pH Theory

What is pH ?

• pH is a number that describes

• the degree of acidity (H+) or
• the degree of alkalinity (OH-)
• of a solution.
Acids and bases

• Acid - dissolves in water to furnish H+ ions

• Base - dissolves in water to furnish OH- ions

HCl -----> H+ + Cl -
HNO3 -----> H+ + NO3-
HF -----> H+ + F -

NaOH -----> Na+ + OH-

KOH -----> K+ + OH-
NH4OH -----> NH4+ + OH-
Why do we measure pH ?

• to produce products with defined properties

• to produce products with less costs
• to avoid damage to people, materials and environment
• to fulfil regulatory requirements
• to protect equipment
• to get new knowledge, research and development
Definition of pH

• The term pH is derived from the Latin language:

pondus Hydrogenii = weight of hydrogen

potenti Hydrogenii = power of hydrogen

• A pH value is defined as the negative logarithm of the

hydrogen ion activity (a)

Sörenson Equation: pH = - log aH+

The pH scale

• The pH scale is determined by the dissociation of water

H2O = H+ + OH-

• The activities (a) of these two ions are interrelated by the ionic
product ( I )

I = (aH+) (aOH-) = 10-14 (at 25oC)

Definition

• Definition : pH = - log aH+

Water: H2O ----> H+ + OH-

Neutral: [H+] = [OH-]

[H+] = 10-7 M
pH 7.0

Acidic [H+] > [OH-]

[H+] > 10-7 M

Alkaline: [H+] < [OH-]

[H+] < 10-7 M
pH range of some products

Orange pure water

juice
Milk of Magnesia
Lemon cheese
juice egg white
beer milk borax

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

HCN 0.27% (0.1N) NaOH 4%

acetic acid 0.6% (0.1N) NH OH 1.7% 1.0N
4
NH OH 0.017% (0.01N)
HCl 0.37% (0.1N) 4
Potassium acetate 0.98% (0.1N)
H SO 4.9% (1N)
2 4
Sodium bicarbonate 0.84% (0.1N)
How does pH work ?

• The pH value is measured by glass reference

Potentiometry electrode electrode
high impedance
pH Meter

The pH measuring system:

pH (also called glass) electrode

Reference electrode

• The difference between the potentials

of the pH and reference electrodes is a
function of the solution being
measured.
Nernst equation for pH electrodes

E = Eo + 2.3 RT/F x log aH+

E = Measured potential
Eo= Standard potential if aH+ = 1 mol/l
R = Universal gas constant
T = Absolute temperature (oK) mV
F = Faraday constant
2.3 RT/F = Nernst potential (slope)
aH+ = H+ activity of measured solution
DmV = 59.16
1 pH = 59.16 mV at 25oC

pH
7 8
Electrodes Design

• Constructrion of glass and reference electrodes

S7 Connector
Filling port
AgCl coated
Lead-off wire Reference electrolyte

Screen AgCl coated

Lead-off wire
Internal
buffer
pH sensitive Ceramic junction
Membrane glass
 Electrode Design

• Construction of a combination pH electrode

Screw Cap,
S7 or MultiPin head
Integrated
temperature
Refill opening, Safe Lock probe
Ceramic
Reference electrolyte junction

ARGENTHAL
Reference System
pH sensitive
Silver Ion Trap
glass membrane
 pH Glass Electrode

• pH sensitive glass membrane

• Thermodynamic equilibrium of hydrogen ions at phase
boundary
• Diffusion of H+ ions to maintain equilibrium

internal buffer
+ H+ = constant - Alkaline
Acidic
--
positive + negative
+ charge medium
medium charge
+
- -
+
+ -
H+ H+
Glass membrane (0.2 - 0.5mm)
Gel layer ca. 100 mm
Diaphragm potential

• Large diaphragm potentials (high diffusion potentials)

• Time required for dynamic equilibrium

• Highly acidic solutions

• Highly alkaline solutions
• Distilled water
• Solution with high salt concentration
• Partly organic and organic solutions
• Certain suspensions

Contaminated diaphragm -----> Number one “ troublemaker ”

Requirements for glass membranes

• An internal buffer which provides a constant zero point

• Special glasses with high selectivity for H+
• Good mechanical properties of the pH Glass

Meeting these requirements

• Internal buffer with constant pH and chloride ion activity
• Different glass compositions for different pH ranges
• Different membrane shapes for different applications
• Strong pH membrane glass
Requirements for pH reference electrode

• Small and constant diaphragm potential

• Stable potential of the reference lead-off system
• No chemical reaction between measuring solution and the
reference electrolyte

Meeting these requirements

• Special reference electrolyte (e.g. FRISCOLYTE)
• Special lead-off system (ARGENTHAL)
• Advanced patented Silver ion trap
• Open junction, solid state polymer electrolyte
Temperature influence

• How does temperature influence pH measurement?

