Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 137

6.2 WET CHEMICAL METHODS AND COMMON
TECHNIQUES IN ENVIRONMENTAL ANALYSIS

6.2.1 Gravimetric and Volumetric Wet Chemical
Methods

The essential measurement involved in gravimetric methods is weighing with an
analytical balance. Gravimetric methods are rarely used in environmental conta-
minant analysis because their concentrations are often low in the range of minor to
trace level. Gravimetric methods are used only for a few parameters such as solids,
moisture, sulfate, and oil and grease. Volumetric (i.e., titrimetric) analysis, on the
contrary, still finds wide applications for the measurement of a variety of important
parameters. Although titrations may be performed automatically with an autotitrator
(mostly electrochemically based instrument), the simplicity of the buret-based
manual titration points to many applications employed in environmental analysis.
Titration is based on the simple stoichiometric relationships of chemical
reactions. While titration can be performed by many electrochemical titration
devices (Chapter 11), the classical titrimetric methods rely on a color change using
indicators. The chemistry underlying color change is based on one of following
four major reactions: (a) acid-base reaction, (b) oxidation-reduction (redox) reaction,
(c) complexation/chelation reaction, and (d) precipitation reaction. Each of these
reactions has its unique applications in environmental measurements (Table 6.3). For
instance, redox-based titrations are applicable to transition metals or elements that
possess more than one oxidation states. Acid-base titrations can be employed for
acidic/basic inorganic or organic compounds, and the indicator changes color at a
specific pH range during titration. Table 6.3 is a list of common wet chemical

Table 6.3 Wet chemical methods commonly used in environmental analysis

Parameters Method/Operation* EPA APHA (SM 20 )

Moisture G/Oven Drying - -

Solids G/Filtration 160.1 to 160.4 2540A to G

Acidity T (acid-base) 350.1 2310 B

Alkalinity T (acid-base) 310.1 & 310.2 2320 B

Hardness T (complexation) 130.1 & 130.2 2340 C

DO T (redox) 360.2 4500 O B to F

BOD T (redox) 405.1 5210 B

COD T (redox)/Reflux 410.1 to 410.3 5220 C

Oil and Grease G/Extraction/Distillation 413.1 5520 B

Residual Chlorine T (redox) 330.2 to 330.4 4500 D

4500-Cl B

Chloride T (precipitation) 325.3

Ammonia T (acid-base)/Distillation 350.2 4500-NH 3 C

4500-CN D

Cyanide T (complexation)/Distillation 335.1

4500-S 2 F Sulfide T (redox)/Distillation 376.1 & 9030 A

* G: Gravimetric; T: Titrimetric; SM 20 : Standard Method, 20th Edition.