Water and Soil Characterization - pH and Electrical Conductivity

What Are pH and Electrical Conductivity?

Davis Mine was originally used to extract pyrites. The waters of the emerging stream are acidic and rich in iron compounds which are then exploited by various bacteria that produce oxidized iron compounds with the typical orange/brown color. The image of the site was provided by Cristine Barreto, courtesy of the microscope web site.

It is often useful to characterize an environment, such as a body of water, by measuring its pH and electrical conductivity (EC). pH is a measure of the acidity of the water or soil based on its hydrogen ion concentration and is mathematically defined as the negative logarithm of the hydrogen ion concentration, or

pH = -log[H+], where the brackets around the H+ symbolize "concentration"

The pH of a material ranges on a logarithmic scale from 1-14, where pH 1-6 are acidic, pH 7 is neutral, and pH 8-14 are basic. Lower pH corresponds with higher [H+], while higher pH is associated with lower [H+].

Electrical conductivity (EC) is a measurement of the dissolved material in an aqueous solution, which relates to the ability of the material to conduct electrical current through it. EC is measured in units called Seimens per unit area (e.g. mS/cm, or miliSeimens per centimeter), and the higher the dissolved material in a water or soil sample, the higher the EC will be in that material.

How Are pH and Electrical Conductivity Measured?

A meter and probe or litmus paper can be used to measure the pH of a sample. The more accurate, but expensive, of these methods is the meter and probe. pH meters are calibrated using special solutions, or buffers with a known pH value. Calibration protocols can be found in manufacturer's instructions, but a simplified protocol can be accessed through the Washington State University Ecology Program pH measurement website.

Using litmus or pH paper is the simpler and less expensive way of measuring pH. This method employs special strips of paper that change color based on the pH of a sample solution. The strips come in a variety of resolutions, from simple acid vs. base comparison to a narrow resolution of pH values. These strips of paper can measure the difference 0.2-0.3 pH in a sample. Litmus paper changes color based on whether the sample solution is acidic or basic, turning red or blue, respectively. pH strips indicate a sample's pH by changing color as well; these colors are indicated on the package and vary for different pH ranges and manufacturers.

Electrical conductivity can be measured using a meter and probe as well. The probe consists of two metal electrodes spaced 1 cm apart (thus the unit of measurement is microSeimens or milliSeimens per centimeter). A constant voltage is applied across the electrodes resulting in an electrical current flowing through the aqueous sample. Since the current flowing through the water is proportional to the concentration of dissolved ions in the water, the electrical conductivity can be measured. The higher the dissolved salt/ion concentration, the more conductive the sample and hence the higher the conductivity reading.

How To - Protocols and Concerns for pH and EC Measurements

The following are general protocols for measuring pH and EC. Manufacturer's instructions and guidelines should be followed, if available.

These pH strips can measure pH in a series of ranges by putting sample on a strip and comparing its color change with colors on the box that correspond to a certain pH. The strip on the left measures pH 0-7 and shows results of a strong acid sample; the center strip is pH range 5-10 and shows results of a 6.97 buffer solution sample; the strip on the right measures a broad range (pH 1-14) and shows results of a 10% bleach water solution sample. This image can be enlarged by clicking on it. Photo by Monica Bruckner.
Measuring pH using litmus paper or pH strips:
1. Place a droplet of sample on the paper - be sure you drop or pour the sample over the paper rather than dipping the paper into the sample, as the latter may contaminate the sample.
2. Observe color change on the paper. If using litmus paper the paper will turn a red or pink color if the sample is acidic, while a blue paper indicates a basic sample. If using pH strips, colors corresponding to pH values should be listed on the packaging.

