithm of Hydrogen Ion Activity
(concentration and activity are closely related)
•
pH = -log [H+]
– Example: if the hydrogen concentration is 1.0 x 10-4
moles/liter, the pH is 4.0
H+
H+
O2-
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H+
H+
+
O2-
4
What is pH?
pH measures the relative amount of hydrogen (H+) ions compared to hydroxide
ions in an aqueous solution:
• H20 = H+ + OH-
H+
H+
O2-
H+
H+
+
O2-
In an aqueous (water based) solution, the concentration of hydrogen ions
multiplied by the concentration of hydroxide ions is a constant number:
• Kw = [H+][OH-], Kw = dissociation constant for water
• At 25ºC, Kw = 1 X 10-14; at 35ºC, Kw = 1 X 10-14
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What is pH?
Acids and Bases
pH
Acid dissolves in water to
furnish H+ ions

HCl
H+ + Cl-
Base dissolves in water to
furnish OH- ions

Hydrochloric Acid

Na+ + OH
-
Sodium Hydroxide
H+ + NO3-
HNO3
NaOH
Nitric Acid

KOH
K+
+ OH
-
Potassium Hydroxide

HF
H+ + F-
Hydrofluoric Acid

NH4OH
NH4+ + OH
Ammonium Hydroxide
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What is pH?
• pH is a measure of hydrogen ions in a water based solution
• It is expressed in hydrogen ion concentration: pH = -log [H+]
• Acids and bases added to water changes the amount of H+ ions.
pH
Acid
0
1
2
3
4
5
6
Neutral
7
8
9
10
11
12
13
Alkaline 14
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Hydrogen Ion (H+)
1
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
0.00000001
0.000000001
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
0.00000000000001
Hydroxyl Ion (OH-)
0.00000000000001
0.0000000000001
0.000000000001
0.00000000001
0.0000000001
0.000000001
0.00000001
0.0000001
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
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How is pH expressed?
• pH measurement is expressed on a scale of 0.0 to 14.0
• pH electrode will produce 59.16 mv per pH unit at 25 °C
• pH of 7.0 is neutral (H+ ions = [10-7] and OH- ions = [10-7])
• More hydrogen ions (H+) is considered an acid
• More hydroxyl ions (OH-) is considered alkaline (base)
Acids
pH
mV
0
1
2
3
Bases
4
5
6
+414 +355 +296 +237 +177 +118 +59
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7
8
00
-59
9
10
11
12
13
14
-118 -177 -237 -296 -355 -414
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pH Values of Common Items
4.0% NaOH
Crackers
Blood
Pure Water
Milk
Beer
Tomatoes
Oranges
Lemons
4.9 % H2SO4
0.0
1.0
NaOH = Sodium Hydroxide
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
pH
H2SO4 = Sulfuric Acid
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pH Electrode Construction
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Traditional pH Sensors
A combination pH sensor consists of:
Reference electrode
pH electrode
Temperature element
Reference electrode
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pH
electrode
Temperature element
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pH Sensing Electrode
Hydrogen sensitive glass
Inert glass stem
A silver/silver chloride wire
Buffered electrolyte solution
Silver/Silver Chloride Wire
Electrolyte
Solution:
Glass membrane
thickness 0.2-0.5 mm
Glass stem
Hydrogen sensitive
glass bulb
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Sol-gel (silica oxide layer)
on both sides of glass
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Traditional Reference Electrode
Silver/Silver
chloride wire
Sealed barrier
Silver wire
Sealed barrier
3.5 M KCL
1st reference chamber:
3.5 M KCL
Plastic tubing
Silver Chloride
2nd Junction
Front Porous
Junction
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2nd reference chamber:
KCl Electrolyte
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Electrode Construction
To make a pH measurement, a pH and a Reference electrode are required.

