Tech Note 514 Detail

Conductivity Sensor Use

Affected Products:

PS-2116 NOT A VALID PART NUMBER
CI-6729 Conductivity Sensor
CI-6739A 10x Conductivity Sensor

Problem/Symptom:
Conductivity Sensor Proper Use

PASCO Solution:

Background

Electrolytic conductivity is defined as the ability of a liquid to conduct electrical current. In conductive solvents, dissolved ions are the principle conductors of electricity. By selecting the appropriate electrode, one can easily measure the electrical conductivity of liquids ranging from ultra-pure water to the most salty solutions.

How well a solution conducts electricity is dependent on the following parameters:

  1. concentration of ions
  2. mobility of ions
  3. valence of ions
  4. solution temperature

PASCO conductivity sensors determine the electrical conductivity of a solution by measuring the AC current flowing through a circuit when a 1 kHz AC voltage is applied to a 2-cell electrode submerged in the solution.

The accuracy of conductivity measurements depends on the following factors:

  1. Absence of contamination
  2. Resistance of the electrodes to polarization
  3. Consistent electrode geometry (cell constant) between calibration and measurement
  4. Consistent temperature between calibration and measurement

Polarization

Polarization occurs because of migration of the ions in the solution to the surfaces of the electrodes. If a DC voltage is applied, then there will be a voltage drop across this region. The AC voltage and platinized electrodes are used to reduce the effects of solution polarization on the conductivity measurements. When a solution becomes completely polarized all current stops because the voltage outside of the boundary layer of oppositely charged ions falls to zero.

A 1 kHz AC voltage and platinization to minimize polarization effects. The electrodes are platinized, which is a process that greatly increases the surface area of the platinum electrodes, resulting in a considerably increase in the time for the surface to saturate with ions and decreasing the effect of ion shielding (polarization resistance) of the electrode.

Contamination

The electrodes perform best when free of contaminants that would reduce the surface area of the electrodes or change the dielectric cell constant.

Contamination will make the probe more susceptible to polarization effects and change the conductivity of the solution being measured.

Temperature Effects

Conductivity has a substantial dependence on temperature. This dependence is usually expressed as percent/Celsius at 25 Celsius. Ultrapure water has the largest dependence on temperature, at 5.2%/oC. Ionic salts run about 2%/oC, with acids, alkalis, and concentrated salts solutions are around 1.5%/oC. Temperature variation causes frequent problems with conductivity measurements when the solution under testing has a rapid varying temperature. The change in conductivity is virtually instantaneous.

Geometry

The length between the sensing elements, as well as the surface area of the metallic electrodes, determine a electrode cell constant, with units of length/area. The cell constant is a critical parameter affecting the conductance value produced by the cell and handled by the electronic circuitry. The cell constant is a geometric factor that converts between conductance and conductivity. The PASPORT Conductivity Sensor - PS-2116A assumes that a probe with a cell constant of 10 is connected to the probe. For solutions of low conductivity, one could use a 1.0 x probe or smaller to obtain a better signal to noise ratio, but one would have to divide the measured values by a factor of ten in software to obtain the correct values.

Below are the optimum conductivity range for cells with different cell constants:

Cell Constant

Optimum Conductivity Range

0.01

0.055 - 20 µS/cm

0.1

0.5 - 200 µS/cm

1.0

0.01 - 2 mS/cm

10.0

1 - 200 mS/cm

Advantages and Disadvantages of Conductivity Measurement

In general, conductivity offers a fast, reliable, nondestructive, inexpensive and durable means of measuring the ionic content of a sample. Reliability and repeatability are usually excellent. Unlike measurement with ion-selective electrodes, such as pH sensors, the response of a conductivity sensor will not drift over time.

The principle drawback of conductivity measurements is that they are not ion-selective, giving a reading proportional to the combined effect of all of the dissolved ions. In order to determine the amount of total dissolve solids, one must have an idea of the ionic composition of the solution being measured.

Units of Conductivity

The units of measurement used to describe conductivity and resistivity 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 conductance, or conductivity. This is the conductance as measured between the opposite faces of a one-centimeter cube of material. This measurement has units of Siemens/cm.

