e D A Q Am p User Manual

Quad pH/mV Amp

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This document was, as far as possible, accurate at
the time of printing. Changes may have been madeto the software and hardware it describes sincethen: eDAQ Pty Ltd reserves the right to alterspecifications as required. Late-breaking information
may be supplied separately. Latest information andsoftware updates can be obtained from our web
site.
Trademarks of eDAQ
e-corder and PowerChrom are registered trademarksof eDAQ Pty Ltd. Specific model names of data
recording units, such as e-corder 201, andPowerChrom 280, are trademarks of eDAQ Pty Ltd.
Chart and Scope are trademarks of ADInstruments
Pty Ltd and are used under license by eDAQ.
EChem is a trademark of eDAQ Pty Ltd.

Product: Quad pH/mV Amp (EA168)
Document Number: U-EA168-1005
Copyright © October 2005
eDAQ Pty Ltd
6 Doig Avenue
Denistone East, NSW 2112
AUSTRALIA
http://www.eDAQ.com
email: info@eDAQ.com
All rights reserved. No part of this document may be
reproduced by any means without the prior written
permission of eDAQ Pty Ltd.

Other Trademarks
Mac OS, and Macintosh, are registered trademarksof Apple Computer, Inc. Windows 98, Windows
Me, Windows 2000, and Windows XP aretrademarks of Microsoft Corporation.
PostScript, and Acrobat are registered trademarks ofAdobe Systems, Incorporated.
All other trademarks are the properties of theirrespective owners.

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Quad pH/mV Amp

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On-lineContents

1 Overview 1
How to Use this Manual 2
Checking the Quad pH/mV Amp 2.eDAQ Amps 2

2 The pH/mV Amp 5
The Front Panel 6
The Electrode and Reference Inputs 6
The On-line Indicators 6
The Back Panel 7
The Signal Outputs 7
I2C Connector 8
Connecting to the e-corder 8
The pH/mV Amp Self Test 9
Using Chart and Scope 10
Setting up the Quad pH/mV Amp 10
Previewing the Signal 10

3 Electrode Calibration 15

A Technical Aspects 27
Quad pH/mV Amp Operation 27
Technical Description 27

B Troubleshooting 29
C Specifications 31
D Electrode Behavior 33
The Nernst Equation 33
Electrode Response 34
The Isopotential Point 34
Non-Linear Calibration 35

Index 37
License & Warranty 39

Signal Calibration 16
Temperature Compensation 21
Manual Temperature Compensation 21
Automatic Temperature Compensation 22
Isopotential Point 24
Viewing Electrode Response 24
Manual Compensation Response 25
Removing the Calibration 26

Quad pH/mV Amp

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iv

Quad pH/mV Amp

C H A P T E R

1 Overview

O N E

The eDAQ Quad pH/mV Amp is designed to work with an e-cordersystem, recording signals from:• pH electrodes (either combination, or half cell electrodes)• potentiometric ion selective electrodes (ISEs)• redox (or ORP, oxidation–reduction potentiometric) electrodes• other high impedance sources up to ±2 V requiring electrometer
input.
Signals from up to four electrodes can be recorded simultaneously.

Quad pH/mV Amp

1

How to Use this ManualThis manual describes how to set up and begin using your pH/mV
Amp. The use of the pH/mV Amp with the Chart software pH
Calibration extension is discussed. The appendices provide technicaland troubleshooting information.

Checking the Quad pH/mV Amp
Before you begin working with the Quad pH/mV Amp, please checkthat:• all items described in the packing list are included; and that• there are no signs of damage that may have occurred duringtransit.
Contact your eDAQ distributor if you encounter a problem.
You should also become familiar with the basic features of youre-corder system, which are discussed in the e-corder Manual on the
Installer CD.

eDAQ Amps.

The Quad pH/mV Amp is part of a family of preamplifiers known aseDAQ Amps.
The Quad pH/mV Amp is designed for recording signals from up to
four pH and other potentiometric electrodes. As with other eDAQ
Amps, it is designed to be operated under full software control and is
automatically recognized by Chart or Scope software which controls its
gain range, signal filtering, and other settings.

2

Quad pH/mV Amp

The range of eDAQ Amps includes the:• Quad pH/mV Amp, suitable for connection of pH, ion selective,
and potentiometric (ORP) electrodes• Potentiostat, a three–electrode potentiostat that can be used forvoltammetric and amperometric experiments. Gain ranges of 20 nA
to 100 mA in 1:2:5 steps.
• Picostat, a high sensitivity three–electrode potentiostat suitable foruse with carbon fibre and other microelectrodes. Current gain
ranges of 10 pA to 100 nA in 1:2:5 steps.
• QuadStat, a four channel, three electrode potentiostat suitable foruse with amperometric biosensors. Current gain ranges of 2 nA
to1 mA/channel in 1:2:5 steps.
• Bridge Amp, suitable for sensors requiring a low drift, high gain
differential amplifier. Also provides DC excitation• GP Amp, suitable for high output sensors requiring a highimpedance single ended or differential amplifier. Also provides DC
excitation.
See our web site at www.eDAQ.com for more information.