Measuring pH of a liquid using a pH meter and probe:
1. Turn on the pH meter and calibrate the probe using two standard solutions (pH 4, 7, and 10 buffers are recommended, dependant on the range you are measuring). Calibration procedures vary by instrument, so following the manufacturer's instructions is highly recommended. BE SURE TO RINSE THE PROBE THOROUGHLY BETWEEN BUFFERS USING DEIONIZED WATER AND CAREFULLY BLOT THE PROBE DRY USING A KIM WIPE. pH meters should be calibrated before each use (before each series of samples, not between each sample itself) or when measuring a large range of pH.
2. Check calibration by measuring the pH of the standard solutions in measure rather than calibrate mode.
3. Collect sample water in a glass or plastic container. Collect enough so the probe tip can be submerged in sample; either rinse the probe with deionized water (and blot dry) or with sample before inserting the probe into the collection vessel.
4. Submerge the probe into the sample and wait until the pH reading on the meter stabilizes. Many meters have automatic temperature correction (ATC), which calculates the pH taking into account temperature, if your meter does not have this feature, you may need to adjust a knob on the meter to correct the pH for temperature. Record the measurement when the pH reading is stable.

Measuring EC of a liquid sample using a meter and probe:
These three meters and probes can measure pH (left) and electrical conductivity (center and right). The image can be enlarged if you click on it. Photo by Monica Bruckner.
1. Turn on the EC meter and calibrate the probe using a standard solution of known conductivity (choose a standard close to what you believe the sample is). Calibration procedures vary by instrument, so following the manufacturer's instructions is highly recommended. BE SURE TO RINSE THE PROBE THOROUGHLY BEFORE AND AFTER CALIBRATION USING DEIONIZED WATER AND CAREFULLY BLOT THE PROBE DRY USING A KIM WIPE. EC meters should be calibrated before each use (before each series of samples, not between each sample itself) or when measuring a large range of EC.
2. Check calibration by measuring the EC of the standard solutions in measure rather than calibrate mode.
3. Collect sample water in a glass or plastic container. Collect enough so the probe tip can be submerged in sample; either rinse the probe with deionized water (and blot dry) or with sample before inserting the probe into the collection vessel.
4. Submerge the probe into the sample and wait until the EC reading on the meter stabilizes. Many meters have automatic temperature correction (ATC), which calculates the EC taking into account temperature, if your meter does not have this feature, you may need to adjust a knob on the meter to correct the EC for temperature. Record the measurement when the EC reading is stable.

Measuring soil pH:

Soil pH can be measured using a pH meter (usually mixing the soil sample with water or a salt solution) or by adding a dye to the soil and observing a color change that can be compared with a chart for pH determination. The latter method can be done using a kit that contains the necessary chemicals. For more information on measuring soil pH and why soil pH matters, please visit The NRCS Soil pH website.

Measuring soil EC:

Soil EC can be measured via electrodes inserted directly into the ground or by extracting soil water using a lysimeter (an instrument that uses suction to extract soil or groundwater from the ground. EC of groundwater can also be measured using a probe inserted into a well (a perforated tube inserted into the ground that can measure water table height) or piezometer (a tube only open at the bottom that measures the water potential at the depth where the opening is located). The electrode method employs a special series of probes, two of which send electrical current through the soil and two of which measure the voltage drop. To measure soil water EC, water is extracted from a lysimeter, well, or piezometer and measured. Alternately, a probe attached to a meter can be lowered into a well or piezometer and the liquid EC can be measured in that manner.

Results Analysis

pH and EC measurements can vary greatly and are affected by several environmental factors including, climate, local biota (plants and animals), bedrock and surficial geology, as well as human impacts on the land. Common values of pH and EC for particular environments can be found in the literature, such as peer-reviewed journal articles or textbooks. In general, pH readings between 1-6 are considered acidic, 7 is neutral, and 8-14 are basic. Relatively dilute waters, such as distilled water or glacial melt water have low electrical conductivities, ranging from zero to the microSeimen range, whereas temperate streams and lakes, especially those with a significant groundwater contribution, generally have higher electrical conductivities.