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The key issue here is to decide upon the test procedure that best meets your requirements for:
a) accuracy
b) cost (initial & cost per test)
c) skill level
d) repeatability
e) portability
f) decision-making information obtained
g) safety and reagent disposal
h) are the results reportable?
Remember, the most complete, reliable, and accurate information obtained from your testing will provide you with the decision-making tools to monitor your water or wastewater systems, make operational changes and meet permit requirements and state mandates.
Colorimetric
Methods
Defined as the measurement of a parameter where its concentration
is directly proportional to color development and intensity after
the addition of known volume of reagent(s) (chemicals). In cases
like chlorine residual the reaction is almost immediate, and results
can be determined right away. Other tests like nitrates and phosphates
may require 5 to 10 minute waiting periods before full color development
is obtained due to the chemistry involved.
Some unique colorimetric tests react in reverse. That is - the greater the color development, the lower the concentration of a particular parameter. Examples here are Fluoride, and some Ozone test methods. To determine concentration, the color developed in the sample is either compared visually with manufacturer supplied standards (color comparator) or inserted into a photometer, colorimeter, or spectrophotometer to give results directly on a meter scale, or digitally via a discreet readout. Results obtained are expressed as parts per million (ppm), milligrams per liter (mg/L), grains per gallon (gpg), etc.
Visual
Comparator Limitations
Individual differences in ones ability to discern color intensity
Background lighting. Most manufacturers formulate their color
standards using natural daylight. Incandescent, fluorescent and
direct sunlight are unacceptable and may produce errors. Color
blindness is a definite problem with visual color comparison methods.
Certain colors are extremely difficult to discern variations.
Example: yellows, and some hues of blue. Even with the limitations
described above, visual color comparison methods are inexpensive,
generally easy to use, conveniently packaged and designed for
simplicity. Some visual test results are reportable for permit
purpose. Check with your local inspector. Calorimetric method
using a photometer, colorimeter, or spectro- photometer offer
a unique advantage. Many meters are battery powered and conveniently
packaged for portability. To briefly describe their operation,
a light beam is passed through the sample. Depending upon the
amount of color present, light is transmitted through the sample
and detected by a photodiode. With aid of electronics, the results
are displayed on a meter, either directly in concentration or
as a percentage of light transmitted. Advantages of instrumentation
are:
- eliminates need for visual interpretation by operator
- eliminates concern for background lighting
- ultimately greater accuracy
Of course, using a meter to "read" color development can be more expensive initially. Colorimetric test methods offer an opportunity to provide on-the-spot results and have the capability to test for a variety of common parameter-~. Tests for Chlorine, Iron, Manganese, Copper, Zinc, Aluminum, Fluoride, Ozone, Nitrates, Phosphates, Sulfides, and many more are available. Weigh the advantages, disadvantages and overall requirements before you make your decision.
Titrimetric
Methods
A sample is taken, and reagent(s) is added to produce a color.
In this case the reagent is known as an indicator reagent. A titrant
or a reacting reagent is added drop by drop until a color change
occurs. The point at which the color changes is called the endpoint.
Titrimetric methods offer a number of titrant dispensing apparatuses: Drop count, where a calibrated dropper dispenses drops of equal size. Once the endpoint is reached, the number of drops required to reach the endpoint is counted and multiplied by a conversion factor. Example, one drop equals 5 ppm.
Laboratory burets, Automatic burets - which are generally not portable. This dispensing apparatus has a calibrated scale on the barrel. Titrant is dispensed until the endpoint is reached. Volume used is then read from the calibrated scale. In many cases the number of milliliters used equals the test result in ppm. Direct Reading Microburet, a syringe size calibrated microburet which dispenses the titrant until endpoint is reached. Results are usually read directly off the calibrated scale in ppm. This procedure is totally portable like drop count methods.
Digital Titrators, where titrant from a cartridge is inserted into a micro dispensing device. The amount dispensed is read on a digital venier, usually in ppm. Titration methods are generally quite inexpensive, and are the preferred method in many procedures. Typical tests for Acidity, Alkalinity, Carbon Dioxide, Hardness, Dissolved Oxygen, and Chlorine are among the most common. Here too, convenient packaging and simplicity are the key to their portability and accuracy. This method is preferred in determining corrosion in water supplies, and offers the operator an easy, inexpensive approach in meeting lead/copper requirements.
