Inorganic Nutrients
93
DISSOLVED SILICA
Dissolved silica (Si0 2 ) usually occurs in moderate abundance in fresh water. Although
essentially unionized and relatively unreactive chemically, dissolved silica is assimilated
in large quantities by diatoms in the synthesis of their cell walls or frustrules. Since
diatoms are major algal components in many lakes, diatom utilization can modify
greatly the concentrations and flux rates of dissolved silica in lakes and streams. The
availability of dissolved silica can have a marked influence on the productivity and
succession of algal populations [cf., Wetzel (1983)].
The solubility of particulate amorphous silica is related inversely to pH between
approximately 3 and 7 (Marshall, 1964) and increases somewhat at alkaline values to a
pH of 9. Carbonic acid in water reacts with silicate materials to form dissolved silica
and carbonates. Utilization of dissolved silica in the trophogenic zone of lakes by
diatoms often markedly reduces epilimnetic concentrations and contributes to a
progressive seasonal succession of diatom and other algal species. Below a concentration of about 0.5 mg Si0 2 /1, most diatoms cannot complete effectively with
nonsiliceous algae.
Silicon in solution as silicic acid (H 4 Si0 4 ) or silicate (SiO/ -) reacts with acidic
ammonium molybdate to form a yellow silicomolybdate complex. This complex is then
reduced by sodium sulfite to form the molybdate blue color. The following method is
accurate to at least ± 0.02 mg/l in the ranges generally encountered in fresh waters
(Rainwater and Thatcher, 1960). The method is not applicable where the dissolved
silica content is greater than 100mg/i.
Sample Storage. Samples in clean polyethylene bottles should be filtered to remove
particulate matter (nonglass fiber filters, 0.5-.um pore size) immediately after collection.
The filtrate is relatively stable, but analyses should be performed without appreciable
delay. When water samples are frozen, the dissolved silica may be converted to an
unreactive form. However, ifthawed samples are allowed to stand for a period of time,
the silicon will again become reactive. These reactions are dependent the salinity of the
sample (Burton et ai., 1970), and evaluation of these effects should be made in each case
when samples are stored in the frozen state.
Reagents
1. Hydrochloric acid, O.25N: Mix 22 ml of concentrated HCI (sp. gr. 1.18) with water
and dilute to 1 liter.
2. Ammonium molyhdate, 5/;,: Dissolve 52 g (NH4)6M07024' 4 H 20 in water and
dilute to 1 liter.
3. Disodium EDT A, 1 %: Dissolve 109 disodium EDT A in water and dilute to 1 liter.
4. Sodium sulfIte, 17%: Dissolve 170 g Na 2 S0 3 in water and dilute to 1 liter.
5. Silica standard: Stabilize sodium silicate pentahydrate by placing approximately 3 g
Na 2 Si0 3 ' 5H 2 0 in a desiccator for 3 h. Dissolve 1.765 g in water and dilute to lliter.
Dilute 100.0 ml (volumetric pipet) of this intermediate solution with deionizeddistilled water to 1 liter; 1.00 ml = 0.050 mg Si0 2 .
Procedures
1. Pipet 10.0 ml of each sample into 25- or 50-ml Erlenmeyer flasks.
2. Pipet 10.0 ml of deionized-distilled water into a flask to serve as a blank.
93
DISSOLVED SILICA
Dissolved silica (Si0 2 ) usually occurs in moderate abundance in fresh water. Although
essentially unionized and relatively unreactive chemically, dissolved silica is assimilated
in large quantities by diatoms in the synthesis of their cell walls or frustrules. Since
diatoms are major algal components in many lakes, diatom utilization can modify
greatly the concentrations and flux rates of dissolved silica in lakes and streams. The
availability of dissolved silica can have a marked influence on the productivity and
succession of algal populations [cf., Wetzel (1983)].
The solubility of particulate amorphous silica is related inversely to pH between
approximately 3 and 7 (Marshall, 1964) and increases somewhat at alkaline values to a
pH of 9. Carbonic acid in water reacts with silicate materials to form dissolved silica
and carbonates. Utilization of dissolved silica in the trophogenic zone of lakes by
diatoms often markedly reduces epilimnetic concentrations and contributes to a
progressive seasonal succession of diatom and other algal species. Below a concentration of about 0.5 mg Si0 2 /1, most diatoms cannot complete effectively with
nonsiliceous algae.
Silicon in solution as silicic acid (H 4 Si0 4 ) or silicate (SiO/ -) reacts with acidic
ammonium molybdate to form a yellow silicomolybdate complex. This complex is then
reduced by sodium sulfite to form the molybdate blue color. The following method is
accurate to at least ± 0.02 mg/l in the ranges generally encountered in fresh waters
(Rainwater and Thatcher, 1960). The method is not applicable where the dissolved
silica content is greater than 100mg/i.
Sample Storage. Samples in clean polyethylene bottles should be filtered to remove
particulate matter (nonglass fiber filters, 0.5-.um pore size) immediately after collection.
The filtrate is relatively stable, but analyses should be performed without appreciable
delay. When water samples are frozen, the dissolved silica may be converted to an
unreactive form. However, ifthawed samples are allowed to stand for a period of time,
the silicon will again become reactive. These reactions are dependent the salinity of the
sample (Burton et ai., 1970), and evaluation of these effects should be made in each case
when samples are stored in the frozen state.
Reagents
1. Hydrochloric acid, O.25N: Mix 22 ml of concentrated HCI (sp. gr. 1.18) with water
and dilute to 1 liter.
2. Ammonium molyhdate, 5/;,: Dissolve 52 g (NH4)6M07024' 4 H 20 in water and
dilute to 1 liter.
3. Disodium EDT A, 1 %: Dissolve 109 disodium EDT A in water and dilute to 1 liter.
4. Sodium sulfIte, 17%: Dissolve 170 g Na 2 S0 3 in water and dilute to 1 liter.
5. Silica standard: Stabilize sodium silicate pentahydrate by placing approximately 3 g
Na 2 Si0 3 ' 5H 2 0 in a desiccator for 3 h. Dissolve 1.765 g in water and dilute to lliter.
Dilute 100.0 ml (volumetric pipet) of this intermediate solution with deionizeddistilled water to 1 liter; 1.00 ml = 0.050 mg Si0 2 .
Procedures
1. Pipet 10.0 ml of each sample into 25- or 50-ml Erlenmeyer flasks.
2. Pipet 10.0 ml of deionized-distilled water into a flask to serve as a blank.