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The concentration of Total Dissolved Solids (TDS) in drinking water can affect the taste of the water.  However, it is not an indication of water safety.  Only determination of the specific inorganic chemicals making up the TDS will help determine whether the water is contaminated and not potable. Measurement of TDS will help us determine whether treatment systems such as reverse osmosis, deionization, or distillation are operating correctly and efficiently.  It may also help you in determining if the water you have is suitable for its intended purpose.  Minerals and mineral compounds, including Calcium, Magnesium, Sodium, Sulfate, Chlorides, Fluoride, and a variety of other inorganic chemicals make up the total dissolved solids.  However, organic chemicals, such as petroleum-based compounds, do not contribute to TDS.

The basic procedure for determining TDS in drinking water is to pass a well-mixed sample through a standard glass fiber filter.  The measured volume is placed in a pre-weighed dish, and the dish and its contents are dried to a constant weight at 180 degrees C.  The increase in total weight of the dish and its contents represents the TDS.

Sample size should yield between 2.5 and 200 milligrams of dried residue, and require no more than 10 minutes of filtration.  A sample yielding more than 200 milligrams may result in a water-trapping crust formed during drying, and trap moisture.

The temperature selected for drying has an important bearing on the results. Volatilization of organic matter, mechanically occluded water, water of crystallization (especially sulfate related), and gas losses due to heat-induced chemical decomposition, as well as weight gains due to oxidation can all be temperature dependent.  Loss of CO2 results from conversion of bicarbonates to carbonates, and carbonates may partially decompose to oxides or salts.  Some chloride and nitrate salts may be lost.

Generally, evaporating at 180 degrees C yields values closer to the sum of individually analyzed components than drying at lower temperatures.  This method requires a fairly well-equipped laboratory, with drying ovens, analytical balances, controlled air systems, and some expertise with laboratory equipment.  Results are calculated and recorded as milligrams per liter (mg/l).

Electrical conductivity can be used as a check against TDS measurements as a general measure of accuracy.  The acceptable TDS ratio is 55 to 70% of the conductivity reading.  The conductivity factors of ions commonly found in water vary according to the specific ion.  These factors can be found in Standard Methods for the Examination of Water and Wastewater.  Since various water samples may contain different ratios of specific ions, it is virtually impossible to accurately calculate TDS based on a conductivity value, nor can you calculate the concentration of any specific ion.

Conductivity bridges or meters can be purchased through water testing supply companies.  Accuracy and sensitivity using a meter will not be as precise as using the evaporation method, however it is quick, easy to use, and relatively inexpensive per test.  Unless the conductivity meter has automatic temperature compensation, the results will be lower at colder temperatures, and the results would need to be adjusted with a compensation factor.  If results are outside the acceptable range, you must look back at both measurements.

Using electrical conductivity to calculate TDS is something that can be done by persons with relatively little technical training, and a decent meter should be able to provide with reproducible results.  If you were to test a good quality distilled or purified water, you would likely receive results indicating TDS levels of 1 to 10 milligrams per liter, or conductivity results of approximately 2 to 20.  If you were to measure ocean water, your TDS results would be 10,000 to 20,000 mg/l, or conductivity readings 20,000 to 40,000.  Most well, lake, stream, and spring waters will run in the TDS range of 100 to 1000, with conductivity readings of 200 to 2000.  It is a measurement of a water sample’s conductivity or specific conductance.  Conductivity tells us the purity of water based on the amount of electricity it can conduct.  For example, if a water sample contains a high mineral level, it will conduct a high level of electricity.  The purer the water, the lower the conductivity.

Using either method to measure the amount of minerals in water allows the monitoring of water treatment systems designed to reduce mineral content.  The basis of reverse osmosis, deionization, and distillation systems is to reduce or attempt to eliminate dissolved minerals.