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Focuses on measurement of pH, ORP (Redox), and Conductivity and aspects related to inline measurement of these critical analytical parameters. Discussion topics include scientific theory, measurement challenges, proper troubleshooting, installation, key applications, and the future of analytical measurements
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pH & Conductivity Parameter TrainingMeasurement, Maintenance & the Future
ISA Boston SectionMarch 15, 2011
U
E2
E3
E1
Reference-electrolyte
E6
E5
E4
InnerBuffer
Internal usage only
What is pH ?
The statements: “Acid” “Neutral” “Alkaline”
are replaced with precise numerical values
Internal usage only
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH
acidic: pH 0-6.9 [H+] > [OH-][H+] > 10-7 M
neutral: pH 7.0[H+] = [OH-][H+]= 10-7 M
alkaline: pH 7.1 - 14[H+] < [OH-][H+] < 10-7 M
Definition: pH = - log [aH+ ]
H20 H+ + OH-
[aH+ ] * [a OH-
] / [aH2O] = 10 -14
pH basics - pH scale
Internal usage only
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
lemonjuice
orangejuice
beer
cheese
milk
pure water
egg white
borax
Milk of Magnesia
Some examples of pH values
H2SO4 (1N)4.9%
HCl (0.1N)0.37%
acetic acid (0.1N) 0.6%
HCN (0.1N)0.27%
sodium bicarbonate 0.84% (0.1N)
potassium ac. 0.98% (0.1N)
NH4OH 0.017% (0.01N)
NH4OH 1.7% 1.0N
NaOH 4%
Internal usage only
pH is a potentiometric measurement via an electrochemical sensor/electrode/probe
U= EpH -Eref (mV)
This potential difference is a function of the solution being measured
EpH
glass electrode
Eref
reference electrode
high impedance pH Meter
How does pH measurement work?
Internal usage only
Combination pH Sensors
Internal usage only
H+ H+
++
++
+
+
------
Acidic AlkalineGlassmembrane
Glass membrane (0.2 - 0.5mm)
Gel layer ca. 1 µm (outer and inner)
positivecharge
negative charge
internal buffer
The surface layer of the glass membrane is the “key performer” in each pH measurement! pH is a measurement of the potential difference between inner and outer layer of glass membrane!
What is special about pH glass?
This is one reason why pH sensors need to be stored in salt solution when not in use!
Internal usage only
pH meter
Referenceelectrolyte
Innerbuffer
E1 Potential of the reference lead-off system
E2 Diaphragm or diffusion potential
E3 Potential of internal lead-off
E4 Potential on the inner surface
E5 pH dependent potential (on the outside of the membrane)
Potential of the glass electrode (E5) can’t be measured individually. A second (reference)electrode is necessary. The potential of this electrode must be independent of the sample solution (buffer)
Combined pH electrode- glass and reference electrode integrated
What is a combination electrode?
E1
E2
E3
E4
E5
Internal usage only
Biotech pH sensors have a reference junction typically composed of ceramic
Reference Junction
Ceramic has uniform pore size and allows for capillary action to connect the reference buffer to the process
Many Mettler-Toledo sensors have pressurized reference systems that physically push buffer out of the sensor
Internal usage only
E = E0 + 2.303 R T log n F
1. Internal Reference Potential2. Inner Glass/Solution Potential 3. External Reference Potential4. LIQUID JUNCTION POTENTIAL
The Nernst Equation
Internal usage only
E = E0 + 2.303 R T log n F
2.303 R T is known as “The Electrode Slope” n F
IDEAL SLOPE = 59.16 mV/pH unit at 25 °CSlope is temperature dependent
The Nernst Equation
Internal usage only
The Nernst Response Curve
mV = 59.16
pH
mV
E = Eo + 2.3 RT/F log aH+
where 2.3 RT/F = 0.198TK = 59.16 mV @ 25oC
Internal usage only
pH Electrode Calibration Curve
pH
0 4 7 10 14
+ 500 mv
0 mv
- 500 mv
mill
ivol
ts
pH 10.00 buffer
0.0 millivolts
pH 7.00 buffer
-177.5 millivolts
IDEAL VALUES!
