Salinity and Conductivitiy
Testing Parameters
Salinity is a measure of the total amount of inorganic solids present in water. Freshwater is classified as having a salinity of less than 0.05 parts per thousand (ppt). Salinity can be measured in water by either a hydrometer or a refractometer. Conductivity is defined as the property by which electricity is produced. Some elements are good conductors, some are not. The inorganic solids present in water, those that make up the salinity, have a specific electric conductance that can be measured. By measuring the conductivity with a conductivity meter, one can also calculate the salinity. Depth and inflow of freshwater influence conductivity and salinity. In deeper areas, the water is not so sensitive to the effects of evaporation, thus the salinity levels stay even. However, in shallow water (in which the benefits of freshwater inflow are not present) salinity will increase as evaporation occurs. Salinity is important to osmoregulation (the control of osmotic processes) in plants and animals. Particular plants and animals are adapted to certain levels of salinity. Some tolerate a wide range of salinity (Euryhaline), others tolerate a narrow range (Stenohaline). Generally speaking, fluids of saltwater fish are "fresher" than the water they live in, they are conserving fresh water. Likewise, fluids of freshwater fish are saltier that the water they live in, as they are conserving salts.
Testing Procedures
SALINITY (coastal locations only)
For reading salinity, participants will be using refractometers. Basically, a refractometer is just a prism in a metal tube. Make sure your lens is rinsed twice with distilled water. Insert the refractometer into the saltwater to cover the lens, remove, and read. The water to be tested will be placed under the little clear plastic lens, and light shines through the lens/prism.
The light rays are bent according to the concentration of salt (or minerals or anything else in the water). The higher the salt concentration, the more the light is bent. The refractive index is then read off the scale inside the eye piece.
Refractometers are extremely fragile, and they must be kept calibrated. They all come with instructions so you may want to consult these.
CONDUCTIVITY (inland locations only)
Conductivity is measured using a conductivity meter. Prior to each field trip, the conductivity meters must be calibrated to ensure accurate field readings. The standard that is used to calibrate the meters is a potassium chloride solution. Inland locations will receive a standard solution as part of their water quality monitoring kit. This is a non-toxic solution so gloves and goggles are not required. To calibrate and use the meter, you will need a small beaker, standard solution, and the conductivity meter.
On your Water Quality Monitoring form, record the pre- and post-calibration as well as the standard solution reading.
The initial reading is what the meter reading is before calibration.
The standard reading is what the solution standard reads.
The post reading is what the meter reads after testing in the field.
To calibrate:
- Remove the protective cap from the conductivity meter probe.
- Thoroughly rinse the beaker and the probe with a small amount of standard solution.
- Dip the conductivity meter probe into the standard solution, making sure not to immerse the meter past the brown line. Do not allow the probe to rest on the bottom of the beaker.
- Stir gently and wait for a few seconds. This is to release any air bubbles on the probe. If air bubbles are present on the probe area of the meter, gently tap the probe against the side of the beaker. Press the on/off button.
- Wait for approximately two minutes to allow the conductivity meter to adjust for the temperature of the solution.
- If the conductivity meter does not read the same value as the standard solution, adjust the meter with the small screwdriver at the back of the meter. Turn very slowly, as small adjustments will greatly alter the value.