Acidification of sediment and tissue samples with Sulfurous Acid
Removing inorganic carbonates by acidification can leach out acid soluble organic compounds
containing N. This can bias results for both d15N and wt.%N. Using HCl forms the hygroscopic salt CaCl2
from CaCO3. The solid phase sample mass changes significantly, so weighing after acidification will lead
to bias in wt% estimates. This experiment will test the utility of H2SO3 (6% solution) as an alternative to
HCl. The acidification is performed within the capsule used for the subsequent analysis.
1) Record the tare weight of a 5x9mm silver capsule.
2) Weigh out approximately 5 – 20 mg sediment or 1 mg animal tissue or 2 mg algae into the
silver capsule and record sample mass. Leave the capsule open and place in tray.
3) Bring tray with weighed samples into the fume hood. While wearing safety goggles, lab coat,
and gloves, pipet 30ul of n-pure into each capsule.
4) Pipet 10ul of 6% H2SO3 into the capsule. If there is any splashing or losses from the reaction,
discard sample and capsule and start over.
5) After all visible reaction has subsided, dry sample in vented 60 deg C oven for 15 minutes.
6) Remove from oven and add 30ul of 6% H2SO3. Dry in 60 deg C oven.
7) Repeat step 6 using 50ul, then multiple additions of 100ul of 6% H2SO3. The total amount of
H2SO3 added will depend on the amount of carbonate present. See NOTE below.
8) Wrap silver capsule and analyze.
NOTE: 6% H2SO3 has approximately 0.094 M [H+] using 1st dissociation only. Each 100ul of 6% H2SO3
will react with 470 ug CaCO3. If the sample weight is 10000ug, and it is 50% CaCO3, then you will need
to add at least 1.1 mls total of 6% H2SO3, which is equivalent to 5170ug CaCO3. The following additions
would result in 1.09 mls total H2SO3: 10ul, 30ul, 50ul and 10 additions of 100ul.