Internal usage only
Understanding Temperature Compensation
There are two temperature effects to account for:
-Nerstian (graph on the left dealing with the physical nature of the sensor) This is what temperature compensation handles
-Solution temperature dependence This is an actual temperature effect that you want to measure.
0 oC 25 oC 30 oC20 oC 40 oC10 oC
+/- 0.6 pH
pH vs. Temperature - "pH 2.00" Buffer
1.97
1.98
1.99
2
2.01
2.02
2.03
2.04
0 20 40 60 80 100
Temperature C
pH
Isopotential point
Internal usage only
Temperature Error (No Temp. Compensation)
Temperature error table for pH signal
pH°C 2 3 4 5 6 7 8 9 10 11 125 0.30 0.24 0.18 0.12 0.06 0 0.06 0.12 0.18 0.24 0.30
15 0.15 0.12 0.09 0.06 0.03 0 0.03 0.06 0.09 0.12 0.1525 0 0 0 0 0 0 0 0 0 0 035 0.15 0.12 0.09 0.06 0.03 0 0.03 0.06 0.09 0.12 0.1545 0.30 0.24 0.18 0.12 0.06 0 0.06 0.12 0.18 0.24 0.3055 0.45 0.36 0.27 0.18 0.09 0 0.09 0.18 0.27 0.36 0.4565 0.60 0.48 0.36 0.24 0.12 0 0.12 0.24 0.35 0.48 0.6075 0.75 0.60 0.45 0.30 0.15 0 0.15 0.30 0.45 0.60 0.7585 0.90 0.72 0.54 0.36 0.18 0 0.18 0.36 0.54 0.72 0.90
No temperature error Temperature error < 0.1 pH units Temperature error > 0.1 but < 0.3 pH units Temperature error = or > 0.3 pH units
( simplified model with Error = /pH (pr.) - 7/ x /(T (pr.) - 25):10/ x 0.03 )
Internal usage only
Causes of Failure or Measurement issues
Bubbles forming at the tip on the inside of the sensor
- Mount sensor 15 degrees to vertical
Reference Junction Fouling due to coating or chemical reaction.
- Keep the reference clean
Reference Poisoning due to ingress and chemical reaction.
Pre-mature aging due to exposure to heat.
- Such as SIP or steam sterilization
Internal usage only
Coating of the glass or even binding of critical ions.
Abrasion of the glass.
Electrical Connection failure due to moisture intrusion, incomplete connection, dirty or corroded contacts, etc.
Calibration issues
Causes of Failure or Measurement issues
Internal usage only
pH Calibration 101- Bracket your process pH
pH
0 4 7 10 14
+ 500 mv
0 mv
- 500 mv
mill
ivol
ts
pH 10.00 buffer
0.0 millivolts
pH 7.00 buffer
-177.5 millivoltsProcess pH value
Cardinal Rules for pH Calibration
Always have your calibration buffers “bracket” your process pH measurement of interest.
-Example; Process pH of 8.2 should utilize 7 and 10 buffers
Never calibrate with two buffers more than 3 pH units apart.
Internal usage only
pH Calibration-Impact of buffers
Importance of good buffers
-Buffer quality is variable Does 7 buffer contain a biostat? Is accuracy +/-0.01 or 2 Is there a pH v. temperature table?
-Buffers are nominal values Choose the right automatic table or a
custom.
-Alkaline buffers >10 readily absorb CO2 and change pH.
-Mind contamination and aging aspects.
Automatic temperature compensation and buffer tables automatically select the proper buffer value at a given temperature increasing calibration accuracy.
Internal usage only
pH Calibration /Justification
1. StepDetermine Asymmetry Potential / Zero
Point
2. StepDetermine the Slope
pH Buffer 7.00
pH Buffer
4.00
[mV]
200
-200
pH
7 14
[mV]
200
-200
pH
7 14pHas
4
[%] Slope
Internal usage only
pH Calibration
Always ensure the electrode is clean before calibration.
Zero point: pH 7 buffer. The E0 point is a critical point in the calibration curve and an indication of sensor status. Should be close to 0 mV
Use fresh buffers.
