94
Exercise 7
3. Prepare a series of standards as follows using the 0.05-mg Si0 2 /ml stock solution;
make up to 10.0 ml with deionized-distilled water:
0.5 ml = 0.025 mg/1 0 ml = 2.5 mg/l
1.0 ml = 0.05 mg/1 0 ml = 5.0 mg/l
2.0ml = 0.10 mg/lO ml ,= 10.0mg/1
3.0ml = 0.15 mg/10ml= 15.0mg/1
4.0 ml = 0.20 mg/1 0 ml = 20.0 mg/l
4. Add 5 ml 0.25N HCl to each flask; swirl.
5. Add 5 ml 5% ammonium molybdate; swirl.
6. Add 5 ml 1 % disodium EDT A; swirl.
7. After 5 min have elapsed following the addition of the molybdate, add 10 ml of 1 T~)
sodium sulfite.
8. Mix and allow to stand approximately 30 mins. The color is stable for several hours
after this time. (Do not use rubber stoppers to close the flasks.)
9. Using a wavelength of700nm and a 1-cm cell, read the absorbance of the samples
and standards against the blank.
10. Determine the mg dissolved silica in the samples from a plot of absorbancies of the
standards versus their known concentrations.
Note: See also the "Automated Analyses" section below.
SALINITY: MAJOR IONS
The salinity of inland waters is usually dominated completely by four major cations
(calcium, Ca 2 +; magnesium, Mg2+; sodium, Na +; and potassium, K +) and the major
anions (carbonates, CO/- and HC0 3 -; sulfate, S042-; and chloride, Cl-). The
concentrations of these ions generally constitute over 99% of the total salinity. The
salinity of the water is variable and is governed by contributions from rock sources of
the drainage basin, atmospheric wet and dry deposition, and balances between
evaporation and precipitation. Concentrations of Mg, Na, and CI ions are relatively
conservative and exhibit minor spatial and temporal fluctuations within lakes and
streams from biotic utilization or biotically mediated changes in the environment. On
the other hand, concentrations and forms of Ca, K, inorganic carbon, and S04 are
highly dynamic and are influenced markedly by metabolic activities.
A number of satisfactory methods exist for the analyses of concentrations of the
major ions [cf., Mackereth (1963), Golterman and Clymo (1969), and the American
Public Health Association et al. (1989)]. Application of the most appropriate methods
depends on the concentrations found in the waters under study and the sensitivity of
methods will be used in the study of freshwater chemistry. See the "Automated
Analyses" section below.
Carbonates-bicarbonates
The methods for analysis of inorganic carbon are discussed separately in Exercise 8.
Sulfate
Several gravimetric, turbidimetric, complexometric, and potentiometric methods are
available for the analysis of sulfate ions in water [cf., Golterman and Clymo (1969) and
Exercise 7
3. Prepare a series of standards as follows using the 0.05-mg Si0 2 /ml stock solution;
make up to 10.0 ml with deionized-distilled water:
0.5 ml = 0.025 mg/1 0 ml = 2.5 mg/l
1.0 ml = 0.05 mg/1 0 ml = 5.0 mg/l
2.0ml = 0.10 mg/lO ml ,= 10.0mg/1
3.0ml = 0.15 mg/10ml= 15.0mg/1
4.0 ml = 0.20 mg/1 0 ml = 20.0 mg/l
4. Add 5 ml 0.25N HCl to each flask; swirl.
5. Add 5 ml 5% ammonium molybdate; swirl.
6. Add 5 ml 1 % disodium EDT A; swirl.
7. After 5 min have elapsed following the addition of the molybdate, add 10 ml of 1 T~)
sodium sulfite.
8. Mix and allow to stand approximately 30 mins. The color is stable for several hours
after this time. (Do not use rubber stoppers to close the flasks.)
9. Using a wavelength of700nm and a 1-cm cell, read the absorbance of the samples
and standards against the blank.
10. Determine the mg dissolved silica in the samples from a plot of absorbancies of the
standards versus their known concentrations.
Note: See also the "Automated Analyses" section below.
SALINITY: MAJOR IONS
The salinity of inland waters is usually dominated completely by four major cations
(calcium, Ca 2 +; magnesium, Mg2+; sodium, Na +; and potassium, K +) and the major
anions (carbonates, CO/- and HC0 3 -; sulfate, S042-; and chloride, Cl-). The
concentrations of these ions generally constitute over 99% of the total salinity. The
salinity of the water is variable and is governed by contributions from rock sources of
the drainage basin, atmospheric wet and dry deposition, and balances between
evaporation and precipitation. Concentrations of Mg, Na, and CI ions are relatively
conservative and exhibit minor spatial and temporal fluctuations within lakes and
streams from biotic utilization or biotically mediated changes in the environment. On
the other hand, concentrations and forms of Ca, K, inorganic carbon, and S04 are
highly dynamic and are influenced markedly by metabolic activities.
A number of satisfactory methods exist for the analyses of concentrations of the
major ions [cf., Mackereth (1963), Golterman and Clymo (1969), and the American
Public Health Association et al. (1989)]. Application of the most appropriate methods
depends on the concentrations found in the waters under study and the sensitivity of
methods will be used in the study of freshwater chemistry. See the "Automated
Analyses" section below.
Carbonates-bicarbonates
The methods for analysis of inorganic carbon are discussed separately in Exercise 8.
Sulfate
Several gravimetric, turbidimetric, complexometric, and potentiometric methods are
available for the analysis of sulfate ions in water [cf., Golterman and Clymo (1969) and