The Total Dissolved Solids Analysis

Determining if water is safe to drink is a monumental challenge, and one that is quite obviously vital to public health. Recently, we have explored the analyses directed at evaluating the biological risks to the safety of the water supply: the Coliform tests. Today we turn to the most common analysis to evaluate chemical risks: Total Dissolved Solids, or TDS. And thanks to Follower William for suggesting this topic.

Drinking water is not “pure,” and we do not “purify” water during treatment. Instead, the water we provide is “potable” — safe for human consumption. But it has lots of chemicals within it. The TDS analysis measures essentially all of those chemicals. You’ll see why I can only say “essentially” a little later.

The TDS analysis is among the simplest tests that are performed in a modern water quality laboratory. A known volume of water — usually 100 mL — is placed in a pre-weighed vessel, such as the “evaporating dish” shown in today’s photo. It is then placed in an oven, where the water will evaporate. We could do the same thing faster by placing the evaporating dish over a flame, as shown here, but the oven method is easier and more common.

After cooling off the now “empty” evaporating dish, we weigh the vessel again. Lo and behold, its weight has increased. That happened because the water was converted into a gas, and is now no longer present in the dish, but most of the chemical constituents in the water are still there in the dish. The only ones that are not present are those that evaporate at even lower temperatures than water. These would be liquids or gases at temperatures below the boiling point of water — 100 degrees C or 212 degrees F. This means that only chemicals that are solids remain in our dish. Normally, there are very few liquid or gas constituents, so “essentially” all of the non-water chemicals in the water are still in our dish.

The result of this test is a weight — more accurately, a “mass,” measured in milligrams — of solid chemicals that were present in the volume of water analyzed, measured in liters. So the units for TDS are mg/L, just like with most constituents we measure in drinking water.

So what are these “solids?” We don’t know. While the test is easy and accurate, it is not very discriminating. It could all be plutonium, or it could be harmless materials like sodium, calcium, magnesium, chloride, sulfate, and bicarbonate. This is the biggest drawback of this analysis.

Because we have no idea what these solids are, there is no primary drinking water standard for TDS. The secondary standard isn’t even clear. Probably the best number for a TDS maximum contaminant level (MCL) is 500 mg/L, but this assumes that this is the water quality the consumer will ingest for a very long term — think decades. Other secondary standards of 1000 mg/L and 1500 mg/L are in place for shorter terms — think days.

With the TDS analysis, we get a decent picture of overall chemical water quality in relatively short order — the test can be done in less than an hour, if necessary. But it is only the beginning of a complete chemical analysis of the water. Such an analysis is likely to take a few weeks, but will tell us with far greater detail what chemical constituents are present in our drinking water.

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