Chapter 1 — Overview

3

4

Quad pH/mV Amp

C H A P T E R

T W O

2 The pH/mV Amp

This chapter describes how to connect and use your eDAQ Quad
pH/mV Amp.

IMPORTANT: Always make sure that the e-corder is turned off before
you connect or disconnect the Quad pH/mV Amp. Failure to do thismay result in damage to the e-corder and/or the pH/mV Amp.

Quad pH/mV Amp

5

The Front Panel
The front panel of the Quad pH/mV Amp is shown in Figure 2–1.

The Electrode and Reference Inputs
The BNC input connectors marked ‘E’ of the pH/mV Amp provide
connection for combination pH, ISE or ORP electrodes. Combinationelectrodes have an internal reference and no separate referenceelectrode is required.
Some electrodes do not have an internal reference (most ISEs and ORP
electrodes and occasionally some pH electrodes). These electrodes,
which are referred to as half cell electrodes, will need to be used with aseparate reference electrode. Half cell electrodes should be connectedto the BNC input connector marked ‘E/2’. The separate referenceelectrode is connected to the 4 mm socket marked ‘Ref’.

The On-line Indicators
When lit, these indicate that the software (such as Chart or Scope) has
located and initialized the pH/mV Amp. If the light does not go on
when the software is run, check that the pH/mV Amp is properly
connected.

Electrode inputs, BNC connectors. For combination electrodes

Figure 2–1
The Quad pH/mV Amp
front panel

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On-lineReference electrode input,
4 mm socket

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BNC connector

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Quad pH/mV Amp

The Back Panel
The back panel of the Quad pH/mV Amp is shown in Figure 2–2.

The Signal Outputs
The Quad pH/mV Amp back panel has four BNC connectors labelled
Channel 1 — Channel 4.
The Quad pH/mV Amp provides four signals, one for each sensing
electrode.

pH/mV signals, BNC connectors

Figure 2–2
The pH/mV Amp backpanel

Channel 4

Channel 3

Channel 2

Chassis ground, 4 mm post

Channel 1

No user serviceable parts inside.
Refer servicing to qualified service personnel.
2

Input

I C
Bus

Made in Australia by eDAQ Pty Ltd.

Output

1

5
6

9
Input

Chapter 2 — The pH/mV Amp

INT

DSD

SDA

DSC

I2C control signals

SCL

Regulated +17 V DC

Regulated +8 V DC

Regulated –17 V DC

Digital Ground

INT
DSD
SDA
DSC
SCL

Figure 2–3
I2C connector pin
assignments

Power lines

Regulated –17 V DC
Digital GroundI2C control signals

Regulated +17 V DC
Regulated +8 V DC

DB–9 pin, I2C connectors

5

1

9

6
Output7

I2C Connector
The back panel, Figure 2–2, has two DB-9 pin ‘I2C bus’ connectors
labelled Input and Output. The Input connector enables connection to
the e-corder (or to the output of other eDAQ Amps). This connection
provides power to the Quad pH/mV Amp and carries the various
control signals (for gain range and filter selection) to and from thee-corder. The pin assignments are shown in Figure 2–3.

Connecting to the e-corderYour Quad pH/mV Amp will have been supplied with an I2C cable(DB-9 pin connectors at either end), and four cables with BNC
connectors at either end.
Channel 1

Figure 2–4
Four combinationelectrodes shown
connected to a Quad
pH/mV Amp ande-corder 401

Channel 2

Channel 3

Channel 4

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Input 2E/2On-lineInput 3

Power
OutputStatus

Input 4

Trigger

Figure 2–5
Back view of Quad
pH/mV Amp connected
to e-corder

Channel 4

Channel 3

Channel 2

Channel 1

No user serviceable parts inside.
Refer servicing to qualified service personnel.

Input

OutputMade in Australia by eDAQ Pty Ltd.

No user serviceable parts inside.
Refer servicing to qualified service personnel.

2

I C Bus8

Made in Australia by eDAQ Pty Ltd.

Rated Voltage: 90 - 250 V AC
50/60 Hz 25 VA

Quad pH/mV Amp

First make sure that the e-corder is turned off. Then connect the I2C
cable to the I2C connector on the back panel of the e-corder, and theother end to the I2C Input connector on the back panel of the pH/mV
Amp. Use BNC cables to connect the signal outputs on the back panelof the Quad pH/mV Amp to the front panel of the e-corder as shown in
Figure 2–4 and Figure 2–5.
Check that all connections are firm. Loose connections may cause
failure to recognize the eDAQ Amp by the software, erratic behavior,or loss of signal.
Multiple eDAQ Amps can be connected to an e-corder. The numberthat can be connected depends on the number of input channels on thee-corder. The initial eDAQ Amp should be connected as shown in
Figure 2–4. The remainder are linked via I2C cables, connecting the
I2C output of one eDAQ Amp to the I2C input of the next, as in Figure
2–5. The signal outputs of each eDAQ Amp are connected to an input
channel of the e-corder.

