Article written by: Jason Danaher Ph.D.
Changes in concentrations of specific water quality parameters can have a direct impact on the growth and health of aquatic species. Maintaining optimal water quality parameters is a necessity because the aquatic organism is in direct contact with the surrounding aquatic environment; therefore, much of aquaculture is about creating, maintaining, and continuously monitoring water quality parameters.
The technologies available to monitor water quality parameters range from simple test strips and titration methods to a variety of handheld and benchtop equipment. Advanced ion-specific sensors and in-situ monitoring equipment are also available for the real‐time monitoring of water quality parameters. All of these technologies have the potential to quantify specific dissolved elements, compounds, and/or gases in solution.
Returning back to “Water Chemistry 101” we remember a solution is the combination of a solvent and solute where the solvent is the compound most prevalent in solution and the solute is the compound dissolved in the solvent. In other words, the water sample is the solution, the solvent is the water and the solute is the parameter we want to measure.
A water quality test kit (Figure 1) is an essential piece of equipment all facilities should maintain on site. They have their advantages and disadvantages, but offer efficient, accurate and affordable methods to calculate concentrations of dissolved oxygen, total ammonia nitrogen, nitrite, pH, total alkalinity, total hardness and carbon dioxide present at time of sampling in the aquaculture system. These kits typically utilize drop count titration and color disc methods to deliver the needed results. One of the disadvantages of these kits are their limited detection limits for the targeted water quality parameter.
Figure 1. A basic multiparameter water quality test kit used to test aquaculture parameters.
Determination of the quantitative minimum and maximum detection limits for a test method are important and only results within the detection range should be reported. For example, the limit of detection for nitrite may range from 0.05 mg/L to 0.8 mg/L. If a test result is below the lowest detectable concentration (i.e. 0.05 mg/L) it should be reported as being less than the detectable limit. Even if a concentration below 0.05 mg/L is reported by the test method, it is at best only an estimate. On the other hand, can you rest assured the nitrite concentration is really 0.8 mg/L if you grab a sample of water from your system, follow the test kit instructions and methodology for nitrite, and observe the test result is 0.8 mg/L? Perhaps the nitrite concentration is greater than 0.8 mg/L. You wouldn’t know because the test kit has a maximum limit of detection of 0.8 mg/L and should not trust test results with concentrations exceeding 0.8 mg/L.
When the maximum limit of detection of a parameter is observed, then this signals an appropriate time to dilute the water sample before repeating the test. A dilution is achieved when extra solvent is added to a solution without adding more solute. Simply put and using our example for nitrite above, a dilution is achieved when distilled water is added to the water sample without adding more nitrite. Distilled water is an ideal solvent because it has been boiled and condensed to remove chemical impurities; thus, it should not add more nitrite (i.e. solute).
When the concentration of the water quality parameter is unknown and the maximum limit of detection has been reached with the test, then it is always good practice to start with a 50% dilution before repeating the test procedure. If the test originally required a water sample of 10 mL then you can create a dilution factor of 50%, 25% and 12.5% by referring to Figure 2.
Figure 2. Mixing the water sample with the correct ratio of distilled water to create the appropriate dilution for water quality testing will require the correct reverse dilution factor to determine the actual concentration of the measured water quality parameter in the original sample.
The dilution factor represents how much of the original water sample from the aquaculture system remains in the entire test sample solution after dilution has occurred. A dilution factor of 50% indicates there is 1 unit of aquaculture system sample water and 1 unit of distilled water, making a total of 2 units. A dilution factor of 25% indicates there is 1 unit of aquaculture system sample water and 3 units of distilled water, making a total of 4 units. A dilution factor of 12.5% indicates there is 1 unit of aquaculture system sample water and 7 units of distilled water, making a total of 8 units.
To perform a 50% dilution for a 10 mL sample, you will need to mix equal volumes of your sample water with distilled water. Pour 5 mL of system water into the test tube and dilute it with 5 mL of distilled water to the 10 mL mark. Avoid the common dilution mistake of adding too little or too much system water to the sample tube when making the dilution. Make sure you first pour 5 mL of system water into the sample tube and adjust the volume with more or less system water if needed. Then, add 5 mL of distilled water to reach the 10 mL volume mark. To create a 25% strength solution, pour 2.5 mL of system water into the test tube and dilute it with 7.5 mL of distilled water to the 10 mL mark. You can then run the test according to the methodology specified by the water quality kit.
It is extremely important you compute the reverse dilution to calculate the actual concentration of the parameter you are analyzing after completing the dilution and obtaining a reading within range of the testing method (Figure 2). If you obtained a nitrite reading of 0.6 mg/L from the 50% solution, then you would have to multiply 0.6 mg/L by its reverse dilution factor of 2 to calculate the original nitrite concentration in the water sample. The test result concentration would be: 0.6 mg/L nitrite x 2 = 1.2 mg/L nitrite. If you had created a 25% or 12.5% strength solution, then you would have had to multiply the 0.6 mg/L by a reverse dilution factor of 4 or 8, respectively, to calculate the original nitrite concentration in the water sample. After computing the reverse dilution, the test result concentration for a 12.5% solution would be 0.6 mg/L nitrite x 8 = 4.8 mg/L nitrite. Table 1 below can be used to help remember the correct reverse dilution factor to multiply the observed reading by to calculate the original concentration.
Table 1. A reverse dilution table showing the dilution factor needed to return to the water sample’s original concentration after a dilution occurred to perform the water quality test.
It is important to note the dilution must be made prior to the test method analysis. You can never dilute the sample solution after adding reagents and observing a color change as the test reagents contain only a specified amount of chemicals to react with the targeted parameter and if the water sample were to have a greater concentration than the test’s maximum detectable limit, no additional reaction would occur to account for the greater concentration. It is incorrect to dilute the resultant color and compare the faded color to the color disc expecting to get a reliable reading. You must always dilute the original water sample to a known dilution factor, add reagents according to the test method, record the parameter’s concentration within the test’s detection limits, and then compute the appropriate reverse dilution calculation and obtain the original concentration.
Also, samples should not be diluted prior to carrying out a pH test. A pH test must use the system water in its original form. If the pH test appears to have reached its maximum limit of detection, then you may need to purchase a high range pH test kit to obtain accurate results or consider a handheld pH pen with a greater range of operation. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com if you have additional questions or require clarification on technologies to test your water quality parameters.