Turbidimetric
Methods
Some unique test procedures do not use color as a way to detaining
results. A sample is taken and a reagent is added which produces
turbidity or cloudiness in the sample. The greater the turbidity,
the greater the concentration. Turbidimetric like colorimetric
methods can be "read" using a visual comparator, or by the use
of a colorimeter (meter). Results here too are expressed in ppms
or mg/L. Typical tests using this method are Potassium and Sulfates.
Again, this method can be totally portable and conveniently packaged
as a kit.
Electrometric
Methods
One of the most commonly used, an electrode is inserted into a
sample. A small current or voltage is produced and electronically
amplified and read on a meter scale. Typical tests here are pH
and conductivity, but a variety of parameters using ion specific
electrodes (ISE) can be measured including Calcium. Nitrates,
Chlorine, etc.
Nearly every electrometric procedure requires meter calibration and/or sample pretreatment. Examples here are the 4, 7 & 10 pH buffers used to calibrate pH meters. Generally, electrometric methods are more costly initially and require a higher degree of care and maintenance due to the electrode systems.
Today, inexpensive pocket pH, conductivity, and ORP meters are on the market. Even though designed to be disposable after a period of time, great care must be exercised in their use and maintenance. Yes, these pocket meters rival the costs of colorimetric or titrimetric methods. They generally are not acceptable for reporting purposes, but are ideal for quick system checks.
Nephlometric
Methods
This method is specific to water turbidity. Suspended matter within
the sample is measured via a specially designed meter which sends
a focused light beam through the water sample. Suspended solids,
dirt, and silt scatter the light. The scatter is measured by a
photodiode at a 90~ angle incident to the light source. Results
here are expressed a~ Nephlometric Turbidity units (NTU's), and
are more qualitative than quantitative. Portable battery powered
units are available for field use. Private and municipal water
treatment systems using surface water supplies such as lakes,
streams, etc. are required to measure turbidity routinely as a
guide to monitoring various water treatment systems like settling
basins, and sand filter performance. Continuous monitoring turbidity
meters and recorders are becoming the rule rather than the exception.
Gravimetric
Test Methods
These are essentially physical test procedures. They include settleable
solids, settle ability tests primarily used as operational guideposts
in both water and waste facilities. A sample is taken (usually
one liter) and mixed and allowed to settle. Imhoff cones, and
settlometers are common containers here. The samples are timed
at various intervals to determine ratio of solids and volume of
solids serried. Results can be transferable to plant operations
to determine proper floccuiant dose, expected sludge volumes,
adjust waste and return sludge in wastewater facilities. These
are relatively simple test methods that require no chemical or
reagent to perform (except when determining flocculant dosage),
and provide valuable current data to a water or wastewater operator.
Sampling
Technique
All test methods described above require a proper sample. Accurate
sample volumes required by the test are important. Some important
points to remember.
- Choose the proper point in the water system for your sample.
Let the spigot run a short period of time to obtain a representative
sample. (Note if this is a first draw sample. For lead or copper
disregard this step.) Pour the correct volume of sample into the
test tube or jar. Accurate results require accurate sample volumes.
- Once test is complete dispose of waste reagent/sample properly,
and clean all test tube thoroughly.
- Follow the test kit manufacturers' directions specifically.
Do not alter the procedure to suit your needs or to take shortcuts
risking skewed results.
- Do not intermix different manufacturers' reagents, particularity
colorimeteric ones, unless they are the exact concentration.
Summary
Briefly, we have looked at six water test methods for use by the
water and wastewater operator. So what's right for you?
- A careful review of tests are needed.
- Choose the test method that suits your testing skill level.
- What accuracy do you need? Know the test's limitations.
- Review test kit (method) cost versus expected results. - Look
to the marketplace for manufacturers of test equipment and kits.
Review their products.
- Safety and reagent disposal requirements. - Are the results
reportable, does the procedure follow standard methods or EPA
manual? State approved?
Safety
& Environmental Factors
Many test kits and instruments contain hazardous reagents. Read
ail safety related instructions. Thoroughly review test procedures
before running the test. Use manufacturer supplied Material Safety
Data Sheets (MSDS) to learn about specific hazards and waste reagent
disposal. Know shelf-life of specific reagents and replace when
required.
Some heavy metal test kit reagents have been banned for water
testing in the home. Tests for Lead, Cadmium, Mercury, etc. may
contain extremely hazardous materials like Carbon Tetrachloride
and Sodium Cyanide. Leave those tests for the outside certified
laboratory to perform. Nearly all test methods defined and described
here are common inorganic tests. Tests for pesticides, aromatic
hydrocarbons (gasolene's), PCB's and the like should also be left
to a qualified certified laboratory with the proper equipment.
Reproduced with premission from the Lamotte Company.