The pH/mV Amp Self Test
After you have installed the software, connected the e-corder to acomputer as described in your e-corder Manual, and connected thepH/mV Amp as described above, a quick check can be performed on
the pH/mV Amp:
• Turn on the e-corder and check that it performs its normal
diagnostic tests, as described in the e-corder Manual• Once the e-corder is ready, open Chart or Scope.
• During initialization, you should see these indicators flash briefly
and then remain lit — the Quad pH/mV Amp is ready for use.
If the indicators do not light, check your cable connections and repeat
the procedure. If this does not solve the problem, contact your eDAQ
distributor.

Chapter 2 — The pH/mV Amp

9

Using Chart and Scope
Setting up the Quad pH/mV Amp
When the Quad pH/mV Amp is connected to an e-corder input andsuccessfully installed, the pH Amp… menu command appears in theChart software Channel Function pop-up menu, Figure 2–6. In Scope
software, thebutton appears in the Input A (orInput B) panel, see Figure 2–6.

Figure 2–6
Accessing the pH/mV
Amp controls
Scope software
Chart software

If the software fails to find the Quad pH/mV Amp, the normal Input
Amplifier… command or button remains. If you were expecting aconnected pH/mV Amp, you should check the connections at this point,
quit the application, then open it again.
Choosing the pH Amp… menu command or clicking the button willaccess the pH Amp dialog box (which replaces the Input Amplifier
dialog box for that e-corder channel — your Chart and Scope Software
Manuals have details on the Input Amplifier dialog box).

Previewing the Signal
The pH Amplifier dialog box allows you to preview a signal so that you
can select the amplification and filter settings. The dialog boxes for
Chart software are shown in Figure 2–7 and Figure 2–8. The Scope
software controls are similar.
The incoming signal is displayed in real time, but is not recorded tohard disk (once the signal moves across the display area it is lost). Clickthe OK button to apply the selected settings.

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Quad pH/mV Amp

Figure 2–7
The pH/mV Amp pH
dialog box, forpreviewing a signal(Chart software on a
Windows Computer)

Select input
channele-corder input
channel number

Pause/Resume buttons

Gain range(sensitivity)
Select filter
Average signal
amplitude

Preview signal

Invert signal

Drag labels and tickmarks to adjust axis

Figure 2–8
The pH/mV Amp pH
dialog box, forpreviewing a signal(Chart software on a
Macintosh Computer)

Select input
channele-corder input
channel number

Average signal amplitude

Gain range(sensitivity)

SelectfilterPreview signal

InvertsignalPause/Resume button

Chapter 2 — The pH/mV Amp

11

Signal Display
The input signal is displayed so that you can see the effect of changing
the settings. The average signal value is displayed at the top left of thedisplay area. Slowly drifting signals (as from pH electrodes) will be
represented quite accurately, whereas quickly changing signals will be
displayed as a solid dark area showing only the envelope (shape) of
the signal formed by the minimum and maximum recorded signals.
You can stop the signal scrolling by clicking the Pause button at the top
right of the display area
. Click the Scroll button
to resumescrolling.
You can shift and stretch the vertical Amplitude axis to make the bestuse of the available display area (see the Input Amplifier dialog in theChart Software Manual for further details). Changes made here update
settings in the main window of the program.

Setting the Range
Use the Range pop-up menu to select the input range (sensitivity) of thechannel.
At 25 °C, a change of one pH unit corresponds to a 59 mV change.
This relationship is used in Table 2–1 which shows the pH ranges
corresponding to various pH/mV Amp range settings, assuming that the
pH electrode has a response of zero millivolts at pH 7. For pH
measurements covering a wide range, a setting of 500 mV should be

Table 2–1
pH ranges
corresponding to pH/mV
Amp settings

Range setting (mV)Resolution† (µV)

Range (pH)Resolution (pH)

±500

15.6

–1.5 to 15.5*< 0.001

±200

6.25

3.6 to 10.4

< 0.001

±100

3.13

5.3 to 8.7< 0.001±50

1.56

6.2 to 7.8< 0.001

±20

0.625

6.7 to 7.3< 0.001† Resolution is 16 bits, or 0.0015%, of range setting.

* In practice this range will be limited by the pH electrode12

Quad pH/mV Amp

used, while the 200 or 100 mV settings are suited to narrow range pH
measurements (for example for biological pH monitoring).

Filtering
Low pass filter settings of 10, 5, 2 and 1 Hz are available. The low
pass filter will remove high frequency noise. pH and ion selective
electrodes have a slow response time, usually in the order of seconds,
and the 1 Hz filter setting is often routinely used. If you are using fast
response electrodes then a higher setting can be used.
The Mains Filter,, removes 50 or 60 Hz interference due tomains hum. See the Chart Software Manual for more information. For
most applications it is recommended to have the Mains Filter on.

Inverting the Signal
Ticking the Invert checkbox,, reverses signal polarity (that is
positive and negative signal values are reversed).

Units Calibration
Clicking the
button access the Units Conversion dialog box.
For more information about Units Conversion, see the Chart Software
Manual or Scope Software Manual.
However, when using Chart software for recording data from pH andion selective electrodes, and where single or double point calibration is
employed, use the pH Calibration extension, see Chapter 3 — do NOT
use Units Conversion.
Where multiple point calibration (more than two points) is required, use
the Multiple Point Calibration extension.
Both the pH Calibration extension and Multiple Point Calibration
extensions can be obtained from the eDAQ website (www.eDAQ.com).

Chapter 2 — The pH/mV Amp

13

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Quad pH/mV Amp

C H A P T E R

T H R E E

3 Electrode Calibration

The pH Calibration extension is a Chart software plug-in that adds the‘pH Calibration...’ command to the Chart channel pop-up menus.
The extension:• performs single or double point calibration of the pH or ISE
electrode;• prepares a calibration graph from the measured electrode signal;• enables manual temperature compensation (MTC) adjustment; or• enables automatic temperature compensation (ATC) if temperature
readings are collected on a second channel
The resulting calibration information is used to convert the raw voltage
signal from the electrode into appropriate units (pH, pF, pNa, etc.),
which will then be displayed in the Chart window.

Multiple point (i.e. more than two points) linear or non-linear calibration
can be performed with the Multipoint Calibration extension.

Both the pH and Multipoint Calibration extensions can be downloaded
free of charge from the eDAQ website (www.eDAQ.com).

Quad pH/mV Amp

15

Signal Calibration
Choosing the pH... command (available after installing the pH
Calibration extension) from a channel pop-up menu accesses theElectrode Calibration dialog box, Figure 3–1 and Figure 3–2.
If a region of the electrode signal has first been selected in the Chart
window then that region will also appear in the display area of theElectrode Calibration dialog box.
The raw signal values (with units of V ormV) and the corresponding pH values areentered in the boxes (at right) to calibrate the electrode. For single
point calibration only the top set of boxes need to have values
entered. The software will calculate the electrode response based on
an assumed isopotential point of zero mV and an ideal 100%
Nernstian response. For more accurate calibration two calibration
points should be used and the actual electrode response will becalculated.
While the raw signal values can be entered directly by typing themin, it is more usual for the electrode to be immersed in a standard
buffer solution of known pH (or ion concentration) and the signal
recorded for a minute or so until it has stabilized. If two pointcalibration is required, then the electrode is immersed in a second
standard buffer solution of a different pH (or ion concentration).
After recording the raw signal from the two buffer solutions, a signal
similar to that shown in Figure 3–3 should be obtained. Select aregion that includes the signal response from both buffer solutions,and choose pH... from the Channel Function pop-up menu. This
opens the electrode calibration dialog box with the selected
waveform displayed, Figure 3–1 and Figure 3–2.
Select a suitable part of the signal in the data display by using themouse to drag the pointer across it. You can also shift and stretch
the vertical Amplitude axis to make the best use of the available
display area.
If you click in the data display area, a vertical line indicates theactive point, and an indicator arrow, , shows the point of

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Quad pH/mV Amp

Figure 3–1
The Electrode Calibration
dialog box of the pH
Calibration Extension(Windows)

Choose the pH
Calibration…
command from thechannel menu

Calibration
reportRaw signal Standard
valuesbuffer values

Temperaturecompensation

Figure 3–2
The Electrode Calibration
dialog box of the pH
Calibration Extension(Macintosh)

Ionic charge
pop-up menu

View electrode response,
Figure 3–11

Raw signal
valuesStandard
buffer values

Units pop-up
menu

Selected signal frommain Chart windows

Units pop-up
menu

Decimal places
selection

Select region of
signal for raw value

Decimal places
selection

Calibration
reportTemperaturecompensation

Choose the pH…
command from thechannel menu

Select region of
signal for raw value

Chapter 3 — Electrode Calibration

Calibration
temperature

Electrode response
dialog button

17

Figure 3–3
Selection of raw mV datafrom two buffer solutions,
for use in electrodecalibration

Figure 3–4
Selecting different
regions of the electrodesignal➊➋➌➍intersection with the signal (Figure 3–4, ➊). If you select an area, dragthe pointer (mouse) across the signal, thearrow shows the average
amplitude of the signal, and two markers,
andindicate themaximum and minimum data points in the selection (Figure 3–4, ➋).
Usually a region of steady signal is selected (Figure 3–4, ➌ and ➍),then clicking the Value button , enters the raw signal value (the18

Quad pH/mV Amp

Figure 3–5
Performing two point
electrode calibration

Click on arrow to enter
average signal valueEnter buffer pH

Select region of
signal for first buffer
Results ofsingle pointcalibration

Click on arrow to enter
Enter second buffer pH
average signal value

Results ofdouble pointcalibration

Select region of
signal for second
bufferaverage of the selection) in the adjacent box, for example. The standard buffer value is then manually entered in
the adjacent box,
, and the desired units
selected (see next section).
This is then repeated for the second buffer solution Figure 3–5.

Choosing Units
The Units pop-up menu,, is used to select the unit
name. Common units are included in the menu but you cans also define
new units, or delete unwanted ones. Unit names are stored in apreferences file and become available to other files.
Choose Define Unit… from the Units pop-up menu to access the Define
Unit dialog box, Figure 3–7, to enter a new unit name.

Chapter 3 — Electrode Calibration

19

Figure 3–6
Defining new unit names

Enter the unit
name
Units pop-up menu

Delete unwanted units by choosing Delete Unit… from the pop-up
menu.
Set the number of decimal places to be displayed for cursor readings ofthe calibrated signal, by clicking the arrows,, (Figure
3–1 and Figure 3–2).
To preview the signal with the new units, click the
button.You
can apply and cancel the units conversion without losing the values you
have entered by clicking the
buttons. To apply thechanges to the channel and return to the Chart window, click the
button. To abandon the changes click the
button.

Ionic Charge
Use thepop-up menu, Figure 3–7, to select the value for thenumber of electrons transferred at the electrode. This usuallycorresponds to the charge of the ion and thus for pH, sodium, or
potassium measurements n = +1; for magnesium or calcium n = +2;
while for halides, or nitrate n = –1.
The value of n is used to calculate the response of the electrode using
the Nernst equation, and must be set correctly to obtain a valid
calibration.

Figure 3–7
The Ionic Charge pop-up
menu

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Quad pH/mV Amp

Calibration Temperature
You must also enter the temperature of the standard buffer solutions,. Please remember that the pH value of a buffer
solution is temperature dependent and that the correct buffer value
should be used at that temperature.

Temperature Compensation
For the most exacting work, both calibration and experimental
measurements are performed in a thermostatted glass reaction vessel
maintained at a known temperature to within ±0.05 °C. Under these
conditions temperature compensation is NOT required.
When pH, or other ISE measurements, will be made at temperatures(Tm) other than the temperature under which calibration was performed(Tc), the Nernst equation can be used as a basis for compensation of
the readings.
The pH extension offers two types of temperature compensation:• manual temperature compensation (MTC). Used where themeasurements are made at a constant temperature, but whereTm ≠ Tc.
• Automatic temperature compensation (ATC). Used where
temperature fluctuations are expected to occur during theexperiment, that is, when Tm is not constant. In these cases
temperature should be monitored on a second channel. Note thatthe calibration must still be performed at a known constanttemperature.
Both MTC and ATC corrections can be removed if required, so that theoriginal, raw data can be re-displayed.

Manual Temperature Compensation
The manual temperature compensation (MTC) option is for use when
you need to measure ionic concentration at a temperature other thanthat at which the electrode was calibrated. For example, the calibration
may have taken place at 25°C, but the subsequent samples might be at

Chapter 3 — Electrode Calibration

21

Figure 3–8
Selecting temperaturecompensation

Manual temperature compensation

Automatic temperature compensation,with temperature measured on channel 2

37°C, or 65°C etc. MTC will alter the electrode response slope to suitthe new temperature on the basis of the Nernst equation.
Make sure that temperature compensation is enabled in the Temp.
Compensation check box by selecting. The temperature at of
the sample data should be also be entered.

Automatic Temperature Compensation
When the temperature of the solution being measured is changing it isnecessary to perform automatic temperature compensation (ATC).
Using a suitable temperature probe and amplifier (for example theeDAQ Thermocouple Pod) the temperature signal can be recorded on aa spare e-corder channel. When the Automatic option is chosen, the pH
extension will use temperature data from one of the Chart channels tocorrect the pH channel for variations in temperature. To choose theautomatic temperature compensation option click in the radio button
next to the Automatic option. The channel selection pop-up menu canselect the channel on which temperature was recorded.

Recording Temperature
To use a temperature channel for ATC, the channel must first be
appropriately calibrated, using the Units Conversion feature, with units
of °C, °F, or K. If you have not calibrated the temperature channel analert will appear, Figure 3–9, because the pH extension needs to know
these units in order to apply the Nernst equation.

22

Quad pH/mV Amp

Figure 3–9
Uncalibrated
temperature channel
warning alert

Select Temperature fromthe channel menu

Figure 3–10
Previewing thetemperature signal andsetting up units for thetemperature channel

Select suitable range:
5 V up to 100 °C
2 V up to 40 °C

Preview oftemperature signal

Access Units Conversion fortemperature signal

0 V = 0 °C and
5 V = 100 °C
are the factory
default settings for
the RTD probe

To set the units of the temperature channel, preview the temperature
signal by using the Temperature command (channel menu) to access the
dialog box shown in Figure 3–10.

Chapter 3 — Electrode Calibration23

Isopotential Point
The isopotential point of the electrode is the potential which remains
constant as the temperature changes, see Figure 3–12. Most pH
electrodes are constructed so that the isopotential point is about 0 mV
(which for most pH electrodes also corresponds to pH 7). But it is notuncommon to find, even in new pH electrodes that the isopotential pointis actually anywhere between ±50 mV and that this value changes as
the electrode ages. Ion selective electrodes may have an isopotential
completely outside the measuring range of the electrode.
Most pH meters assume an isopotential point of 0 mV (at pH 7) when
they apply MTC or ATC. However, with the Chart pH extension, you
can enter any isopotential value. To calculate this value you must do
two calibration runs at different temperatures. Refer to Appendix D for
more information on how to determine the isopotential point.

