University of New Hampshire Discrete Total Alkalinity/ Dissolved Inorganic
Carbon Sampling method
by: Chris Hunt
1. Introduction
This procedure describes a method for the determination of the pH and Total Alkalinity
(TA) of discrete samples taken from either Niskin bottles on cruises or bucket samples
during a river sampling trip. Samples are collected, poisoned, stored in a cool dark place,
and analyzed in this laboratory. Using two of pH, pCO2, or alkalinity, calculations of the
entire carbonate system can be performed. Alkalinity and pH determinations are
performed on the same aliquot of water, so only one bottle needs to be taken to perform
both analyses.
2. Apparatus
2.1 Sample Bottles
Samples are collected in glass BOD bottles with ground-glass stoppers, generally
60 mL in volume. The bottles should be triple-rinsed with DDW, then filled
again with DDW before the sampling trip. The stoppers are held on with a
positive closure device, usually a plastic cap with a foam insert that keeps the
stopper securely closed, or a rubber band with plastic clip.
2.2 Collection Equipment
At sea, Niskin bottles are used to collect water samples from various depths, and
then brought back to the surface. On a river trip, or for surface seawater
collection, a rinsed plastic bucket with a rope is fine.
2.3 Poisoning Equipment
Each sample is poisoned with a small amount of a saturated mercury chloride
solution. The DOE Handbook of Methods (1994) calls for a volume of saturated
mercury chloride of about .02% by volume for each sample. This translates into
roughly 10 µL for our 60 mL samples. This poison is kept in small brown glass
vials with caps that have a septum built-in. A syringe is used to draw the mercury
chloride from the vial and dispense it into the sample bottle.
3. Equipment Checklist
-Glass BOD bottles, rinsed and ashed, with stoppers and positive closure caps, taped
before the cruise
-Length of tubing for Niskin Bottle
-Mercury Chloride solution (saturated)
-Pipettor (10 µL total volume) with tips
-Gloves
-Label tape and marker
4. Procedure
Note: If a Niskin bottle is being used, and no Winkler oxygen samples are being
collected, the TA sample should be the first withdrawn from the bottle. This ensures
a minimal amount of air-water gas exchange, which can shift DIC values:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
From a Niskin bottle, ensure that the white knob near the top of the bottle is
completely shut, and then push in the stopcock near the bottom of the bottle.
Very little water should come out of the stopcock.
Make sure the bottle is already labeled, as this is hard to do when the tape is wet.
Dump the DDW from the sample bottle.
Rinse the bottle three times with a small amount of water from the Niskin, using
the white knob near the top to start and stop flow from the stopcock.
Fill the bottle slowly, letting the water run down the side of the bottle. The goal is
to prevent bubbles from forming in the sample. Alternatively, and short piece of
tubing can be attached to the stopcock and used to fill the bottle from the bottom.
Fill the bottle until a very small headspace will remain with the stopper in (about
1% headspace). Flush out the tubing first, squeezing it if necessary to ensure
there is no air in the tubing before filling the TA bottle.
Stopper the bottle and clamp the positive closure cap over the stopper.
When ready, open the bottle again and poison the sample with the Mercury
Chloride solution. For the 60 mL BOD bottles, we use 10 µL of poison. Be sure
to wear gloves when you do this!
Re-cap the bottle and put into the cooler or other cool dark place for transport
back to the lab.
If a sample is being taken from a bucket, the procedure is essentially the same as
with the Niskin. Just remember, the whole goal is to get a bubble-free sample in
the bottle.
Refrigerate the samples after the poison has been added.