PASCO CI-6729 User Manual

Includes

Teacher's Notes

and

Typical

Experiment Results

CONDUCTIVITY SENSOR

Instruction Manual and Experiment Guide for the PASCO scientific Model CI-6729 (1X) and

Model CI-6739A (10X)

012-06485B

12/97

CONDUCTIVITY

SENSOR

CI-6729

CI-6739A

CONDUCTIVITY

699-06620

CONDUCTIVITY

699-06621

10X

Conductivity Sensor 012–06485B

Always use eye protection,

gloves, and an apron when

working with chemicals.

Caution: Follow all local

regulations for the safe handling,

use, storage, and disposal of the

chemicals used in the experiments

described in this manual.

012–06485B Conductivity Sensor

Table of Contents

Section Page

Introduction ...................................................................................................................1

Equipment .....................................................................................................................1

Maintenance

Cleaning ...................................................................................................................2

Storage .....................................................................................................................2

Specifications ...........................................................................................................2

Theory—Principles of Operation of the Conductivity Sensor .....................................3–6

Setup and Calibration

Set up Science Workshop

..................................................................................................................................... 7

Calibration ...............................................................................................................7

Single-Point Calibration ...........................................................................................8

Experiments

Experiment 1: Introduction to the Operation of the Conductivity Sensor ................. 9–10

Experiment 2: Concentration Dependence of Conductivity in Aqueous Solutions . 11–12

Experiment 3: Temperature Dependence of Conductivity in Dilute

Aqueous Solutions ......................................................................... 13–15

Experiment 4: Acid-Base Titration with the Conductivity Sensor ..........................17–18

Teacher’s Notes ................................................................................................... 19–22

Appendix

Table 1: Conductivity of Various Water Samples at 25°C ...........................................23

Table 2: Table of Conductivity vs. Concentration for Common Solutions ................... 24

Table 3: Table of Conductivity vs. Concentration for Common Solutions ................... 25

Table 4: Conversion Chart to Estimate TDS of Aqueous Solutions .............................26

Table 5: Table of High Accuracy Reference Solution for Calibration ..........................26

Table 6: Sample Illustrating the Application of Conductivity to Agriculture ................27

Technical Support ..................................................................................... Back Cover

Conductivity Sensor 012–06485B

Please—Feel free to duplicate this manual subject to the copyright restrictions below.

The PASCO scientific Conductivity Sensor manual (012-06485A) is copyrighted and all rights reserved. However, permission is granted to nonprofit educational institutions for reproduction of any part of the Conductivity Sensor manual providing the reproductions are used only for their laboratories and are not sold for profit. Reproduction under any other circumstances, without the written consent of PASCO scientific, is prohibited.

Limited Warranty

PASCO scientific warrants the product to be free from defects in materials and workmanship for a period of one year from the date of shipment to the customer. PASCO will repair or replace at its option any part of the product which is deemed to be defective in material or workmanship. The warranty does not cover damage to the product caused by abuse or improper use. Determination of whether a product failure is the result of a manufacturing defect or improper use by the customer shall be made solely by PASCO scientific. Responsibility for the return of equipment for warranty repair belongs to the customer. Equipment must be properly packed to prevent damage and shipped postage or freight prepaid. (Damage caused by improper packing of the equipment for return shipment will not be covered by the warranty.) Shipping costs for returning the equipment after repair will be paid by PASCO scientific.

Equipment Return

Should the product have to be returned to PASCO scientific for any reason, notify PASCO scientific by letter, phone, or fax BEFORE returning the product. Upon notification, the return authorization and shipping instructions will be promptly issued.

➤➤

➤ NOTE: NO EQUIPMENT WILL BE

➤➤

ACCEPTED FOR RETURN WITHOUT AN AUTHORIZATION FROM PASCO.

When returning equipment for repair, the units must be packed properly. Carriers will not accept responsibility for damage caused by improper packing. To be certain the unit will not be damaged in shipment, observe the following rules:

➀ The packing carton must be strong enough for the

item shipped.

➁ Make certain there are at least two inches of packing

material between any point on the apparatus and the inside walls of the carton.

➂ Make certain that the packing material cannot shift in

the box or become compressed, allowing the instrument come in contact with the packing carton.

Address: PASCO scientific

10101 Foothills Blvd. P.O. Box 619011 Roseville, CA 95678-9011

Credits

Authors: Dominic Calabrese and Steve Miller Editor: Sunny Bishop

Phone: (916) 786-3800

FAX: (916) 786-3292

email: techsupp@pasco.com

web: www.pasco.com

012–06485B Conductivity Sensor

CONDUCTIVITY

SENSOR

CI-6729 CI-6739A

20K

RANGE

SELEC

699-06620

TIV

ITY

699-06621

TIV

ITY

10X

Introduction

The PASCO CI-6729 and CI-6739A Conductivity Sensors are used with a PASCO Science Workshop

computer interface to investigate factors that influence the electrical conductivity of liquids. The Conductivity Sensor consists of a signal conditioning amplifier and either a 1X or 10X conductivity electrode. The signal conditioning amplifier box is capable of working with either electrode. Students can use this sensor to explore the effects of temperature and concentration on the electrical transport properties of aqueous solutions, especially in applications relating to ecological systems.

