Environmental Engineering Reference
In-Depth Information
taking care not to spill the NaOH. Also, prepare microcosm blanks containing
alkali traps only.
3. Securely attach the covers of the microcosms, first purging the headspace with
N
2
gas for anaerobic treatments. Place the microcosms under the desired incu-
bation conditions.
4. Collect samples for analysis at time intervals dictated by the experimental
objectives, typically sampling more frequently early in the study in order to
obtain an estimate of CO
2
evolution rate and thereby avoid exhausting the alkali
traps by too infrequent sampling. Retrieve the alkali traps from the microcosms
and attach air-tight covers to the vials to stop CO
2
collection.
5. After a brief period of time to allow for aeration, place fresh uncovered alkali
traps in the microcosms. As noted previously, the NaOH used in the traps should
be newly standardized. Replace microcosm lids.
6. For analysis of alkali traps, the solution in a trap is transferred quantitatively,
using water rinses, to a titration flask; this procedure and the titration itself
should be performed on one sample at a time to minimize spurious CO
2
collection. Add 25 ml of 1 M BaCl
2
to the flask, the contents of which should
form a white precipitate of BaCO
3
. A few drops of phenolphthalein indicator are
added and should result in a pink coloration, indicating that the NaOH originally
added to the trap was not exhausted during the soil incubation (i.e., excess NaOH
remains). This excess NaOH is titrated from a pink suspension to a milky white
endpoint using standardized 1.0 M HCl.
7. Carbon dioxide evolution is calculated as mg CO
2
¼
(meq base - meq acid) (22),
where meq base is the milliequivalents of NaOH originally present in the
traps, meq acid is the milliequivalents of HCl required to titrate the excess
NaOH, and 22 is the milliequivalent weight of CO
2
. Milliequivalents of acid or
base
are
calculated
as
the
product
of
concentration
expressed
as
¼
(concentration of HCl
as normality used in titration)] (ml of HCl used in titration). For NaOH and
HCl, molarity concentration is equal to normality concentration (e.g., 1.5 M
NaOH
¼
1.5 N NaOH).
8. Commonly calculated parameters in respiration studies are cumulative CO
2
evolution with time and mean rate of CO
2
evolution between sampling times.
Depending on experimental objectives, it may be desirable to convert values
from a CO
2
basis to an elemental carbon basis.
normality and volume expressed as ml [e.g., meq acid
References
Thien SJ, Graveel JG (2003) Laboratory manual for soil science, 8th edn. McGraw Hill, Boston
Zibilske LM (1994) Carbon mineralization. In: Weaver RW et al (eds) Methods of soil analysis.
Part 2: Microbiological and biochemical properties. SSSA Book Series No. 5. SSSA, Madison,
pp 835-863