• The pH value of the solution being measured
• The slope of the electrode
• The isothermal point of the electrode
• The response time of the electrode
• The lifetime of the pH electrode
Temperature influence

1. The pH value of a solution

• The specific temperature coefficient of each solution being
measured causes the pH values to be different at different
temperatures

20oC 30oC
0.001M HCl pH 3.00 pH 3.00
0.001M NaOH pH 11.17 pH 10.83
Phosphate buffer pH 7.43 pH 7.40
TRIS buffer pH 7.84 pH 7.56

• Inherent properties of solution cannot be compensated with

meters
Temperature influence

• The pH value of a solution

• pH in vessel ≠ pH in beaker
• Sample solutions show variation of pH value with temperature
(temperature coefficients)

50oC 20oC

Same sample
Temperature Influence

5. Lifetime of a pH electrode
• The approximate lifetime of a pH electrode at different
temperatures (with good storage and maintenance)

Measurements performed in aqueous solutions

at room temperature 1 - 3 years
at 90°C 3 - 9 month
at 120 °C few weeks only
Temperature Influence

4. Response time
• Equithal behavior of pH electrodes
• Due to a revolutionary design and production technology pH
electrodes have... Equithal behavior !

80oC

Equithal
30oC

conventional
0 10 20 30 t (min)

Conventional electrode Equithal electrode

Calibration

• “Easy cal”
Cal
Buffer 4.01

Buffer 7.00
pH calibration

• How is a pH electrode calibrated?

1. Zero point : pH 7
2. Slope : pH 4.01, pH 9.18 or other buffer
3. Always use fresh buffers
4. Rinse the electrode and dab dry between calibration
measurements

Do not rub the electrode!

pH calibration

• Zero-point calibration

+mV Buffer pH
7.00

pH
4 7 9

Calibrated Before
calibration

-mV
pH calibration

• Slope adjustment

+mV
178 Buffer pH 4.01 or 9.21

Before calibration

4 7 9 pH

Calibrated

-mV 178 mV = theoretical slope at 25oC

pH calibration

• How frequently should the electrode be calibrated?

• The frequency depends upon:

• The nature of the solution being measured
• The wanted accuracy
• The quality of the electrode
Accuracy of pH measurement

• Accuracy of pH measurement depends on :-

• Calibration
• correct procedure
• new and accurate buffers
• Frequency
• wait for stable value
• Temperature
• differences between calibration and measurement
• temperature measurement and control
• Electrode
• clean diaphragm
• correct type

• Realistic accuracy
• Laboratory: + 0.02 pH
• Process : + 0.1 pH
Accuracy of pH measurement

• Symptom - process reading differs from lab reading

• Grab samples - do they agree?
• Sample pH not stable
• Laboratory reading in error
• Sample not representative of process
• Sample temperature different from process
• Sample hot, electrode cool

50oC 20oC

Same sample
Aging and storage of pH electrode

• Combined electrodes
• definite aging with gel-filled reference systems
• slow drift of zero point because KCl diffuses continuously via
diaphragm
• Good storage and maintenance habits
• Never store electrodes dry
• Always store in the appropriate reference electrolyte
• Never store in distilled water!
 Seven
The New Mettler-Toledo Benchtop Meter Family
Overview of Products

• SevenEasy pH

• SevenEasy Conductivity

• SevenMulti
Meters of the Seven Family

• SevenEasy‘s: Basic level meters

• S20 SevenEasy pH
• basic pH/mV meter and complete with RS interface

• S30 SevenEasy Conductivity

• basic conductivity meter
Meters of the Seven Family

Seven Multi‘s :
• High Level
• All with dual channel capabilities

• S40 SevenMulti pH
• pH/mV meter
• S70 SevenMulti Conductivity
• conductivity meter
• S80 SevenMulti Ion /Ion
• dual channel meter for 2 Ion Measurements
• S47 SevenMulti pH/Conductivity
• dual channel meter for pH and Conductivity
Seven Multi‘s