A combination pH sensor consists of a pH electrode and a reference
electrode built into one body.
–
–
A pH sensor has a glass membrane which is sensitive to H+ ions
A reference sensor has a porous membrane with electrolyte on the inside
pH Sensor
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and
Reference Sensor
= Combination
Sensor
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Temperature Measuring Elements
• Pt 100, Pt 1000, 3K Balco, etc…
• Glass error of 0.03 pH /per pH unit/per10 degree C
0 pH error range
0.1 pH error range
°C
2
3
4
5
6
7
8
9
10
11
12
5
.30
.24
.18
.12
.06
0
.06
.12
.18
.24
.30
15
.15
.12
.09
.06
.03
0
.03
.06
.09
.12
.15
25
0
0
0
0
0
0
0
0
0
0
0
35
.15
.12
.09
.06
.03
0
.03
.06
.09
.12
.15
45
.30
.24
.18
.12
.06
0
.06
.12
.18
.24
.30
55
.45
.36
.27
.18
.09
0
.09
.18
.27
.36
.45
65
.60
.48
.36
.24
.12
0
.12
.24
.36
.48
.60
75
.75
.60
.45
.30
.15
0
.15
.30
.45
.60
.75
85
.90
.72
.54
.36
.18
0
.18
.36
.54
.72
.90
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pH and Sensor Calibration
pH
•
•
•
•
Theoretical slope; 59.16 mV/pH unit at
25C.
Calibrate in pH buffer solutions to
determined actual slope.
Slope indicates the true performance of
the pH glass.
A slope of 49 to 59 is typically
acceptable for a working electrode.
Below 49, the pH glass is bad.
millivolts
•
Theoretical output
500
177 mV
0
400
1
300
2
0 mV
3
200
4
5
100
6
0
-100
7
0
1
2
3
4
5
6
7
88
9
10
11
12
13
14
9
10
-200
11
12
-300
13
-400
14
-500
pH value
3 pH units
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Installation Recommendations
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Preamplification
Preamplifiers are required for pH sensor
•
Preamplifiers boost high impedance signals
•
Preamplification allows separation for hundreds of feet
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Installation Tips
Hints for Installing sensors with preamplifiers
• Mount electrodes in a location with ample clearance for removal for
periodic cleaning and recalibration.
• Choose a location that keeps the electrode glass completely
submerged at all times.
• Place the electrode tip in pH 4 buffer during system maintenance or
storage to avoid dehydration and reduce chemical activity.
• Be aware of mounting positions-some sensors must be mounted +/45 degrees of the vertical.
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Installation – In-Line
In larger diameter tee’s watch for stack height of tee
fitting, and flush style reducer bushings making sure
sensor tip is in process
< 1.5”
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Installation : In-Line
1½ X ¾ in
Flush style
reducer bushing
2 ½ X 1½ in
Flush style
reducer bushing
2 ½ in Tee
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Installation : Submersion
Use “o”-ring
lubricant
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and seal tape
threads
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pH Sensor In-Line Installation
Caution: Air Bubbles
and Air Pockets
Caution: Sediment
and debris Build-up
Caution: Sensor
needs to stay wet
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pH Calibration
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pH Sensor Calibration
Two point calibration:
Standardize / Slope
Single point calibration:
Grab sample
pH 4
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pH 7
pH 10
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pH / ORP Sensor Calibration
Two Point Buffer Calibration
•
Calibrate when:
•
The sensor is initially installed to insure proper functioning
•
pH sensor is replaced
• Calibration establishes sensor slope; ensures linear output

Slope decreases with:
– age, coating, elevated temp.
– pH glass erosion by abrasion, strong NaOH, KOH or HF
– coating from grease and oils

