103
sea water. Allowing for these limitations, the
recipes summarized under the above internetaddress have proven quite useful for the determination of phosphate, ammonium, nitrate, nitrite,
and iron(II) in pore water.
Figure 3.1 shows the characteristic profiles of
ammonium and phosphate concentrations that
were measured by photometrical analysis of pore
water obtained from reactive sediments possessing a high amount of organic matter. Both parameters demonstrate quite similarly shaped curves
compared to alkalinity. Here, the ratio of the concentrations derived from both profiles lies close to
the C:N:P Redfield-ratio of 106:16:1 and clearly
documents their release into the pore water due to
the decomposition of organic matter. A typical
photometric nitrate profile in reactive sediment
zones near the sediment surface is shown in the
quantitative evaluation of fluxes and reaction
rates presented in Figure 3.7.
Alkalinity
Usually, alkalinity is actually the ultimate parameter that is subject to a procedure of ‘genuinely
chemical’ titration, in this regard as a proxy
parameter for carbonate. A new spectrophotometric method for the determination of alkalinity
was proposed by Sarazin et al. (1999). Mostly,
alkalinity will be, for reasons of simplification, set
equal to the total carbonate concentration, although a number of other substances in pore
water will contribute to the titration of alkalinity as
well. Most geochemical model programs (cf. Chap.
15) foresee the input of titrated alkalinity as an
alternative to the input of carbonate. The model
program will than calculate the proportion allocatable to the different carbonate species.
Another problem arising from titration of alkalinity in a pore water sample usually consists in
the fact that samples with very small volumes
cannot easily be used. Most ocean chemists will
be accustomed to the titration of a volume of 100
ml, or at least 10 ml. Under certain circumstances,
a pore water sample obtained from a definite depth
might contain in total not more than 10 ml, hence,
at best, merely 1 ml needs to be sacrificed for the
titration of alkalinity. This requires that markedly
pointed and thin (thus easily breakable) pHelectrodes are used. These are immersed, together
with an electronically controlled micropipette, into
a small vial so that a tiny magnetic stirrer bead still
has enough room to fit inside as well.
Mostly, an adequate amount of 0.001 M
hydrochloric acid will be added to a previously
pipetted volume of 1 ml, thus the pH will be
brought to a value of about 3.5. Since the dilution
of the pre-pipetted volume must be considered,
the alkalinity (Alk) is calculated according to the
following equation:
Alk = [(V HCl ·C HCl ) - 10 -pH · (V 0 +V HCl ) · f H+
-1 ] · V 0
-1
(3.29)
in which V HCl describes the volume of added
hydrochloric acid, C HCl represents the molality of
the added hydrochloric acid, pH denotes the pH
value the solution attains after the addition of
hydrochloric acid, V 0 is the pre-pipetted sample
volume, and f H+ the activity coefficient for H
+
-Ions
in solution. The activity coefficient f H+ can be
determined according to the method described by
Grasshoff et al. (1999), or calculated by employing
a geochemical model (e.g. PHREEQC, cf. Chap. 15).
The Equation 3.29 was also formulated in the first
edition of the textbook published by Grasshoff et
al. (1983) [equation 8-25 on page 108]. It must be
pointed out that a very misleading error has unfortunately found its way into the equation in this
reference. This error is hardly noticeable when low
concentrations are prevalent in ocean water, or
when the sample volumes are comparably large. In
the case of high concentrations in combination
with small sample volumes, however, alkalinity
values will be calculated that are prone to an error
of more than a factor of 2. For this reason, the
equation is presented here in its correct form.
Flow Injection Analysis
A very interesting method to analyze total carbon
dioxide and ammonium in pore water was introduced by Hall and Aller (1992). In this method, a
sample carrier stream and a gas receiver stream flow
past one another, separated only by a gas-permeable
PTFE (Teflon
®
) membrane. For determining the total
CO 2 , the sample carrier stream consists of 10-30 mM
HCl. To this stream, the sample in a volume of about
20 µl is added via a HPLC injection valve. The
carbon dioxide traverses the PFTE membrane and
enters the gas receiver stream which, in this case,
consists of 10 mM NaOH. The CO 2 taken up by the
gas receiver stream causes an electrical conductivity
change that can be determined exactly in a microsized continous flow cell.
