The chemical species that are commonly dissolved in freshwater include calcium (Ca^(2+)), sodium (Na^(+)), iron (Fe^(2+) / Fe^(3+)), magnesium (Mg^(2+)), nitrates (NO3^(-)), sulfate (SO4^(2-)), phosphate (PO4^(2-)), chlorides (Cl^(-)) and bicarbonates (HCO3^(-)). In addition to these inorganic species, a wide variety of organic compounds are present.
There are many sources of these dissolved solids. These sources include fertilisers from agricultural, residential and forestry zones, chemical spills, industrial effluent, forest detritus, and rubbish tips. Some of these chemical species, such as phosphates and nitrates, provide the essential elements required by plants and animals for essential life processes such as metabolism and protein synthesis. In low concentrations, the freshwater ecosystem will be "limited" but in high concentration, the water can become excessively enriched in nutrients resulting in eutrophication. Eutrophication can often cause extensive algal growth or "blooms" that can choke freshwater systems by filling the stream with aquatic vegetation and demanding excessive oxygen during the night. Phosphorous and nitrogen levels are often influenced by human activities such as effluent discharge and land clearing. It should be noted that TDS is normally directly related to water conductivity. High ionic concentration contributes to high conductivity levels.
Although several ionic and non-ionic compounds will be a part of the total dissolved solids base of the residue collected, it is possible and worthwhile for students to attempt to determine which anions and cations are present. Common procedures are flame tests and precipitation detection by the addition of selected reagents. Use some of these common procedures to attempt to determine the anions and cations present in your dissolved solid residue.
TDS = [Increase in weight (g) / Volume (mL)] X [1000 mg / 1 g] X [1000 mL / 1 L] = ? mg/L (ppm)
Calculating TDS
| BEAKER 1 | BEAKER 2 | BEAKER 3 | BEAKER 4 | |
| Final Mass of Beaker (g) | ||||
| Initial Mass of Beaker (g) | ||||
| TDS / 100 mL (=final-initial) | ||||
| TDS (ppm) (use formula above) | ||||
| Average TDS (ppm) |
Closely related to Total Dissolved Solid Levels (TDS) in waterways is conductivity level. If the dissolved solids are predominantly ionic, conductivity levels will be high. If there are low dissolved solid levels or if the dissolved solids are non-ionic compounds, conductivity levels will be low. A conductivity meter can be used to determine conductivity levels. It is valuable to begin testing the conductivity levels of your water samples by, first of all, testing the conductivity of distilled or deionized water in order to have a comparative base for your samples.
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