Acceptable mV/pH slope is typically 49 mV/pH to 59 mV/pH
– Theoretical pH/mV slope @ 25ºC is 59.16 mV/pH
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pH Sensor Calibration
Single Point pH Calibration (Standardize)
To Perform a grab sample evaluation
•
Take sample at or near pH sensor installation point
•
•
Analyze grab sample ASAP using a portable or laboratory
analyzer (avoid temperature change)
•
Change the reading in the instrument to match the portable or
laboratory analyzer
Single point standardization:
•
Compensates for minor coating of the pH electrode.
•
Compensates for small offsets in the liquid junction potential
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pH System Start-up
Recommendations for simple start-up:
•
Place the sensor in 7.0 pH Buffer solution
•
Wait 10 minutes for temperature to equilibrate and cal temp
•
Using 4 and 7 pH buffers cal meter
•
Use fresh off-the-shelf buffer
•
Triple rinse and place in process
•
Allow minimum time to equilibrate
•
Repeat Buffer calibration in 24 hours.
•
Determine appropriate cleaning and calibrating schedule for your
application. Try weekly and based upon observed results
increase or decrease frequency for desired results.
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Sensor Storage & Maintenance
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pH Storage & Care
• When storing boxed sensors, lay the sensor flat to maximize
hydration of the reference surface. Keep the reference surface wet
at all times. Store sensors at a stable room temperature.
• Remove storage boot that sensor is shipped in; keep boot and reboot with pH 4 buffer to keep sensors wetted during storage and
transportation.
• If the sensor dehydrates: Clean sensor, and soak the sensor tip
in pH 4 buffer for 24 to 48 hours, then visually inspect the electrode
for surface cracks, swelling, or discoloration. Severely dehydrated
electrodes cannot be restored to reliable operation.
• Never expose electrode to temperatures below 0° C or allow it to
dehydrate. These conditions may damage the electrode.
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pH Storage & Care
Store between 10 and 30°C
Use protective caps with potassium chloride KCl solution or pH 4 buffer
Do not allow to dry out, else problems
 Slow response
 High glass resistance
 To fix, re-hydrate for 24-48 hours in 4 buffer or KCl solution
Factors that decrease sensor lifetime
• Heat
• Extreme cold
• Vibrations
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Sensor Cleaning
Rinse with DI or warm tap water
Soak for a few minutes in one of the solutions below
Rinse with DI or tap water
Re-hydrate in pH 4 buffer or KCl solution, 1-24 hours
Gentle use of toothbrush is ok
Cleaning Solvent/Solution
Application
Warm water & detergent
For normal applications
Less than 5% HCl acid
For coatings of lime and hydroxides
Acetone or isopropyl alcohol
For coatings containing oils & fats
Dilute bleach, 1-5%
For coatings with algae, bacteria or
molds
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Cleaning Frequency
Keep sensor reasonably clean to maintain accuracy
Frequency depends on…
 Process
 Accuracy required
 Experience with application
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pH/ORP System Troubleshooting
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Possible Problems in Existing Installations
•
•
•
•
Electrode cleaning and calibration. Electrodes should be removed
and cleaned on a regular maintenance schedule depending on the
application.
Faulty electrode. Sensor life is application specific. High temperature
and chemicals shorten lifetimes.
Faulty amplifier assembly
Electrical wire problems. Some environments can cause the wiring to
corrode and cause faulty connections.
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Potential Measuring Problems-Troubleshooting
•
•
•
Use simulator to check electronics
Use raw mV values to investigate sensors
Use buffers to gauge:
response time
accuracy
stability
•
Best Practice: Use logs to record maintenance intervals and events
•
Check critical connections  make sure they are dry
•
Check seal areas
•
Check sensor surfaces for breakage or fouling
•
Clean sensor and calibrate; if the sensor cannot be calibrated, it is
probably time to replace it
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Potential Measuring Problems
What can Affect a pH Measuring or Reference Electrode Output?
Reference Cell
Silver Chloride
Billet
Heavy metals like
Hg++, Cu+, Pb++,
ClO4- Penetrate the
Liquid Junction and
react with KCl
KCl solution
Silver Chloride
Billet
pH (glass)
pH Measuring Electrode: Internal solution
and silver chloride (AgCl) wire is protected
from outside elements.
pH Sensitive Glass
Extreme pH or temperature values, harsh
chemicals, and hairline cracks will affect the
reading
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Porous
Reference
Junction
Stray electrical
currents
Br-, I-, CN-, Scompounds
penetrate liquid
junction and react
with Ag+
Oils, grease, dirt,
from the outside and
silver compounds
formed on the inside
will clog the
reference junction
Reference Electrode: There are many elements that
can affect the reference electrode.
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Potential Measuring Problems

pH Measurement Electrode
– Process coating
– Mechanical failure
– Chemical attack