3.4
Analyzing Constituents in Pore Water, Typical Profiles
sea water. Allowing for these limitations, the
recipes summarized under the above internetaddress have proven quite useful for the determination of phosphate, ammonium, nitrate, nitrite,
and iron(II) in pore water.
Figure 3.1 shows the characteristic profiles of
ammonium and phosphate concentrations that
were measured by photometrical analysis of pore
water obtained from reactive sediments possessing a high amount of organic matter. Both parameters demonstrate quite similarly shaped curves
compared to alkalinity. Here, the ratio of the concentrations derived from both profiles lies close to
the C:N:P Redfield-ratio of 106:16:1 and clearly
documents their release into the pore water due to
the decomposition of organic matter. A typical
photometric nitrate profile in reactive sediment
zones near the sediment surface is shown in the
quantitative evaluation of fluxes and reaction
rates presented in Figure 3.7.
Alkalinity
Usually, alkalinity is actually the ultimate parameter that is subject to a procedure of ‘genuinely
chemical’ titration, in this regard as a proxy
parameter for carbonate. A new spectrophotometric method for the determination of alkalinity
was proposed by Sarazin et al. (1999). Mostly,
alkalinity will be, for reasons of simplification, set
equal to the total carbonate concentration, although a number of other substances in pore
water will contribute to the titration of alkalinity as
well. Most geochemical model programs (cf. Chap.
15) foresee the input of titrated alkalinity as an
alternative to the input of carbonate. The model
program will than calculate the proportion allocatable to the different carbonate species.
Another problem arising from titration of alkalinity in a pore water sample usually consists in
the fact that samples with very small volumes
cannot easily be used. Most ocean chemists will
be accustomed to the titration of a volume of 100
ml, or at least 10 ml. Under certain circumstances,
a pore water sample obtained from a definite depth
might contain in total not more than 10 ml, hence,
at best, merely 1 ml needs to be sacrificed for the
titration of alkalinity. This requires that markedly
pointed and thin (thus easily breakable) pHelectrodes are used. These are immersed, together
with an electronically controlled micropipette, into
a small vial so that a tiny magnetic stirrer bead still
has enough room to fit inside as well.
Mostly, an adequate amount of 0.001 M
hydrochloric acid will be added to a previously
pipetted volume of 1 ml, thus the pH will be
brought to a value of about 3.5. Since the dilution
of the pre-pipetted volume must be considered,
the alkalinity (Alk) is calculated according to the
following equation:
Alk = [(V HCl ·C HCl ) - 10 -pH · (V 0 +V HCl ) · f H+
-1 ] · V 0
-1
(3.29)
in which V HCl describes the volume of added
hydrochloric acid, C HCl represents the molality of
the added hydrochloric acid, pH denotes the pH
value the solution attains after the addition of
hydrochloric acid, V 0 is the pre-pipetted sample
volume, and f H+ the activity coefficient for H
+
-Ions
in solution. The activity coefficient f H+ can be
determined according to the method described by
Grasshoff et al. (1999), or calculated by employing
a geochemical model (e.g. PHREEQC, cf. Chap. 15).
The Equation 3.29 was also formulated in the first
edition of the textbook published by Grasshoff et
al. (1983) [equation 8-25 on page 108]. It must be
pointed out that a very misleading error has unfortunately found its way into the equation in this
reference. This error is hardly noticeable when low
concentrations are prevalent in ocean water, or
when the sample volumes are comparably large. In
the case of high concentrations in combination
with small sample volumes, however, alkalinity
values will be calculated that are prone to an error
of more than a factor of 2. For this reason, the
equation is presented here in its correct form.
Flow Injection Analysis
A very interesting method to analyze total carbon
dioxide and ammonium in pore water was introduced by Hall and Aller (1992). In this method, a
sample carrier stream and a gas receiver stream flow
past one another, separated only by a gas-permeable
PTFE (Teflon
®
) membrane. For determining the total
CO 2 , the sample carrier stream consists of 10-30 mM
HCl. To this stream, the sample in a volume of about
20 µl is added via a HPLC injection valve. The
carbon dioxide traverses the PFTE membrane and
enters the gas receiver stream which, in this case,
consists of 10 mM NaOH. The CO 2 taken up by the
gas receiver stream causes an electrical conductivity
change that can be determined exactly in a microsized continous flow cell.
3.4
Analyzing Constituents in Pore Water, Typical Profiles