Environmental Engineering Reference
In-Depth Information
There are additional considerations when alkali traps are being used to collect CO
2
.
Care must be taken that excess alkali is used in the traps so that the CO
2
-trapping
ability is not exhausted between samplings. A reasonably conservative starting point
when working with soil amended with organic matter at a 1 % rate is the equivalent of
15 ml of 1.5 M NaOH per 50 g of soil (Thien and Graveel 2003). It is absolutely
essential that the alkali used in the traps and the acid used for titration be accurately
standardized; the use of potassium hydrogen phthalate or a commercially available
primary standard is strongly recommended. The concentration of NaOH used in the
traps should be determined each time they are replenished. Additionally, it is critical
that microcosm blanks (i.e., microcosms containing alkali traps only) be employed to
account for any CO
2
in the headspace originally or added when the microcosms are
opened for aeration; the CO
2
measured in the blanks should be subtracted from the
values obtained from the microcosms containing soil.
Objective: To monitor soil respiration under laboratory conditions using acid-base
titration to measure CO
2
evolution.
Materials and Equipment Needed
500-ml soil microcosms (16-oz canning jars with air-tight lids)
Balance
1.5 M NaOH, standardized
Vials for alkali traps (plastic, 50-ml centrifuge tube with air-tight cap)
1.0 M HCl, standardized
1.0 M BaCl
2
Phenolphthalein indicator solution
Burets
125-ml Erlenmeyer flasks
Procedures
Overview
: The general approach outlined here is suggested for its simplicity and
adaptability to a wide range of experimental objectives and is based on the
procedure described by Thien and Graveel (2003) for use in instructional soil
science laboratories.
1. Weigh 50 g of soil into each microcosm. Adjust moisture content and add a
bulking agent and or organic amendment if desired. For anaerobic studies, the
soil can be flooded with water at this time.
2. Dispense a precisely measured volume (~15 ml) of standardized NaOH into
vials to be used as alkali traps. Place uncovered alkali traps into the microcosms