Bracket your process measurement when calibrating
Rinse the electrode and dab dry between calibration measurements. Do not rub the electrode. Rubbing could cause static charges and disrupt sensor function.
Make sure you have an acceptable slope and fast response time. Good transmitters will tell you how your sensor is doing.
-Good slopes range from 90% to 101% (possibly even lower)
-A pH sensor should reach a stable value in buffer in 30S or less
Internal usage only
pH Simulator (mV input)
Simulates an ideal electrode in different buffers and at different temperatures, also accounting for the resistance of the pH-glass membrane. Using the simulator it is possible to check the amplifier for:
Calibration and linearity pH cable viability Temperature compensation Quality of input circuitry (resistance and current) Presence of ground loops
Internal usage only
How frequently should the sensor be calibrated?
Depends on a few factors:
The nature of the solution being measured
The accuracy required by your SOP
The quality of the sensor
Internal usage only
Grab Samples- Why don’t they agree?
Physical changes to the grab sample
Sample pH not stable
Sample reacted with CO2 or other atmospheric gas
Sample temperature different from process
Sample hot, electrode cool
37oC
20oC
Same sample
Internal usage only
pH Sensor Installation (How to Install)
All of our pH sensors use liquid buffer in the sensing electrode OR liquid electrolyte in the reference. Therefore proper installation of a pH sensor is 15 degrees above horizontal!
15°
Internal usage only
pH Sensor Installation (How NOT to Install)
Sensors mounted horizontally / parallel to the ground or worse UPSIDE DOWN tend to form an air bubble between the glass and internal electrolyte of the sensor
NO!
HECK NO!
Air Bubble
Internal usage only
pH Recap
Take care of the glass tip and reference junction!
Shake down the pH sensor
Remove the silicone bead from the reference junction
Keep the sensor hydrated in 3 molar KCl or even 4 buffer
Remember: These sensors have a shelf life
Install the sensor 15 degrees to vertical
Bracket your process pH measurement when calibrating with buffers
Be wary of buffer shelf life and alkaline buffers absorbing CO2
Make sure temperature isn’t a factor
Buffer 7 should be darn near 0 millivolts
Each pH measurement unit shift should account for 59.16mV
Use a pH simulator if you suspect a problem with the cable or transmitter
Internal usage only
What is ORP ?
The qualitative statements, such as
“Oxidizing” “Reducing”
are replaced with Redox (ORP) potential values
You may also hear the term “Redox”
Internal usage only
What is REDOX measurement tell you?
The REDOX potential is a measurement of the affinity of a solution to either gain or lose electrons when it is subject to change by introduction of a new species.
-A solution with a higher (more positive) reduction potential than the new species will have a tendency to gain electrons from the new species (i.e. to be reduced by oxidizing the new species).
-A solution with a lower (more negative) reduction potential will have a tendency to lose electrons to the new species (i.e. to be oxidized by reducing the new species).
Internal usage only
ORP (Oxidation Reduction Potential)
Also known as “redox”
Oxidation— reaction w/ loss of electrons, higher potential
Reduction—reaction w/ gain of electrons, reduced potential
Measurement is again a combination electrode but this time it is the reference system of a pH sensor and platinum indicator electrode.
Still have to keep the reference diaphragm happy but platinum is more resilient than glass
Internal usage only
ORP (Oxidation Reduction Potential)
Potential is generated by the relative concentration of chemical oxidants and reductants
Oxidation can happen even in the absence of oxygen- Mg + Cl2 -> Mg2+ + 2Cl-
- In this reaction Mg is oxidized because it loses electrons and Cl is reduced because it gains electrons
Since the sensor is non-selective for Oxidants or reductants, the presence of either will contribute to the overall ORP value.
E = E0 + kT log[Oxidants][H+][Reductants]
Internal usage only
pH effect on ORP
• Potential (E) increases as [H+] increases (pH decreases)
• Potential (E) decreases as [H+] decreases (pH increases)
E = E0 + kT log[Oxidants][H+][Reductants]
Internal usage only
Temperature effect on ORP
• Potential (E) increases as temperature (T) increases
• Potential (E) decreases as temperature (T) decreases
E = E0 + kT log[Oxidants][H+][Reductants]
Internal usage only
Questions?
