A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS

silicate

(less than about

12 Acg-at Si/liter), the factor F („ c. )

may be assumed to be equal to

0.1 x

Note: The factor F is a function of the salinity of seawater samples. Between salinities 25 and 35%, the variation is less than 3% and may be neglected. The factor F, at a salinity S%0 is related to the factor, F, obtained as described above, by the approximate formula:

F ___ F x (1 4- 0.003 S) , 1.08

This correction should be used for the most precise work when the salinity varies by more than about 10%,, from a value of 28%,. The factor for pure water (zero salinity) is thus some 8%

less than the value obtained by the present method using synthetic sea water.

11.6. DETERMINATION OF REACTIVE NITRATE

INTRODUCTION

Since publication

of

the first edition

of

this manual several new procedures

have

been proposed

for the

determination

of nitrate in

sea

water. The

method based

on the

work

of

Mullin

and

Riley (Anal. Chim. Acta, 12: 464, 1955), previously described

in the

first edition has proven moderately satisfactory

but a

lengthy re-duction period

and

sensitivity to

motion are

serious disadvantages.

The latter

we now bclieve is caused by

the

presence

of air

above

the solution

during reduction

and

can be minimized by carrying

out the

reduction

in a

small closed bottle

of

some 60-ml capacity rather than

in the

recommended 125-ml Erlenmeyer flasks.

The

method

of

Chow

and Johnson

(Anal. Chim. Acta, 27: 441, 1962) based

on a zinc

dust reduction, has

the

disadvantage that it involves

a

magnetically stirred

solution in an ice-bath

followed by

filtration. This

is neither cheap nor convenient when handling many samples at

sea. The

rapid

and

elegant procedure described by

Armstrong

(Anal. Chem., 35: 1292, 1963) lacks sensitivity, can be troublesome

in waters of

high "humic

acid" content and

involves unpleasant

concentrations of

sulphuric

acid. This

method, however, is ideal

for

determinations

on culture solutions

containing high

concentrations of nitrate,

especially when only small

volumes of solution are

available.

The

following procedure is based

on a

method by

Morris and

Riley (Anal.

Chim. Acta, 29: 272, 1963) with some modifications.

At

the

suggestion of

Grasshoff (Kiel. Meeresforsch., 20: 5, 1964) we

use ammonium

chloride.

A

cadmium-mercury column has been replaced by

a

cadmium-copper column based

on

^the,work

of Wood, Armstrong and Richards

(J. Marine biol. Assoc. U.K., 47: 25, 1967), although we

have

had

trouble

with

the use of

EDTA suggested by these workers

and have

reverted to

ammonium

chloride

as an

activator. Reduction

of nitrate

nitrite

is nearly complete

and the

method described below is probably

as sensitive as

is practicable by

a routine

spectrophotometric procedure.

METHOD

A.

CAPABILITIES

Range: 0.05-45 pg-at/liter 1. PRECISION

AT THE

20 pG-AT/LITER LEVEL

The correct value lies in the range:

Mean

of

n determinations -±0.50/n1 14-at/1iter (using 1-cm cells).

2. PRECISION

AT THE

1 ittG-AT/LITER LEVEL

The correct value lies in the range:

Mean

of

n determinations -±-0.05/re itg-at/liter (using 10-cm cells) . 3. LIMIT

OF

DETECTION

The

smallest amount

of nitrate

nitrogen that can be detected with certainty is

about

0.05 ,ag-at/liter using 10-cm cells.

72 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS B. OUTLINE OF METHOD

The nitrate in sea water is reduced almost quantitatively to nitrite when a sample is run through a column containing cadmium filings loosely coated with metallic copper. The nitrite thus produced is determined by diazotizing with sulphanilamide and coupling with N-(1-napthyl)-ethylenediamine to form a highly coloured azo dye the extinction of which is measured. A correction may be made for any nitrite initially present in the sample.

C. SPECIAL APPARATUS AND EQUIPMENT

The reduction columns may be conveniently prepared by joining end-to-end three pieces of glass tubing, viz: 10 cm of 5-cm internal diameter tube on to 30 cm of tubing 10 mm in internal diameter (which is to contain the metal filings, see Sect. E below) which in turn is joined to 35 cm of a tube 2 mm in diameter. The last tube is bent just below this join into a U so that it runs up parallel to the 10-mm diam tube and then its end is bent over to form an inverted U syphon. This last bend should be just level with the top of the 10-mm diam tube when the assembly is held upright to form a column. With this arrangement, liquid placed in the top reservoir tube should flow out of the system and stop when the level of liquid is such that it just covers the metal filings (see below). Place the reduction columns inside large glass or plastic cylinders (for protection) and fix to the outside of these a small cylinder of glass, drawn to a tube at one end and closed by a rubber tube and pinchclip. This cylinder should hold about 75 ml and be arranged under the exits of the reduction columns to collect effiuents. Mark the cylinder at 40 and at 50 ml.

50-ml graduated cylinders.

125-ml Erlenmeyer flasks. These should be kept grease-free so that the minimum amount of liquid is retained when the flasks are drained dry.

D. SAMPLING PROCEDURE AND SAMPLE STORAGE

Sample (100-!-2 ml) should be measured from sea-sampling bottles by a 100-ml measuring cylinder and placed in a 125-ml Erlenmeyer flask. Samples are stable for several hours cold and in the dark but the analysis should not be delayed for more than about 12 hr. If greater delays are unavoidable the samples should be frozen to -20 C in a deep-freezer where no detectable changes should occur for many weeks. Unless the number of samples is large it may be desirable to store samples at sea in this fashion and return them to a shore-base laboratory for analysis.

E. SPECIAL REAGENTS

1. CONCENTRATED AMMONIUM CHLORIDE SOLUTION

Dissolve 125 g of analytical reagent quality ammonium chloride in 500 ml of distilled water and store in a glass or plastic bottle.

2. DILUTE AMMONIUM CHLORIDE SOLUTION

Dilute 50 ml of concentrated ammonium chloride solution to 2000 ml with distilled water. Store the solution in a glass or plastic bottle.

II.6. REACTIVE NITRATE 73

Im Dokument A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS (Seite 82-85)