pH Reference Electrode
– Poisoning
– Junction Blockage
– Depletion
– Ground Loops
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pH Troubleshooting
Measuring Offset:
•
Thoroughly decontaminate electrode with clean water
•
Clean electrode tip in beaker of 5% HCl solution, < 30 seconds
•
Thoroughly rinse off HCl residue with clean water
•
Place electrode in beaker of fresh 7.0 pH Buffer solution
•
Using electrode gently stir buffer for about 10 seconds.
•
Wait for mV on controller to stabilize (should only need a few seconds)
•
mV value on controller display should be 0 mV, +/- 50 mV
•
Make note of mV value and retain for determining Span.
Example: mV value measured in 7.0 pH Buffer = - 10 mV
Caution: Always wear appropriate safety gear and exercise proper safety
practices when working with or near liquids & chemicals
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pH Troubleshooting / Maintenance
Measuring Span:
•
Remove electrode from 7.0 pH Buffer solution
•
Thoroughly rinse electrode with clean water
•
Place electrode in beaker of fresh 4.0 pH Buffer solution
•
Using electrode gently stir buffer for about 10 seconds.
•
Wait for mV on controller to stabilize (should only need a few seconds)
•
mV value on controller display should be +177 mV away from the
value measured in 7.0 pH Buffer, +/- 50 mV.
Example: Offset value was -10 mV, therefore Span value should be
+167 mV, +/- 50 mV (+117 to +217 mV)
Caution: Always wear appropriate safety gear and exercise proper safety
practices when working with or near liquids & chemicals
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pH Troubleshooting
Occurrence:
•
•
•
•
•
•
•
•
•
•
•
Natural aging
Clogged Reference
Coated pH Glass ORP Tip
Damaged pH Glass
Sodium Ion Error
Attacked Reference
Depleted Reference
Cracked Glass
Shorted Wiring
Moisture intrusion
Glass pH Etched
Effect:
mV slowly drift more positive
Slow response in same direction
Erratic drifting multiple direction
Short Span
Reads low above 12.5 pH
Large Offset (prematurely)
Large Offset (over time)
mV value always around +55, or 0mV
mV value always around 0 mV
Very wrong mV values, erratic, stuck
>2% HF, or >12.5 pH
Note: Offset problems are typically associated with Reference half
Span problems are typically associated with Measurement half
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Potential Measuring Problems
Ground Loop Problems

Sensor picks up current from improperly grounded circuit, typically from
pumps, motors, or other closely located electrically driven equipment
NOTE: Plastic piping can be a source of this problem because:
1. It picks up current or static charges from liquid flow
2. And/or it picks up electrical current from pumps and other
equipment but may not have grounding




Reference Element can be stripped, shortening sensor life
Differential Sensors eliminate the issue of stripped reference element
Symptoms include sensor reading drifting, noisy, or erratic
Eliminating Ground Loops:
1. Find and Fix Grounding Problem
2. Use a Solution Ground
3. Use a Differential Sensor with Solution Ground
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pH: Assessing pH Application (step 1)
pH / ORP application:
• Application (scrubber adjust, waste treat adj, process adj, effluent monitoring, etc)
• Process range e.g., 3 – 5 pH
• Desired process value range e.g., 7.5 – 9.5 pH
• Process conductivity
• Process temperature range
• Solution chemistry (what, percentage)
• Solution quality (DI water, oils / grease, sludge, crystals, solids)
• Chemical compatibility of all wetted components
• Distance from sensor to indicator
• Inline (pipe size, material, flow rate, pressure, recirculation or single pass)
• In tank (tank size, flow rate, type of mixing, retention time, flow through or batch)
• Reagent type and concentration
• Chemical dosing (proportional, on/ off)
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pH Things To Watch For
•
•
•
•
•
•
•
•
•
•
Dry Glass / Junction (keep submerged in liquid)
Clogging of reference (avoid build-up of solids, oils/grease, salts, etc)
Coating of Glass (avoid build-up of oils/grease, solids, salts, etc)
Etching of Glass (avoid >2% hydrofluoric, caustics above 12.5 pH, etc)
Stray electrical currents (plating baths, poor grounding)
Chemical attack of Reference Fill (minimize exposure to mercury,
copper, lead, perchlorate, etc)
Chemical attack of Reference Element (minimize exposure to
bromine, ammonia, iodine, sulphur, cyanide, etc)
Moisture intrusion at back of pre-amp (properly seal seal
chemistry, condensate)
Moisture intrusion between electrode & pre-amp (select proper
“O”-Ring and lubricant to properly seal out chemistry)
Cracked Glass (mount in an easily accessible area for maintenance, away
from mixer blades, do not drop)
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pH Things To Watch For - continued
•
•
•
•
•
•
Controlling pH (should only be performed in tank or recirculation loop)
Retention time (in flow through tank, size the tank to allow at least 10
minutes for every 3 pH units of neutralization)
Improve pH adjustment (use proportionally controlled metering pumps,
and provide proper mixing)
Elevated Temperature (increases ionic activity, decreasing electrode
life)
Temperature fluctuations (cause pumping of fill solution, decreasing
electrode life)
Low conductivity solutions (deplete electrode fill solution, decreasing
electrode life)
•
Mounting angles (use plunger type electrodes for mounting greater than
70° from vertical, or less than 30° from horizontal)
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pH Questions & Answers
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