Internal usage only
What is Conductivity?
Measures only ionic (conductive) species
Fundamental measure of water purity
Mineral content
Chemical concentration
Conductivity is the ability of a solution to carry an electric current.
Internal usage only
Conductivity Measurement
1 cm
1 cm
d = 1 cm
Cond
Solution
Electrode plate
VAC
A conductivity “cell”, sensor, or electrode is an “electro-mechanical” measurement
Internal usage only
Conductivity Advantages and Limitations
ADVANTAGES- Simple
- Fast responding
- Low cost
- Reliable
LIMITATIONS- Non-specific
- Limited sensitivity
Internal usage only
Electrolytes
Acids- Substances which ionize in solution and produce hydrogen
ions, H+
- Hydrochloric Acid (HCl) dissociates into H+ + Cl-
Bases- Substances which ionize in solution and produce hydroxide
ions, OH-
- Sodium Hydroxide, NaOH, dissociates into Na+ + OH-
Salts- Substances which ionize in solution and produce neither
hydrogen or hydroxide ions
- Sodium Chloride, NaCl, dissociates into Na+ + Cl-
Internal usage only
Conductivity- An Electro-mechanical Measurement
Conductivity Calculation:Conductance (meter) x Cell Constant (sensor) = Conductivity
14.13 mS x 0.1 /cm = 1413 µS/cm
Known Known
Back Calculate
Internal usage only
Conductance and Resistance
Conductance is the reciprocal of resistance
Conductance = 1/Resistance
1 µS = 1 Meg ohm
0.055 µS = 18.3 Meg ohm
Units of resistance are in Ohms ()
Units of conductance are measured in Siemens
1,000 µS (micro-Siemens) = 1 mS (milli-Siemens)
1,000 mS (milli-Siemens) = 1 S (Siemen)
1,000,000 µS (micro-Siemens) = 1 S (Siemen)
[Don’t confuse micro and milli !]
Internal usage only
Conductivity Units of Measure
Resistance ohm
Resistivity ohm-cm
Conductance siemens = 1 / ohm
Conductivity siemens/cm microsiemens/cm
(µS/cm)
millisiemens/cm (mS/cm)
microsiemens/m (µS/m)
millisiemens/m (mS/m)
Total Dissolved Solids (ppm TDS)
Internal usage only
Conductivity vs. Concentration
•Conductivity is non-specific; it responds to the sum of all ions in solution
•Can be used for concentration:
•In binary systems (1 chemical in water)
•Other chemicals contribute little conductivity compared to chemical of interest
•Background of other chemicals remains relatively constant
Internal usage only
Chemical Concentration Control
0
100
200
300
400
500
600
700
800
900
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
% by Weight
Mill
iSie
men
s/cm
Sodium Chloride Potassium Chloride Sodium Hydroxide
Hydrochloric Nitric Acid Sulfuric Acid
Internal usage only
Chemical Concentration Control
Internal usage only
Cell Geometry
1 cm
1 cm
d = 1 cm
Cond
Solution
Electrode plate
VAC
The measured resistance will be dependent on the spacing of the electrode – cell geometry “cell constant”
Therefore, units measurement has dimension component, ex. mS/cm
Internal usage only
H
HO
~
Measuring Conductivity
Na+
Cl-
H+OH-
Internal usage only
Typical Conductivity Values
Surface water ~ 250 µS/cm
Well water ~600 µS/cm
RO Water ~5 µS/cm
USP Water ~1 µS/cm
UPW 18 meg ohm
CIP Solution~60 mS/cm
Internal usage only
Common Measurement Challenges
Temperature Compensation
Calibration
Coating
Internal usage only
Concentration
As concentration increases, conductivity generally increases.
Internal usage only
H+
H
HO
OH-
~
Conductivity Temperature Effects
Na+
Cl-H+OH-
Na+ Cl-
Internal usage only
Temperature Coefficients
Temperature effects vary by ion type. Some typical temperature coefficients:
Sample %/ oC (at 25 oC)Salt solution (NaCl) 2.125% NaOH 1.72Dilute Ammonia Solution 1.8810% HCl 1.325% Sulfuric Acid 0.9698% Sulfuric Acid 2.84Sugar Syrup 5.64
Internal usage only
Sensor Cell Constant
1 cm
Conductivity Cell Constant = Length Area
1 cm1 cm2= = 1 cm-1
INSULATOR
ELECTRODE
ELECTRODE
INNER
OUTER
1 cm
1 cm
0.1 cm
0.1 cm-1
0.1 cm
Internal usage only
Contacting Conductivity
Two-Electrodes (Concentric Design)- Conductivity < 10 mS/cm
- Polarization errors - high conductivity
- Electrode coating errors significant
Internal usage only
Conductivity Traceable Calibration
Accuracy of cell constant
Accuracy of temperature measurement
Internal usage only
Conductivity Instrument Calibration
NIST Traceable Resistances to cover ranges of measurement for conductivity and temperature- Decade Boxes
- Instrument-specific Calibrators
Internal usage only
Contacting Conductivity
Four-Electrode Cell- AC voltage applied to outer electrodes,
voltage induced upon inner electrodes
- Measure induced voltage
- Minimizes polarization and electrode coating effects
- Conductivity: 0.02 to 800 mS/cm
V
Internal usage only
AC CurrentSource
AC VoltageMeasurement
Drive Electrodes
MeasuringElectrodes
Four Electrode Sensor
Four-ElectrodeMeasuring Instrument
Four-Electrode Conductivity Measurement
Internal usage only
Four-Electrode Conductivity Measurement
Four-electrode sensors and instruments can tolerate poor measuring conditions because:
- Electrode metal surface condition is less important. - Electrode fouling or coating has less effect. - Four-electrode sensors do not have the narrow
channels of high, two-electrode cell constants. The resulting flat surface design is much less vulnerable to fouling.
Internal usage only
Inductive Conductivity
“Electrode less”- Sending coil induces a
conductivity dependent current in receiving coil
- High conductivity solutions
- Electrode coating effects eliminated
- Conductivity: 0.05 to 2000 mS/cm
G D
induced current
Energized Measured
Internal usage only
Inductive Conductivity Measurement
Virtually non-fouling so it is great for sludge, oils, high particulate matter
No metal/solution contact
Reliable high conductivity measurements
Relatively large sensor size
Cell constant can be affected by surrounding pipe
Internal usage only
Inductive Sensor Installation
What distance from a wall should be kept when installing an inductive sensor in the pipe?
metalsynthetic
Measuring value
too low
Measuring value
too high
Measuring value correct
30m
m /
1.18
”30
mm
/ 1.
18”
Internal usage only62
Cell Installation -2 electrode
Recommended Cell Installation...
OUTLET
INLET
Flow should be directed at the end of the sensor
Conductivity cell installation must assure that the cell is completely immersed in water.
No bubbles can be within the annular space between electrodes or erroneously low conductivity (high resistivity) readings will result.
Upward flow is desirable so air can easily escape.
Internal usage only63
Cell Installation -2 electrode
NOT Recommended Cell Installation...
INLET
OUTLET
OUTLET
AIR
INLET
Avoid dead legs and air traps
Internal usage only64
Cell Installation -4 electrode
OUTLETINLET
Recommended
Maintain a minimum clearance between sensor and pipe
Internal usage only65
Cell Installation -4 electrode
NOT Recommended
Maintain a minimum clearance between sensor and pipe
Internal usage only
Conductivity, Resistivity, TDS Ranges
Conductivity
100M 10M 1M 100K 10K 1K 100 10 1
Ultrapure waterDeionized water
Distilled waterCondensate
Drinking waterCooling tower water
Percentage of acids, bases and saltWaste water
Brackish water, Sea waterWater for Industrial Process
5% Salinity2% NaOH
20% HCl
0.01 .1 1 10 100 1000 10k 100k 1000k 0.021 0.4 4.6 46 460 4.6k 46kTDS ppm
Conductivity and resistivity are measured at 25C; TDS is expressed as Sodium Chloride (NaCl)
Resistivity ohm-cm
µS-cm
Internal usage only
Main Applications and Measuring Range
0.01 0.1 1.0 10 100 1000 10k 100k 1000k Conductivity
(µS/cm)
100 M 10M 1M 100k 10k 1000 100 10 1 Resistivity (Ohm-cm)
Ultra pure water
Pure water
Make up water
Drinking water
Diluted acids, bases, saltsWaste water
Brackish water
Industrial process water
Acids, basesWater Processes
Biotech/Food and Beverage
Chemical Processes
Inductive
4 Elec
2 Elec
Internal usage only
So….where is analytical measurement today?