The Conductivity Sensor is designed for use in aqueous solutions at temperatures ranging from 0 °C to 80 °C. The 1X conductivity electrode provided with the CI-6729 Conductivity Sensor has a range of up to 20,000 microsiemens per centimeter. The 10X conductivity electrode, provided with the CI-6739A, has a range up to 200,000 microsiemens per centimeter. In order to achieve maximum performance of the Conductivity Sensor, soak the electrode in distilled water for 5 to 10 minutes before use to assure complete wetting of the electrodes. The electrode may also be calibrated at approximately the same temperature as the test solution for concentration or total dissolved solids (TDS) measurements.

➤ Conductivity Sensors are used

in many practical applications including:

• Environmental Monitoring

• Oceanography-Salinity

• Agriculture

• Waste Systems

• Beverage Industries

• Electroplating

interface cable with 8-pin DIN connectors

connector for amplifier box

Equipment

connector for

computer

interface

Provided With CI-6729 (1X)

• 1X conductivity electrode with 3-foot BNC cable

• Conductivity Sensor Amplifier box with 8-pin DIN connector and BNC connector

• interface cable with DIN connector

amplifier box

Provided With CI-6739A (10X)

• 10X conductivity electrode with 3-foot BNC cable

• Conductivity Sensor Amplifier box with 8-pin DIN connector and BNC connector

• interface cable with DIN connector

Additional equipment required

• Any PASCO Science Workshop® computer interface

• computer

Replacement Electrodes

• 1X conductivity electrode: 699-06620

• 10X conductivity electrode: 699-06621

conductivity

electrodes

Conductivity Sensor 012–06485B

Maintenance

Cleaning

To ensure accurate and reproducible results, the electrode cell must be clean. A dirty cell will contaminate the solution and change the conductivity of the liquid.

Cells can be cleaned with detergent and/or dilute nitric acid (1%) by dipping or filling the cell with cleaning solution and stirring for three minutes.

Storage

Although the best method to store the electrode is by immersing it in deionized water, the electrode can be stored dry in its container. If the cell is stored dry, it should be soaked in distilled water for 5 to 10 minutes before use.

Specifications:

CI-6729 (1X conductivity electrode)

• Range: 20k scale: 0–20,000 µS/cm 2k scale: 0–2,000 µS/cm 200 scale: 0–200 µS/cm

• Conductivity electrode cell constant: 1.0 ± 0.1

• Accuracy: without calibration < 10% after calibration < 1%

• Resolution: 20k scale, ± 10 µS 2k scale, ± 1.0 µS 200 scale, ± 0.1 µS

• Temperature range 0–80°C

• Electrode body: Epoxy

CI-6739A (10X conductivity electrode)

• Range: 20k scale: 0–200,000 µS/cm 2k scale: 0–20,000 µS/cm 200 scale: 0–2000 µS/cm

• Conductivity electrode cell constant: 10 ± 1.0

• Accuracy: without calibration < 10% after calibration < 1%

• Resolution: 20k scale, ± 100 µS 2k scale, ± 10 µS 200 scale, ± 1.0 µS

• Temperature range: 0–80°C

• Electrode body: Epoxy

012–06485B Conductivity Sensor

Theory—Principles of Operation of the Conductivity Sensor

What is Conductivity?

Conductivity (or specifically, electrolytic conductivity) is defined as the ability of a substance to conduct electric current. It is the reciprocal of the more commonly encountered term, resistivity. All substances possess conductivity to some degree, but the amount varies widely, ranging from extremely low (insulators such as benzene and glass) to very high (silver, copper, and metals in general). Most industrial interest is in the conductivity measurement of liquids that generally consist of ionic compounds dissolved in water. These solutions have conductivities approximately midway between insulators and metallic conductors. This conductivity can be measured quite easily by electronic means, allowing a simple test that can tell much about the quality of the water or the makeup of the solution. A broad line of conductivity equipment is available to measure liquids ranging from ultrapure water (low conductivity) to concentrated chemical streams (high conductivity).

Units of Conductivity

The units of measurement used to describe conductivity and resistively are quite fundamental and are frequently misused. Once the units are known, various waters can be quantitatively described.

