Analytical Measurements: Troubleshooting, Maintenance and the Future

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)


Internal usage only

Temperature effect on ORP

• Potential (E) increases as temperature (T) increases

• Potential (E) decreases as temperature (T) decreases


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.



NOT Recommended Cell Installation...

INLET

OUTLET

OUTLET

AIR

INLET

Avoid dead legs and air traps



OUTLETINLET

Recommended

Maintain a minimum clearance between sensor and pipe



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?

Analytical Measurements: Troubleshooting, Maintenance and the Future

Technology

process ph ph

ph mv e

e phe refmv

ph signal ph c

ph calibration

ph acidic

ph basics ph scale

ph isopotential