CONDUCTIVITY / RESISTIVITY / Total Dissolved Solids Conversions

CONDUCTIVITY (MICROMHOS-CM)

RESISTIVITY (OHMS-CM)

DISSOLVED SOLIDS (PPM)

.056

18,000,000

.0277

.084

12,000,000

.0417

.167

6,000,000

.0833

1.00

1,000,000

.500

2.50

400,000

1.25

20.0

50,000

10.0

200

5,000

100

2000

500

1,000

20,000

50

10,000

Note: ppm x 2 = conductivity

CONDUCTIVITY OF VARIOUS AQUEOUS SOLUTIONS AT Temp = 25 C

Solution

Conductivity [µS/cm]

Pure water

0.05

Power plant boiler water

0.05-1

Distilled water

0.5

Deionized water

0.1-10

Demineralized water

1-80

Mountain water

10

Drinking water

0.5-1

Wastewater

0.9-9

KCl solution (0.01 M)

1.4

Potable water maximum

1.5

Brackish water

1-80

Industrial process water

7-140

Ocean water

53

10% NaOH

355

10% H2SO4

432

31% HNO3

865

Cleaning

Select an appropriate solvent for the contaminants to which the electrode is exposed:

  • For oils, hot water with dish detergent can be used for cleaning.
  • For lime and other hydroxide containing solutions, clean with a 5-10% solution of hydrochloric acid. or when a stronger cleaning solution is required, use concentrated hydrochloric acid mixed into 50% isopropanol.
  • For algae and bacteria containing solutions, use chlorine bleach.
  • Rinse with 0.1 M nitric acid and then rinse several times with distilled water.

Clean cells by dipping or immersing the cell in the cleaning solution, agitating for two or three minutes, and rinsing first with tap water and then several times with distilled or deionized water.

Before measurement, immerse the probe in distilled water, gently tap out any trapped air bubbles, soak for at least an hour in distilled water and recalibrate.

Calibration

  1. A calibration standard is required. If you wish to prepare your own calibration solution follow the recipe below:
  1. Select a desired conductivity from the table below and place the corresponding mass of NaCl in a 1-liter flask:

Mass of NaCl (mg)

Conductivity at Temperature = 25 C (μS/cm)

10

21.4

100

210

1,000

1,990

10,000

17,600

100,000

140,000

  1. Add 500 mL of deionized water to the flask and stir until the salt is completely dissolved.
  2. Add the remaining 500 mL of deionized water and stir the solution.
  1. Clean the electrode.
  2. Soak the conductivity electrode in distilled or deionized water for 5 to 10 minutes.
  1. Dry off the probe.
  2. Immerse the probe in a calibration solution* beyond the level of the holes on the electrode.
  3. Tap the probe against the side of the vessel to remove any air bubbles trapped inside.
  4. Monitor the conductivity while stirring the probe in the solution. Continue stirring until the value stabilizes.
  5. Calibrate the sensor.
  6. Rinse the conductivity probe with distilled or deionized water between samples.
  7. Wipe dry.

Storage

For short-term storage, leave the cell immersed in deionized water. Any cell that has been stored dry should be soaked in distilled water for one hour before use to assure complete wetting of the electrodes.

If the black platinized coating appears to be wearing or flaking off the electrodes or if the cell constant has changed by 50%, the cell should be replaced or re-platinized.

Replatinization (Advanced Chemists Only)

If platinum black appears degraded, or if the values have changed by over 50% from nominal, replace the probe or replatinize according to the following procedure:

  1. Clean the platinum electrode thoroughly in aqua regia being careful not to dissolve the platinum.
  2. Prepare a solution of 0.025 N HCl with 3% chloroplatinic acid (H2PtCl6) and 0.025% lead acetate.
  3. Connect the cell to 3 - 4 V battery to which a variable resistor has been connected.
  4. Immerse the cell in the chloroplatinic acid solution and electrolyze at 10 mA/cm for 12 minutes.
  5. Reverse polarity every 15 - 20 seconds until a black soot coat the plates.
  6. Rinse with distilled water and electrolyze in 0.1 M sulfuric acid to remove excess chlorine.
  7. Rinse with distilled water and allow electrodes to stand in distilled water until ready for use.

Creation Date: 12/7/2004
Last Modified: 07/3/2014
Mod Summary: