CHEMICAL DATA BASE METHODS

(Instituto de Investigacións Marinas. CSIC. Spain)

Aida F. Ríos

**aida@iim.csic.es**

A Metrohm E-654 pH-meter equipped with a Ross (Orion 81-04) combined
glass electrode was used to determine **pH** on the NBS scale. The
temperature was measured using a platinum resistance thermometer and
finally pH was referred to a standard temperature of 15°C (pH~15~)
according to Pérez and Fraga (1987a). The method has a shipboard
precision of ±0.002 pH~15~ (Ríos and Rosón, 1996) and an accuracy of
±0.004 pH~15~ using samples of Certified Reference Material (CRMs)
provided by Dr. Dickson from the Scripps Institution of Oceanography
(Ríos and Pérez, 1999; Ríos and Rellán, 1998).

**Alkalinity** was determined by automatic potentiometric titration with
HCl at a final pH of 4.44 (Pérez and Fraga, 1987b). The electrodes were
standardised using an NBS buffer of pH 7.413 and checked using an NBS
buffer of 4.008. This method has a precision of 0.1% (Pérez and Fraga,
1987b), and an accuracy of 1.4µmol.kg^-1^ (Ríos and Pérez, 1999; Ríos
and Rellán, 1998).

## Carmen G. Castro

**ccarmen@iim,csic.es**

**Nutrient** samples were filtered through 0.45 µm Millipore filter
prior to analysis and were analysed within 12 h after collection; and
were stored in the refrigerator prior to analysis and in the dark.
Nutrient concentrations were determined by segmented flow analysis with
Technicon AAII systems, following Hansen and Grasshoff (1983) with some
improvements (Mouriño and Fraga, 1985; Álvarez-Salgado *et al*., 1992).
The analytical error was 0.05 µmolkg^-1^ for nitrate, 0.05 µmolkg^-1^
for silicic acid and 0.01 µmolkg^-1^ for phosphate. **Dissolved oxygen**
was determined by Winkler potentiometric titration. The estimated
analytical error was 1 µmolkg^-1^. Oxygen saturation was calculated
following Benson and Krause equation (UNESCO, 1986). **Chlorophyll *a***
was measured using 90% acetone extraction in a 10,000 R Turner
fluorometer (Yentsch and Menzel, 1963). The precision was 0.05 mgm^-3^.
**Particulate organic matter** (filtration volume 1 l) was collected on
Whatman GF/F filters and analyses were performed in a PE 2400 elemental
analyser, with a precision of 0.04 µmolkg^-1^ for nitrogen and 0.1
µmolkg^-1^ for carbon.

**M.D. Doval**

**<marylo@iim.csic.es>, mdoval@cccmm.cesga.es**

Seawater samples for **DOC** analysis were collected with 100 ml
polyethylene syringes with teflon plunger tips and filtered by hand
through Whatman Puradisc GF/F disposable filter devices (0.7µm pore
size) on polypropylene housing. The filtrate was drawn eventually into
50 ml polyethylene containers. The filtering system and the containers
used for DOC had been previously soaked on 0.1 N HCl, and rinsed with
Milli-Q water. In addition, the containers were rinsed three times with
50 ml of sample. Samples were immediately stored at -70ºC until analysis
in the base laboratory, eight months later. This storage technique has
demonstrated no artefactual results on the micromolar scale (Hansell and
Carlson, 1998b).

DOC determination was performed by high temperature catalytic oxidation
(HTCO) with a commercial Shimadzu TOC-5000. The combustion quartz tube
was filled with a 0.5% Pt on Al~2~O~3~ catalyst. Three to 5 replicate
injections of 200 µl were performed per sample. The concentration of DOC
was determined by subtracting the average peak area from the instrument
blank area and dividing by the slope of the standard curve. The
instrument blank is the system blank plus the filtration blank. The
system blank was determined by subtracting the DOC in UV-Milli-Q to the
total blank. Measurements made with the high sensitivity catalyst (Pt on
silica wool) produced values \<2 µmol C l^-1^ for fresh UV-Milli-Q
water. The filtration blank (determined by filtering UV-Milli-Q water
through the filtration system) was \<2 µmol C l^-1^. Before sample
analyses, the catalyst was washed by injecting UV-Milli-Q, for at least
12 h, until the system blank was low and stable. The system blank was
\<8 µmol C l^-1^. The device was standardized with Potassium Hydrogen
Phthalate (KHP). The coefficient of variation (C.V.) of the peak area
for the 3-5 replicate analyses of each sample was \~1%. The accuracy of
our HTCO system has been tested within the international
intercalibration exercise conducted by J. Sharp (Univ. of Delaware),
with very satisfactory results (within 10%; J. Sharp, pers. com.).