= S40
 pH

 Conductivity = S70

Ion
 + = S80
Ion

Cond
+
 pH = S47
Customized combinations

• According to your needs (examples)

• S40 + ISFET Exp. Unit

• High end dual channel meter pH/ISFET
• S40 + Ion Exp. Unit
• High end dual channel meter pH/Ion
• S70 + Ion Exp. Unit
• High end dual channel meter conductivity/Ion
• S70 + ISFET Exp. Unit
• High end dual channel meter conductivity/ISFET
• S70 + Conductivity Exp. Unit ???
• Not possible!
SevenEasy - Technical Data

• Outputs
• RS232 : baud rate 1200, 8 data bits, 1 stop bit, no parity, no
handshake
• pH input : impedance > 1012 Ohm
• Ambient conditions:
• Relative humidity: 5 - 80 % (non-condensing)
• Temperature: 5 to 40 oC
• Height above sea level: max. 2000 m
• Dimensions: 180 * 180 * 65 mm
• Weight : 610 g
• Material: mainly ABS, reinforced with PC
SevenMulti - Technical Data

• Outputs
• RS232: adjustable baud rate (1200, 2400, 4800, 9600), 8 data bits, 1
stop bit, no parity, no handshake
• Analog output: output impedance < 10kW
• Ambient conditions:
• Relative humidity: 80 % at 31 oC, 50 % at 40 oC
• Temperature: 5 to 40 oC
• Height above sea level: max. 2000 m
• Dimensions: 190 * 240 * 65 mm
• Weight:
• Basic device: 850 g
• 1 Expansion Unit: 135 g
• Material: mainly ABS, reinforced with PC
 Standard and Kit Versions

•Standard-Version includes:
•Meter
•Operating instructions in five languages
•Test certificate with date, name and signature of testing
person
•Complete electrode arm with base

•Kit-Version includes additionally:

•S20K InLab413 and buffer sachets
•S30K InLab730 and buffer sachets
•S40K InLab413 and buffer sachets
•S70K InLab730 and buffer sachets
•S80K InLab413 and buffer sachets
•S47K InLab413, InLab730 and buffer sachets
 Seven
Features and Benefits
Seven Easy pH
SevenEasy pH – User Friendly Display

• All calibration data at a glance

• Calibration buffer group used
• Electrode offset (in mV) and electrode slope (in % of the theoretical
value)
• Electrode condition icon
• Calibration points during calibration

• All measurement data at a glance

• pH/mV reading
• Endpoint format
• Stability icon
• Temperature
• Temperature compensation mode (ATC or MTC)
SevenEasy pH - Display

• All meter data at a glance

• Data transfer icon
• Self-diagnosis icon
• Battery status icon
• Auto-off override icon
• Error index field

Benefits : Time saving

Ease of use
SevenEasy pH - Calibration

• 1, 2 and 3-point calibrations

• 3 predefined buffer groups
• Auto buffer recognition
• Programmed buffer tables
• 1 user-defined buffer group
• Up to 3 buffers
• Calibration points on display during calibration

Benefits: Flexibility
Ease of use
SevenEasy pH - Endpointing

• Auto endpoint - for most applications

• Manual endpoint - for delicate samples
• Stability icon - indicates stable signal

SevenEasy pH - Temperature Compensation

• Automatic temperature compensation (ATC) from -5 to
105oC
• Sensor integrated temperature probe
• External temperature probe
• Manual temperature compensation (MTC) from -5 to
105oC
SevenEasy pH - Modes

• pH mode
• mV mode

SevenEasy pH - Self-Diagnosis
• Find out whether a problem you might have is caused by
the meter or by something else

Benefits : Time saving

Traceability
SevenEasy Conductivity
SevenEasy Conductivity - Display

• All calibration data at a glance

• Used calibration standard
• Reference temperature

• Cell constant can be viewed by pressing just one button

• All measurement data at a glance

• Conductivity / TDS / Salinity / Resistivity reading
• Endpoint format
• Stability icon
• Temperature
• Temperature compensation mode (ATC or MTC)
• Temperature correction method (linear, non-linear)
SevenEasy Conductivity - Display