The basic unit of resistance is the familiar ohm. Conductance is the reciprocal of resistance, and its basic unit is the siemens (S), formerly called mho. In discussions of bulk material, it is convenient to talk of its specific conduct-ance, now commonly called its conductivity. This is the conductance as measured between the opposite faces of a 1 cm cube of the material (Figure 1).

This measurement has units of siemens/centimeter (S/cm). The units microsiemens/centimeter (µS/cm) and millisiemens/centimeter (mS/cm) are most commonly used to describe the conductivity of aqueous solutions. The corresponding terms for specific resistance (or resistivity) are

ohm - centimeter (Ω · cm), megaohm - centimeter (M Ω · cm) and kiloohm - centimeter (k Ω · cm).

Users of ultra pure water prefer to use resistivity units of M measurement in these units tends to spread the scale out in the range of interest. These same users frequently use k

Ω

· cm when dealing with less

pure water such as tap water.

Ω

· cm, because

conducting surface

Figure 1

Conductivity Cube

Conductivity Sensor 012–06485B

However when dealing with a chemical solution ranging from extremely dilute to very concentrated chemical, use of conductivity units of µS/cm and mS/cm are common. In these applications, conductivity has the advantage of an almost direct relationship with impurities, especially at low concentration. Hence, a rising conductivity reading shows increasing impurities, or a generally increasing concentration in the case of a chemical stream (with some exceptions in concentrated solutions). See Table 1 for a comparison of resistance and conductivity.

Table 1. Conductivity Of Various Aqueous Solutions At 25 0C

Solution Conductivity Resistivity

Pure water 0.05 µS/cm 18 MΩ · cm Power plant boiler water 0.05–1 µS/cm 1–18 MΩ · cm Distilled water 0.5 µS/cm 2 MΩ · cm Deionized water 0.1–10 µS/cm 0.1–10 MΩ · cm Demineralized water 1–80 µS/cm 0.01–1 MΩ · cm Mountain water 10 µS/cm 0.1 MΩ · cm Drinking water 0.5 –1 mS/cm 1–2 kΩ · cm Waste water 0.9–9 mS/cm 0.1–1 kΩ · cm KCl solution (0.0l M) 1.4 mS/cm 0.7 Ω · cm Potable water maximum 1.5 mS/cm 0.7 Ω · cm Brackish water 1–80 mS/cm 0.01–1 Ω · cm Industrial process water 7–140 mS/cm rarely stated Ocean water 53 mS/cm rarely stated 10% NaOH 355 mS/cm rarely stated 100/0 H

2504

31% HNO

432 mS/cm rarely stated 865 mS/cm rarely stated

Conductivity Electrodes (Cells)

Simple conductivity sensors are constructed of an insulating material imbedded with platinum, graphite, stainless steel or other metallic pieces. These metal contacts serve as sensing elements and are placed at a fixed distance apart to make contact with a solution whose conductivity is to be determined. The length between the sensing elements, as well as the surface area of the metallic piece determine the electrode cell constant, defined as length/area (Figure 1). The cell constant is a critical parameter affecting the conductance value produced by the cell and handled by the electronic circuitry.

A cell constant of 1.0 will produce a conductance reading approximately equal to the solution conductivity. For solutions of low conductivity, the sensing

012–06485B Conductivity Sensor

electrodes can be placed closer together, reducing the length between them and producing cell constants of 0.1 or 0.01. This will raise the conductance reading by a factor of 10 to 100 to offset the low solution conductivity and give a better signal to the conductivity meter. On the other hand, the sensing electrodes can be placed farther apart to create cell constants of 10 or 100 for use in highly conductive solutions. Table 2 lists the optimum conductivity range for cells with different cell constants.

Table 2. Optimum Conductivity Range For Cells

With Different Cell Constants

Cell Constant Conductivity

0.01 0–20 µS/cm

0.1 0–200 µS/cm

1.0 0 –2000 µS/cm

10.0 0–200,000 µS/cm

to amplifier box

glass rod

The conductivity electrode provided with the CI- 6729 has a cell constant of

1.0 and is designed to achieve optimum performance over a range of 0 to 20,000 µS (Figure 2). This performance is achieved by using a cylindrical cell geometry and platinized platinum conductors embedded on a glass rod. For measurements of conductivity greater than 20,000 µS, the 10x electrode should be used.

Conductivity Sensor Amplifier

The Conductivity Sensor amplifier has two distinct functions. First, it provides a signal or voltage used to drive the conductivity electrode, and second, it senses the electrical current the electrode passes when placed in the solution to be tested.

If the voltage and current are known, then the resistance of the cell can be determined. If the resistance of the cell is known, then the conductivity can be determined by taking the inverse of the resistance and multiplying by the conductivity cell constant. While it is instructional to know how the sensor determines conductivity, Science Workshop handles the calculations and reports conductivity for the 1x cell directly. If the 10x conductivity electrode is used, the value reported by Science Workshop should be multiplied by ten.

platinized platinum conductors

Figure 2

Schematic view of the cell for the CI-6729 Conductivity Electrode.

