Titration TT131
Published on May 8, 2014
 

Titration - Tech Talk 130

Ammonia is the primary waste byproduct of protein consumption, and is excreted across fish gills as Titration is a commonly placed wet laboratory chemical analysis method. This method uses a reagent titrant to find a concentration of an unknown in a given volume of a sample. Most aquaculture test kits that use titration as a method of testing show results by means of an endpoint. The endpoint is measured by a change in color due to the use of an indicator. A sample solution with the indicator will begin one color before the use of the titrator is added. With the addition of the titrator, neutralization or EDTA-complexing occurs, while the sample solution gradually changes color until the color no longer changes. When the color no longer changes, the endpoint is reached and the concentration can be calculated. Common test kits in aquaculture that use titration as a method of testing are alkalinity, pH, mineral hardness and dissolved oxygen.

Testing in this manner relies on sample volume and end point recognition. Titration can be a very reliable way to test. This type of test can be verified with the use of a standard known solution. The reagents can be susceptible to freezing, heating or evaporation. Since the test can be verified with a standard, titration test kits are a great investment.

Alkalinity and pH test kits use acid-base titration with the use of indicators to reach an endpoint. The indicators used in these test change color when the solution is neutralized by an acid at a known concentration. Different indicators are used for various pH ranges due to the indicators varying efficiency of color change in a given pH range. The use of different indicators means most freshwater pH test kits will not provide proper results in marine water samples, and vice-versa.

Mineral hardness tests use indicators specific to the desired mineral being tested and EDTA to form an EDTA-mineral complex. This test is volume-specific as well. The sample solution has the required amount of indicator added. The EDTA titrator is slowly added until a distinct color change is reached. At this point, with most aquaculture test kits, the volume of titrator used provides a concentration of the mineral such as calcium or magnesium.

Titration of dissolved oxygen is the Winkler Test. The Winkler method uses a few more complex steps when compared to the alkalinity, pH and mineral tests. Manganese (II) sulfate is mixed with a hydroxide and iodide precipitate by the dissolved oxygen. The solution is acidified to convert iodide to iodine. The iodine is titrated with thiosulfate. The amount titrated is read to yield a concentration of dissolved oxygen because the dissolved oxygen is proportional to the iodine concentration.