•All meter data at a glance

•Data transfer icon
•Self-diagnosis icon
•Battery status icon
•Auto-off override icon
•Error index field

•Meter menu can be entered just by pressing one

button

Benefits : Time saving

Ease of use
SevenEasy Conductivity - Calibration

• 3 predefined calibration standards (84 S/cm; 1413 S/cm and

12.88 mS/cm) with programmed buffer tables
• Alternative to a calibration, the known cell constant of your
electrode can be entered
• 2 different reference temperatures (20 and 25oC)

Benefits : Flexibility
Ease of use
SevenEasy Conductivity - Endpointing

• Auto endpoint - for most applications

• Manual endpoint - for delicate samples
• Stability icon - indicates stable signal

SevenEasy Conductivity - Temperature

Compensation
• Automatic temperature compensation (ATC) from -5 to
105oC for sensor integrated temperature probe
• Manual temperature compensation (MTC) from -5 to
105oC

Benefits : Flexibility
SevenEasy Conductivity - Temperature Correction
Methods
•Linear correction for samples which show a linear
dependance of conductivity on the temperature (in % / oC)
•If linear corrections coefficient is set to 0 % / oC,
measurements according to USP (26) are possible

Benefits : Flexibility
SevenEasy Conductivity - Modes

• Conductivity mode
• TDS mode (e.g. benchmark for life in water)
• Salinity mode (to measure the salt content of water)
• Resistivity mode (raw data of conductivity measurement)

Benefits : Flexibility
Traceability
Other SevenEasy Features
• Self – Diagnosis
• Find out whether a problem you might have is caused by the meter
or by something else
Benefit : Confidence
• Maintenance
• Service mode allows service technician to adjust sensor inputs
Benefit : Cost saving
• Battery Operation
• Make a portable meter out of your benchtop meter
• Auto-off function after 10 minutes
• Override auto-off function for timed interval readings
Benefit : Flexibility
Other SevenEasy Features

• Interface
• Every endpointed reading is sent to a PC or a printer through the
RS232 interface if a printer or a PC is connected
• Timed interval reading to transfer a reading every second can be
switched on and off
• Settings:
• Baud rate 1200
• Data bit 8
• Stop bit 1
• Parity none
• Handshake no

Benefits : Time saving

Data security
Seven Multi Meters
Seven Multi Meters

•General Concept
•Hotkeys and System Menu
General Concept

•One meter for all your pH / mV / rel.mV / Ion / Conductivity / TDS /

SAL / Res / ISFET measurements *USP

Benefits : Integrated measuirement solutions for pH,

conductivity and ISE
Save costs and bench space
Multiple measurements with the same meter

•Full modularity - turn your pH meter into a Conductivity meter by

simply exchanging Expansion Units *USP

Benefits : Upgradeable, Expandable, Futuristic

High flexibility in applications
 Seven Multi - General Concept

•Dual channel meter

•SevenMulti performs measurements with two Expansion Units
simultaneously (all combinations of Exp. U. possible, except Cond. /
Cond.)
•Automatic recognition of Expansion Units
Benefits : Ease of Use

•Large Display - shows all necessary measurement data in single and

dual channel mode, makes graphical illustration of measurement and
calibration data possible, allows full text menu description
Benefits : Ease of use and easy on the eyes
SevenMulti - General Concept

•Service mode – allows adjustment of sensor inputs, complete

testing for recertification and resetting of factory settings *USP
Benefit : confidence in results
•Alphanumerical keypad for easy data entry
Benefit : identification of samples and users
•Fixed hotkeys for simple and fast meter operation in all
configurations
Benefit : saves time
•Easy menu navigation using flexible softkeys
Benefit : user friendly, less training
SevenMulti - General Concept

•Full GLP support with user name, sample ID, sensor ID, time and
date, meter ID etc.
Benefits : Compliance with standards
•System menu for settings that are independant of the connected
Expansion Unit
Benefits : No worries for wrong setting
•PIN-code protected Lab manager‘s menu for operations that
require special authorization
Benefits : Security of settings
•Analog data output for pH and Ion Expansion Units to transfer
your mV reading to another device
Benefits : Flexibility
SevenMulti - General Concept