When a potential is a applied to the conductivity cell, the ions in solution are influenced by the charge on the cell’s electrodes and begin to migrate toward the electrodes (Figure 3).

Figure 3

Conductivity Cell in operation

Conductivity Sensor 012–06485B

Figure 4

Polarized Electrode

➤ Approximate conductivities

°°

(at 25

C) and impurity

°°

concentrations (in ppm) of various water samples are given below.

Sample Conductivity

Pure H20 0.055 µS/cm (0.027 ppm)

Distilled H20 0.5 µS/cm (0.206 ppm)

City H20 50 µS/cm (25 ppm)

Ocean H20 53 mS/cm (35,000 ppm)

After a fairly short period of time most of the ions in solution will move to the electrodes and the current flow through probe will begin to decrease. This process is called polarization (Figure 4). Polarization causes the conductivity probe’s output to be low if not corrected.

One method used to reduce the effects of polarization is to alternate the polarity of the voltage applied to the electrode. Polarization of the electrode and the associated build up of oxidation/reduction byproducts is prevented if the voltage is alternated quickly enough.

Conductivity Measurements

The conductivity of an electrolytic solution is dependent on several factors, including the concentration of the solute, the degree of dissociation of the solute, the degree of dissociation of the molecules present in the solution, the valiancy of the ion(s) present in the solution, the mobilities of ions that are formed upon dissociation, and the temperature of the solution. In general, ion mobilities remain constant over a specific concentration range, depending on the solute in solution. Within the specific concentration range, the conductivity of a solution will increase in proportion to an increase in solute concentration.

Since temperature is a measure of the average kinetic energy of an atom, ion, or molecule, any change in temperature of a solution will strongly affect the mobility of the species present in the electrolyte. Therefore, the conductivity of the solution will also change. As a result, the conductivity of a solution whose concentration is known is always quoted for a specific temperature. Several tables in the Appendix illustrate the temperature dependence of conductivity for specific electrolytic solutions.

Concentration or total dissolved solid (TDS) measurements can also be accurately determined by correlating the conductivity of a solution with reference tables or graphs. TDS measurements are generally used to estimate the salt content of water samples. A rule of thumb used to estimate TDS in mg/L (ppm) is to divide the measured conductivity in microsiemens per centimeter (µS/cm) by two. Table 4 in the Appendix is a conversion chart for TDS measurements involving NaCl and CaCO Appendix.

. Additional references of conductivity data are also listed in the

012–06485B Conductivity Sensor

Setup and Calibration

Set up

Science Workshop

1. Attach electrode to the amplifier box (Figure 5a).

2. Plug the 8-pin DIN connector of the amplifier box into any analog

channel on a PASCO computer interface (Figure 5b).

3. Launch Science Workshop, drag the analog plug icon to the analog channel icon, and select Conductivity Sensor.

4. In the Experiment Setup window, drag a Graph display to the Conductivity Sensor icon (

), and select the appropriate conductivity range to be displayed. (The range setting selected on the amplifier box must match the display range for the graph.)

➤➤

➤

A Digits display may also be used to display conductivity.

➤➤

Calibration

Prepare one of the weight percent NaCl solutions given in the table below by weighing out in air the desired mass (mg) of NaCl in a 1-liter flask. Add 500 ml of deionized water and stir the solution to dissolve the salt. Add the remaining 500 ml of deionized water and stir the solution.

RAN

SEL

20K

ECT

CONDUCTIVITY

CONDU

SENSOR

CTIVITY

I-6729 I-6729

C C

I-6739A

R S

C 2 K

T 2 0

CONDUCTIVITY

SENSOR

Figure 5

Plug the sensor into the computer interface.

%Weight

(approx.)

0.001

0.01

0.1

1.0

10.0

➤➤

➤

For direct measurements of conductivity greater than 20,000

➤➤

Mass of NaCl (mg)

100

1,000

10,000

100,000

TDS ppm or mg/liter

100

1,000

10,000

100,000

Conductivity (µS/cm) at 25 °C

21.4

210

1,990

17,600

140,000

µS the 10x electrode should be used. The 1x electrode may be used if a 10:1 dilution of the solution to be measured is made. If the 10:1 dilution method is used remember to multiply the value indicated by 10.

➤➤

➤

For measurements that require very high accuracy, a KCl

➤➤

standard should be used (see Table 5 in the Appendix).

Other calibration solutions can be prepared using the tables and graphs in the Appendix.

+ 23 hidden pages

PASCO CI-6729 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual