AUTOMATIZACIÓN DE LAS REPRESAS DE LOS CANALES

A common way of obtaining a quick estimate of the concentration of dissolved solids in water is to measure the electrical conductivity (EC) of the water. The EC meter actually measures the electrical resistance of the water between two electrodes suspended in the sample. The instrument readout is in microhms per centimeter at 25°C. In general, every 10 units of EC represents 6 to 7 mg/L of dissolved solids.

FIGURE 3-39 Marble balance table Courtesy of Thomas Scientific, Inc.

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Photometers

The photometer is an electronic device that performs the same function as a colorimeter or color comparator. Photometers are far more accurate and precise than visual colorimeters.

A photometer can measure small differences in color intensities not easily seen by the human eye. Other advantages over visual colorimeters include freedom from variable light conditions and elimination of errors because of color blindness or color bias of the analyst.

The photometer is versatile, easy to use, and relatively inexpensive. US Environmental Protection Agency (USEPA) drinking water regulations require the use of photometers in testing for nitrate, arsenic, fluoride, and chlorine residual.

A basic photometer, such as the one in Figure 3-41, has five main components:

1. White-light source 2. Wavelength control unit 3. Sample compartment 4. Detector

5. Meter

The white light passes through the wavelength control unit (a simple colored filter, a diffraction grating, or a prism) to produce a single-color light (light of a specific wave-length). The single-color light then passes through the treated sample, which is contained in a glass tube (cuvette) in the sample compartment. The amount of light that passes FIGURE 3-40 Pocket Colorimeter

Courtesy of Hach Company

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through the sample is sensed by the detector and indicated on the graduated scale or digi-tal meter. The measurement can be expressed in terms of percent transmittance or in terms of absorbance. Finally, the concentration of the measured constituent is found from a previously prepared calibration curve (Figure 3-42). Such a curve must be prepared for each constituent to be measured. At regular intervals, or any time results appear suspect, a complete new set of standards should be prepared to check the calibration curve.

There are two basic types of photometers, the electrophotometer and the spectrophoto-meter. The basic difference between the two is the method used to produce the single-color light.

Electrophotometers. An electrophotometer uses a simple colored-glass filter. A spe-cific filter color is required for each constituent measured. Electrophotometers are gener-ally used for just a few difficult constituent determinations. A newer type of this instrument, used for individual tests such as chlorine residual and nitrates, employs a cali-brated diode that produces the proper wavelength of light used in the measurement of the reagent-reacted sample. Also available are on-line photometers that allow for the continu-ous monitoring of varicontinu-ous parameters, most commonly chlorine residuals. Current regula-tions require their installation at various locaregula-tions.

Spectrophotometers. A spectrophotometer uses either a diffraction grating or a prism to control the light color. When the angle of the grating or the prism is adjusted, different light colors (different wavelengths of light) can be selected. Thus, one adjustable grating or prism provides a continuous spectrum of color selections. A spectrophotometer is particularly useful when a wide variety of constituents is being measured. Its versatility allows convenient selec-tion of the best light color for any test.

A special type of spectrophotometer, the atomic absorption spectrophotometer (AA unit), is used for analyses of most heavy metals in water. It is a sophisticated and expensive analytical tool that must be operated by specially trained laboratory technicians.

FIGURE 3-41 Photometer

Courtesy of Bacharach, Inc., Photo by Dick Brehl

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In the AA procedure, the sample is vaporized either in a flame or graphite furnace. A special light source, from a specific type of hollow-cathode lamp or an electrodeless dis-charge lamp, emits light at a wavelength that is characteristic of the element being mea-sured. The amount of light absorbed by the vapor is meamea-sured. This absorbance is directly proportional to the concentration of the element in the sample.

Another method being used to measure metals is inductively coupled plasma–mass spectrometry (ICP–MS). Instruments provide a reading for many metals at the same time from one sample injected into an argon plasma flame. This procedure creates ions of the metals that are separated in the mass spectrometer and provides a determination of the concentration of the metal present in the sample.

pH meters

A pH meter is a sensitive voltmeter that measures the pH (acidity or basicity) of samples.

Meters having graduated scales indicate pH units from 0 to 14. More sophisticated meters have expanded scales that allow more precise pH measurement within a narrower range and a millivolt scale that allows measurement of specific ions, such as fluoride. Many instrument types are available; a typical meter with digital readout is shown in Figure 3-43.

One or two electrodes are supplied with the meter. One electrode, a standard calomel reference type, develops a constant voltage to be compared against the changing voltage of the second. The voltage of the second electrode, a glass type, changes as the pH changes. The second electrode is designed so that a change of one pH unit produces a volt-age change of 59.1 mV at 25°C. In some units, the two electrodes are mounted in a single unit called a combined or combination electrode. There may also be an additional probe for temperature compensation as changes in temperature can affect the pH results. This type of pH system is falling out of use with the use of the combination electrodes mentioned in the next paragraph.

On-line pH meters that contain combination electrodes with temperature compensa-tion are in common use. They are used to monitor pH at various stages of the treatment FIGURE 3-42 Calibration curve

WATER LABORATORY EQUIPMENTANDINSTRUMENTS 101

train so that pH levels can be monitored and if needed maintained within certain optimal ranges.