•RS232 interface (bidirectional)

•transfer meter settings, calibration and measurement data, methods
to a PC or a printer (GA42 or SP24)
•import data from e.g. a barcode reader
•futuristic: full LabX control and LIMS integration, operation via e-
box *USP
Benefits : Proper documentation, sample
and data storage capability, future
reference
SevenMulti - General Concept

•USB communication module *USP

•transfer meter settings, calibration and measurement data, methods to
a PC or a printer (and use the RS232 interface for data input at the same
time)
Benefits : More choices for interfacing

USB or TTL module

•TTL communication module *USP

•control external devices with your SevenMulti
•make your system fully automated using a Rondolino and a
Seven/Stirrer Driver
Benefits : Increase efficiency and productivity with Automation
SevenMulti - Hotkeys

•Hotkeys have the same functionality for all Expansion Units

•‘I‘ hotkey - context sensitive on-line help to support you (explanations,
tips etc.) in whatever menu you are in
•‘ID‘ hotkey - to enter your sensor and sample ID via keypad or via
barcode reader
•‘Cal‘ hotkey - to start / continue your calibration
•‘Read‘ hotkey - to start / stop a measurement
•‘Data‘ hotkey
•Store the current reading
•View the last 25 readings in
tabuar or graphical form
•View the current calibration
Benefits : Ease of Use
SevenMulti - System Menu

• Select Language
• English, German, French, Spanish and Italian
• Set Time and Date
• different time and date formats
• Screen Settings
• adopt the screen contrast to the environment and activate the screen saver
• Register User Name
• only one user can be registered at a time
• Baud Rate Setting
• set the baud rate for the COM - communication
• Activate Rondolino Sample Changer
• perform fully automated measurements
• Start Instrument Self-test
• find out whether a problem you might have is caused by the meter or by
something else
SevenMulti - System Menu

•Access Memory functions

•Review/Transfer Measurement Data - up to 1000 readings
can be stored in total
• Display All
• Search by Date
• Search by User
• Search by Sample ID

Benefits : Data recall

 SevenMulti - System Menu

•Access Memory functions

•Review/Transfer Calibration Data
• up to 10 calibration data records per sensor for up to 10 sensors per
Expansion Unit can be stored in total
• pH
• Ion
• ISFET
• Conductivity

•Review/Transfer Method Data

• up to 10 methods for each of the following Expansion Units can be
stored in total: pH, Ion, Conductivity

Benefits : Saves time, less repetition

SevenMulti - System Menu

• Lab Manager‘s Menu - PIN-code protected

• Set Instrument ID
• Set Expansion Unit ID
• Set Instrument Login Password - protect the meter from
unauthorized access
• Change PIN-code
• Delete Records in Memory - only the lab manager can delete data
• Empty Memory
• Delete Measurement Data (delete all, delete by date, delete by
user name, delete by sample ID)
• Delete Calibration Data (pH, Ion, ISFET, Conductivity)
• Delete Stored Methods (pH, Ion, Conductivity)

Benefits : Data security, user control

General features of SevenEasy and SevenMulti

• Electrode arm and base

• Can be used stand alone or connected to the meter
• Solid base for stability, also when used stand alone
• Arm can be connected to three different positions on the base
• Base can be connected to the left or to the right of the meter
• Arm can be rotated through 360o
• Height of arm freely adjustable
• Electrode holder stays horizontal at all heights
Benefits :
Freely adjustable to the customer‘s space requirements
Establishes order in the lab
Helps to prevent accidents
General features of SevenEasy and SevenMulti

Operating Instructions
Bedienungsanleitung
Mode d'emploi
• Operating Instructions Instrucciones de manejo
Istruzioni d'uso
• 5 languages
• All information for installation, operation and maintenance at a
new level of clarity
• Additional background of pH, ion, ISFET and conductivity
measurements
• Explanations on how the meter works internally

Benefits : Saves time for the customers

Saves support effort for us
General features of SevenEasy and SevenMulti

• Test certificates
• Meter serial number
• Production date
• Date, name and signature of testing person
• Test report for various tests
(keypad, sensor input, interface etc.)

Benefits : Quality assurance

Fully traceable