Viewing Electrode Response
The pH extension plots the voltage response of the electrode versus pH(or other ion) giving a calibration graph of the electrode.
Click thebutton in the pH dialog box, Figure 3–1, andFigure 3–2. Typical electrode responses are shown in Figure 3–11. The
isopotential point will be set to zero unless temperature compensation isturned on and a new value entered.

Adjusting the Scaling
The vertical axis will automatically scale to the full range at which thedata was recorded. For example, if you were recording on the 1 mV
range then the scale would have a range of ±1 mV. You can then dragthe axis tick marks, and tick labels to stretch and offset the axis andobtain a better scaling.
The horizontal axis represents the ionic concentration. In the special
case of pH, the horizontal scale will be automatically scaled between 0
and 14. The axis tick marks and labels can then be dragged to new
positions to better scale the axis.

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Quad pH/mV Amp

Calibration point

Figure 3–11
The Electrode Response
dialog box

Single point calibration uses
an ideal Nernstian slope

Calibration points
Print
calibration
reportCalibration
pointsDouble point calibration
Copy
calibration
reportThe
buttons andused for scaling.

pop-up menu next to the axes can also be

Manual Compensation Response
When the manual temperature compensation mode is selected and atemperature other than the calibration temperature is entered, theelectrode response dialog will display two electrode plots: one
representing the response at the calibration temperature; and a second
plot showing the response at the specified temperature.
The calibration response can be identified as the line with one or two
calibration points on it. These are represented as dots. The information
displayed on the right of the dialog shows the calibration points andthe values used for the Nernst equation.

Chapter 3 — Electrode Calibration25

Response at 65 °C

Isopotential point

Response at 25 °C

Figure 3–12
Electrode Response
dialog box when using
manual temperaturecompensation.

Printing or Copying
Use theorbuttons to copy the electroderesponse graph for pasting to a word processor document, or to print a
quick report.

Removing the Calibration
The calibration can be turned off at any time by choosing thecommand from the channel pop-up menu, Figure 3–1
and Figure 3–2. This displays the raw electrode signal (in millivolts or
volts) again.

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A Technical Aspects

This appendix describes the some of the important technical aspects of
the Quad pH/mV Amp. You do not need to know the material here to
use the pH/mV Amp. Please remember that user modification of theequipment voids your rights under warranty.

Quad pH/mV Amp Operation
The Quad pH/mV Amp and other eDAQ Amps have been designed to
be used with an e-corder system. The pH/mV Amp is effectively an
extension of the e-corder’s analog input channels. It provides:• very high impedance, electrically isolated inputs suitable for directconnection to pH or other ion selective electrodes.
• software controlled filtering of the signal.
• reference electrode input.
• software selectable gain ranges.

Technical Description
As with other eDAQ Amps, all internal functions of the Quad pH/mV
Amp are controlled from the e-corder by sending information on a
special communications connection called the I2C (eye-squared-sea)bus. This connection supplies power to the eDAQ Amp as well. eDAQ
Amps are connected to the analog input channels of the e-corder via

Quad pH/mV Amp

27

BNC-to-BNC cables, through which the amplified and filtered signal issent to the e-corder. The overall operation of the pH/mV Amp can bebetter understood by referring to Figure A–1.
The E input is used for combination electrodes where the referenceelectrode is connect to the cable shield. The electrode signal is fed to ahigh input impedance amplifier of unity gain.
The E/2 input is meant for use with ‘half cell’ electrodes (that is thosethat require the use of a separate reference electrode). In these cases,
the reference electrode should be connected to the Ref socket. The
output of the electrometer amplifier is fed to a 3 pole, 10 Hz low passfilter.
The E/2 signal is protected with a actively driven shield. If you use ahalf cell electrode with a coaxial cable then the shield will be extended
to the electrode. This will minimize signal leakage and allow the use of
longer cable runs (typically several metres) than would otherwise be thecase.
The pH/mV Amp output signal is fed to the e-corder for secondary
amplification and filtering.
Figure A–1
Block Diagram of
Channel 1 of the Quad
pH/mV Amp
Front Panel

Back Panel
Hi Z

E inputx1

x1

Electrodesignal10 Hz
Low pass filter

Isolation

Ref input
E/2 input

Driven shield
I2C output
I2C interface
I2C input

Online indicator

To channels 2, 3 and 4

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B TroubleshootingB

If you experience any difficulty with the Quad pH/mV Amp use thissection to try to resolve the problem. In the majority of cases, theproblem can usually be fixed by ensuring that the connecting cables
are correctly connected and firmly attached and the re starting theChart or Scope software.
The On-Line indicator fails to light when Chart or Scope is opened• If the e-corder is off check switches and power connections.
• The BNC to BNC cable from the pH/mV Amp to the analog input
channel of the e-corder may not be properly connected. Check thatthe cable is firmly connected to the BNC connector on the backpanel of the pH/mV Amp and to a BNC input on the front panel ofthe e-corder.
All or some of on-line indicators fail to light when the Chart software is
opened, and the pH/mV Amp controls are not accessible• If all the lights are off, then the I2C cable from the pH/mV Amp to
the e-corder may not be properly connected. Check to see that theI2C cables are firmly seated and screwed in.
• Try a new cable to check that the BNC and/or I2C cables areworking correctly.
• If you have tried the above suggestions and still cannot get the
Quad pH/mV Amp to work properly, then try using it on anothere-corder if you have access to one. If the on-line indicators still fail tolight on the second e-corder, the unit may be faulty and should be
returned for repair.

