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Why do we need to analyze water?

If water is badly polluted-- like raw sewage--- it might be obvious from its appearance or odor.
It might be colored or turbid (cloudy), or have solids, oil or foam floating on it.
It might have a rotten odor, or smell like industrial chemicals.
A lot of dead fish floating on the surface of a lake would be a clear sign that something was wrong.
But many harmful-- and beneficial-- materials in water are invisible and odorless. In order to go beyond the obvious, to determine what materials are in the water, and how much, we need to be able to conduct chemical or microbiological analyses.

Analysis of a natural body of water will tell us how clean or polluted it is. If there is damage to wildlife, the measurements will help pinpoint the cause-- and the source. In a wastewater treatment plant, analyses are necessary for monitoring the effectiveness of the treatment processes. In the United States, the Clean Water Act requires wastewater dischargers to have permits. These permits set limits on the amounts of specific pollutants which can be discharged, as well as a schedule for monitoring and reporting the results. Usually, the reports must be filed monthly, while the measurement frequency for a particular parameter (measurable property) can run anywhere from "continuously" to just once a year. Only standard analytical procedures specified in the "Code of Federal Regulations" may be used, so that the government agencies can feel reasonably confident that results from different laboratories are comparable.

Similar considerations apply to drinking water. The purity of the water we drink is of more concern to the average person than the quality of the wastewater discharged by the sewage plant. But we should not forget that in many places, especially along a river, one town's wastewater discharge may be part of the next town's water supply...

There are two aspects to water analysis that we need to consider:

  1. what substances or organisms are we interested in testing for-- and why?
  2. what procedures and equipment do we use to make the measurements, and how do they work?
Let's look at the "procedures and equipment" first:

(If you want to read about the "substances and organisms" first,
click here,-- or here for no-frames version-- but the procedures on that page refer to methods discussed below.)

Analytical Methods

Water analyses are done by several methods. The most common types of measurements are gravimetric (weighing), electrochemical (using meters with electrodes) and optical (including visual). Instrumental methods are becoming increasingly popular, and instrumentation is getting "smarter" and easier to use with the inclusion of microprocessors. In the simplest case, a sample may just be placed in an instrument and a result read directly on a display. More often some physical separation technique or chemical procedure is needed before a measurement is made, in order to remove interferences and transform the analyte-- the target of the analysis-- into a form which can be detected by the instrument.

Since even raw sewage is generally more than 99.9% water, most environmental analyses are measuring very low concentrations of materials. The results of these measurements are usually expressed in the units "milligrams per liter," abbreviated as mg/L. Since a milligram is one thousandth of a gram, and a liter of water weighs about a thousand grams, a mg/L is approximately equal to one part per million by weight. A part per million ("ppm") is only one ten thousandth of one percent. For toxic metals and organic compounds of industrial origin, measurements are now routinely made in the part per billion (microgram per liter) range or even lower. At such low levels, sensitive equipment and careful technique are clearly necessary for accurate results. Avoiding contamination of the sample and using methods which prevent interferences from other substances in the water are crucial requirements for successful analyses.

Separation Techniques:

Some measurements require separating the analyte from other substances in the water which may interfere with the measurement. Some measurements even require separating the analyte from the water entirely. Separation techniques include:

Measurement Techniques:


What do we test for, why, and how? Next
(click here for no-frames version...)


Other Choices:
Intoduction: "What is Water Pollution?" What Happens at the Treatment Plant? Links to Other Information Flush Gordon's Secret Identity
Pollution Treatment links author