Internal usage only
Previous Analog Technology
Digital technology provides better sound quality
Internal usage only
Welcome to the Digital World!
Same electrochemical end of the sensor converted to digital signal which is more robust and gives more information
Internal usage only
Analog Sensor Technology
Limited information for troubleshooting
Most analog sensors provide the user with one piece of information to determine the health of the sensor:
Slope
Internal usage only
ISM: The Evolution of the Sensor
The processing of sensor diagnostics is fully integrated in the sensor electronics
Sensors can be pre-calibrated for easy and effective maintenance
More connectivity options
- Transmitter
- PC with iSense Suite
- Cableless module
Diagnostics data always updated by the sensor
- DLI: Dynamic Lifetime Indicator
- ACT: Adaptive Calibration Timer
- TTM: Time to Maintenance
All information is processed in the sensor for connectivity flexibility
Internal usage only
How Does Digital Sensor Technology Work?
With the ISM digital technology we have transferred the technical sensor experience and know how that we have collected over the years into the sensor and the sensor head.
A small microprocessor in the sensor head stores and processes all relevant data.
This information is digitally transferred to the instrument. ISM sensors are able to perform their own diagnostics in real-time
and in conjunction with our transmitters they can achieve predictive maintenance
Internal usage only
iSense™ ISM Asset Suite
Intuitive interface
No transmitter as interface required
Key performance indicators for fast sensor diagnosis
Sensor status is visualized
74
iSense allows verification and calibration of pH and DO sensors in lab conditions
iSense is key to maximize the benefits of the ISM technology
Internal usage only
UniCond® Conductivity Sensors with ISM®
Conductivity measuring circuit built into sensor body
- UniCond eliminates cable resistance and capacitance effects Traditional systems may have to transmit the analog AC
conductivity signal through cable 50 meters long.
With UniCon, the analog signal goes only 50 mm! It eliminates analog signal interference.
- THORNTON’s unique conductivity measurement method optimizes sensor accuracy.
With no signal degradation along the cable, UniCond delivers higher accuracy over greater distances.
Measurement range is greatly expanded without loss of accuracy.
UniCond® sensors provide breakthrough performance!
50 mm
50 meters
Internal usage only
UniCond® Conductivity Sensors with ISM®
UniCond delivers enhanced system accuracy that out-performs analog conductivity sensors
- Analog conductivity systems calibrate sensor and measuring circuit separately, with contributions to error from both, e.g.
Sensor cell constant accuracy: ± 1% Transmitter accuracy: ± 0.5% System accuracy, ± 1.5% plus cable effects
- UniCond System accuracy No error contributed by transmitter No error contributed by cable or noise pickup System accuracy = cell constant accuracy = ± 1%,
a 33% improvement in accuracy
UniCond® provides accuracy at least 33% better than analog conductivity sensors!
Internal usage only
Applications and Measurement Range
0.01 0.1 1.0 10 100 1000 10k 100k 1000k Conductivity
(µS/cm)
100 M 10M 1M 100k 10k 1000 100 10 1 Resistivity (Ohm-cm)
Ultra pure water
Pure water
Make up water
Drinking water
Diluted acids, bases, saltsWaste water
Brackish water
Industrial process water
Acids, bases
CURRENT 4E RANGE
CURRENT 2E RANGE
UniCond® extends the range of measurement to cover UPW to seawater with a single sensor!
UPW to seawater with a single UniCond® sensor !
Expanded range with enhanced accuracy!
Internal usage only
Thank you!
Internal usage only
Questions?