Quad pH/mV Amp

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C Specifications

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Quad pH/mV Amp

Quad pH/mV Amp

pH/mV Amplifier:

Four.
High impedance, electrometer type.
Galvanically isolated.

Input range:±2 V

Secondary ranges:±1000, 500, 200, 100, 50, 20, 10, 5,
2 mV, supplied by e-corder

Input resolution:

16 bits, 0.0015% of range

Input impedance:

1013 Ω typical

Amplifier noise:< 1µV rms (4 µV p–p) @ DC – 10 Hz

Maximum Input:±10 V

Input Connection type:

Insulated BNC (E and E/2)
4 mm socket (Ref)

Bandwidth/Low pass filter:-3 dB @ 10 Hz
3 pole Bessel

Secondary filters:

5, 2, 1 Hz (digital filters of e-corder)

31

Control PortI2C input and output:

Male and female DB–9 pin connectors.
Provides control and power.

Power requirements:±17 V DC
+8 V DC~3 W

Physical Configuration
Dimensions (h × w × d):

60 × 150 × 200 mm
2.4 × 5.9 × 7.9 inches

Weight:

1.5 kg (3.3 lb)

Operating conditions:0 – 35˚C
0 – 90% humidity (non–condensing)eDAQ reserves the right to alter these specifications at any time.

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A P P E N D I X

DD Electrode Behavior
The Nernst Equation
Most pH and ion selective electrodes exhibit behavior that closely
follows the Nernst equation which describes the relationship between
potential, ion activity (≈ ionic concentration), and temperature.
RT
E = E 0 – 2.303  ------ pY
 nF
where
• E is the observed potential (in volts)• E0 is the potential when pY = 0 (in volts)• R is the gas constant (8.314 J K-1 mol-1)• T is the temperature in Kelvin• F is the Faraday constant (96487 C mol-1)• n is the number of electrons transferred at the electrode (usually
the charge of the ion)
• pY = –log10[Y] where [Y] is the activity of the ionic species
Note that if pY is plotted against E, then the Nernst equation predicts
that a straight line should be obtained of intercept E0 and a slope of–(RT/nF). For pH measurements (n=+1) at 25°C, the term 2.303RT/nF
has a value of 0.05916 V. This is referred to as the Nernstian slope ofthe electrode.

Quad pH/mV Amp

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Electrode Response
If two solutions of known pH are measured then these values can beused to construct a linear plot to determine E0 and electrode slope. The
percentage Nernstian response of the electrode is given by:
response = 100% × slopeobserved/slopecalculated
where:• slopeobserved is the observed slope and• slopecalculated = –2.303(RT/nF)
High quality pH electrodes will have a response in the range
95 – 102%. Cheaper or old electrodes will most likely have a response
outside this range. Ion selective electrodes often exhibit a wider
variation.

The Isopotential PointAt isopotential point, the electrode potential remains constant as
temperature changes. Most pH electrodes are constructed so that theisopotential is about 0 mV (which for most pH electrodes also
corresponds to pH 7). However, it is not uncommon to find that even in
new pH electrodes this value may be anywhere between ±50 mV andthat the value also changes as the electrode ages. Ion selective
electrodes may have an isopotential completely outside the measuring
range of the electrode.
Most pH meters assume an isopotential point of 0 mV (at pH 7) when
applying manual or automatic temperature compensation. However,
the Chart pH Calibration extension allows you to adjust the isopotential
point if required. In order to calculate the isopotential, two calibration
runs must be made at two different temperatures, T1 and T2.
From the Nernst equation:
E = E01 + (slope1) pH
E = E02 + (slope2) pH
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on rearranging terms:
pH = (E – E01)/slope1
pH = (E – E02)/slope2
whereslope1 = –2.303RT1/nF; andslope2 = –2.303RT2/nF
At the isopotential point both potential and pH are constant for both T1
and T2, that is E = Eip, and pH = pHip. Combining equations gives:
E ip – E 01
E ip – E 02---------------------- = --------------------slope 1
slope 2
which can be solved for Eip:
E 02 slope 1 – E 01 slope 2
E ip = ---------------------------------------------------------slope 1 – slope 2
or, simplifying terms:
E 02 T 1 – E 01 T 2
E ip = ----------------------------------T1 – T2
For best results ensure that T1 and T2 are at least 20 degrees apart.

Non-Linear Calibration
Both single and two point calibration assume that the electrode has alinear response to ion activity. Experimentally there will always be somedeviation from linearity in the response. This is most noticeable at very
high or low concentrations. In these instances a non–linear calibration
technique must be used for accurate work.
Also, if you need to calibrate an electrode directly in terms of ionicconcentration (or log of concentration) rather than activity, a non-linear
calibration is needed.
Chapter D — Electrode Behavior

35

In either case, the Chart Multipoint calibration extension can be used
for non-linear electrode calibration. See our web site, atwww.eDAQ.com for more information.

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E

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Ref

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ted

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On-lineIndex

A

ATC 22
MTC 25non-linear 35
report 26
undoing 26

automatic temperature compensation 22
recording temperature 22

Belectrode connectors 6

back panel 7

electrode isopotential point 24

block diagram 28
Bridge Amp 3

Ccalibration 15
ATC 22
MTC 25non-linear 35
report 26
undoing 26
calibration temperature 21
checking the Quad pH/mV Amp 2
connecting multiple eDAQ Amps 9E
eDAQ Amps
Bridge Amp 3
GP Amp 3
Picostat 3
Potentiostat 3
Quad pH/mV Amp 2, 3
QuadStat 3

electrode response 24–26
determination 34
electrode response dialog
axis scaling 24
printing or copying 26

Ffiltering 13
front panel 6

G
GP Amp 3I
I2C connector 8
Ionic Charge menu 20
ISE electrodes
isopotential point 24
isopotential point 24, 34–35

electrode calibration 15

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M

U

manual temperature compensation 21–22, 25

Units pop-up menu 19
user modification 27

N
Nernst equation 20, 21, 22, 25, 33

V

Nernstian response 34

View Response button 24

non-linear calibration 35

WP

warranty conditions 27, 39

pH Ampdialog box 11, 12
filtering 13
setting the sensitivity 12
setup 10–13
Use with Chart or Scope 10
pH calibration extension 15–26
pH electrodeisopotential point 24
pH range settings 12
Picostat 3
Potentiostat 3
preferences file 19
problems 29

Q
Quad pH/mV Amp 2, 3
QuadStat 3

R
reference electrode connector 6

Ttechnical specifications 31–32
Temperature Amp
dialog box 11, 12
temperature compensation 21–24
automatic 22
manual 21–22
troubleshooting 29

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License & Warranty

On-lineTrademarks
e-corder and PowerChrom are registeredtrademarks of eDAQ Pty Ltd. All eDAQ software,
hardware, and documentation is protected bycopyright. eDAQ retains the exclusive ownership of
the trademarks and registered trademarks
represented by its company name, logo, andproduct names.

Responsibilities
You and any others using any eDAQ product agree
to use it in a sensible manner for purposes for which
it is suited, and agree to take responsibility for theiractions and the results of their actions.
If problems arise with an eDAQ product, eDAQ will
make all reasonable efforts to fix them. This service
may incur a charge, depending on the nature of the
problems, and is subject to the other conditions inthis Agreement.

Hardware Warranty
eDAQ Pty Ltd warrants the Quad pH/mV Amp, tobe free of defects in material and workmanship for
one year from the date of purchase. eDAQ Pty Ltd
will repair or replace defective equipment as
appropriate.
To obtain a warranty repair/replacement you mustfirst notify us before return of the instrument and wewill issue you with a RAN (return authorization
number). You must ship the defective product at your

Quad pH/mV Amp

expense. We will pay return shipping. The productshould be packed safely (preferably in its original
packaging) and have the RAN on the shippinglabel. Returns sent without a RAN may be refuseddelivery.
This warranty does not cover hardware that has:• been modified by the user in any way;
• been subjected to unusual physical, electrical,
or environmental stress. This includes damage
due to faulty power sockets, inadequate
earthing, or power spikes or surges;• been damaged because of incorrect wiring toancillary equipment, or because of substandard,connectors or cables; or• had the original identification marks removed
or altered.

Software License
You have the non-exclusive right to use the suppliedeDAQ software (Chart, Scope etc). Your employees
or students, for example, are entitled to use it,provided they adhere to this agreement. Eachseparate purchase of the eDAQ software licenses it
to be used on two computers at any given time (onone computer for data acquisition with a e-corder
hardware unit, and on a second computer for the
analysis of existing data files). Although multiple
copies of a program may exist on several
computers, more than two copies must not be usedsimultaneously. Departmental/company licences are39

available if you wish to run more than two copies
simultaneously.

Technical Support
Please register your unit to receive technical support.
Technical assistance is available via email. Please
describe the problem with as much detail as
possible. Include a small example data file, if
appropriate. Please also state:• the model and serial number of your e-corderunit and Quad pH/mV Amp.
• the type of computer and operating systembeing used (for example Windows XP, or
Macintosh OS 10.2)• the software version you are using (forexample Chart v5.1.2)
We endeavor to answer all your questions, but insome cases, for example where the problem relates
to the other equipment that you are using, a nominal
fee may be charged.

Jurisdiction
eDAQ Pty Ltd is bound by the laws of New South
Wales in Australia, and any proceedings shall beheard by the Supreme Court of New South Wales inAustralia.

Disclaimer
eDAQ reserves the right to alter specifications ofhardware and software without notice.
No liability can be accepted for consequential
damages resulting from use of eDAQ products.

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