POTASSIUM PERIODATE SOLUTION

Dissolve 1.0 g of analytical reagent quality potassium periodate, KI0 4, in 500 ml of distilled water and add one small pellet (about 0.2 g) of sodium hydroxide. Store the solution in an amber glass bottle out of direct sunlight. The solution is stable for several weeks if stored in the dark, but slowly decomposes when exposed to strong light, forming undesirable oxidants. For the best results the solution should be made up fresh every few days.

3. 4,4"-BISDIMETHYLAMINOTRIPHENYLMETHANE SOLUTION

Dissolve 0.20 g of alcohol recrystallized leuco-malachite green in 250 ml of pure acetone. This solution is stable but should not be allowed to evaporate. The development of a slight green colouration does no harm.

11.13. MANGANESE 113

F. EXPERIMENTAL

P ROCEDURE

1. Add 30 ml of sample to a 50-ml stoppered graduated measuring cylinder.

For precise work (see Sect. H) fill another cylinder with 30 ml of the same sample and add 1.0 ml of manganese standard.

2. Measure in 15 ml of acetate buffer from a polyethylene wash bottle, making the total volume (as measured in the cylinder) to 45 ml (Note a). Place the cylin-ders in a thermostatically controlled water bath at a temperature between 23 and 26 C, held constant to ±1 C or better (Note b).

3. After the solutions have come to temperature (allow 15-30 mm) add 5.0 ml of periodate solution from an automatic pipette. Mix the solutions thoroughly and allow them to stand for a further 10-15 min (Note c).

4. Add, with rapid mixing, 1.0 ml of leuco-base solution from an automatic pipette and return solutions to the thermostatically controlled bath.

5. Between 4 and 5 hr after adding the leuco-base measure the extinction of a sample solution in a 2-cm cell (rarely a 1-cm cell may be required) against a "blank"

determination (see Sect. G), using light of wavelength 6150 A (Note d). If a filter-type absorptiometer is used choose a filter having a peak transmission including a wavelength of 6200 A. Correct for any cell-to-cell blark (Sect. G) and record extinction values (E) to the nearest 0.001. Unless adjacent samples are known to have extinction values within about 25% of each other the absorptiometer cell should be rinsed with each new solution before filling.

6. Calculate the manganese concentration in microgram-atoms of manganese per liter (ag-at Mn/liter) from the expression:

,ag-at Mn/liter -=

Es— E X 0 '05

where E is the extinction measured against a "blank" solution but corrected for any cell-to-cell blank (Sect. G) and Es the extinction of the sample to which 1.0 ml of manganese standard has been added. Of the numerous compounds which can interfere only iron is likely to have any significant effect in sea water. The extinction resulting from the presence of iron is about 1% of that obtained from the same molecular concentration of manganese. The greatest interference to be expected in most sea areas will therefore flot exceed about the equivalent of 0.005 pg-at Mn/liter and can generally be neglected. A small correction can be made if the particulate reactive iron value is known (see Part 11.12.1) by assuming that

1 pg-at Fe/liter -.---. 0.01 eg-at Mn/liter

NOTES

(a) Acetate is essential for the reaction, as well as providing a buffer. The amount of dye formed per unit of manganese is a sensitive function of pH, decreasing rapidly as the pH de-creases below about 4. At a pH greater than about 4.2 the amount of dye produced by a manganese-independent reaction (Sect. G) becomes undesirably high.

(b) The amount of dye produced per unit of manganese, also the rate of formation of the dye, depends upon temperature. A decrease in sensitivity of about 4% per 1 C decrease in

114 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS

temperature is to be expected. 25 C is a suitable temperature and fluctuations of less than -± 1 C during colour development may be neglected as they are largely allowed for by the procedure of internal standardization.

(c) The exact concentration of periodate is not critical but should be maintained to 5% or better. The reproducibility and sensitivity of colour development appears to improve somewhat if the periodate reacts for a few minutes with the manganese before the dye-base is added. Ten minutes is a generous excess time and is not critical.

(d) The time for full colour development increases with the salinity of a sample. For salinities of 25%, or less, 2 hr is sufficient but a period of at least 4 hr should be allowed to ensure completion with samples of salinity exceeding 33%,. The manganese-independent reaction affecting the blank (Sect. G) increases with time rather rapidly for the first 1 or 2 hr, but the rate of change is less marked at 4-5 hr and the blank is then less erratic and less dependent on salinity.

G. DETERMINATION OF BLANK

1. REAGENT BLANKS

a. Manganese-free Sea Water

To

about

1000 ml

of HA

Millipore-filtered sea water

of

approximately known salinity

(an open

ocean

source of

high salinity is preferable) add

about

5% w/v

sodium

hydroxide

solution

dropwise with vigorous stirring until

a

slight

permanent

precipitate is formed. Bring

the solution

to

the

boil

and

boil

for about

5 min. Allow

the

precipitate to

stand,

with occasional stirring,

as

it cools to room temperature (2-3 hr)

in a

well-covered beaker. When

the

precipitate has settled decant

the

bulk

of the

liquid through

a

12.5-cm No. 42 Whatman

or

similar fine-grain filter paper

but do

not wash. Add approximately 2% v/v hydrochloric acid to

the

filtrate dropwise until

the pH of the

filtered sea water is anywhere between 7.8

and

8.2.

For

general

use

(see below) dilute

the solution

until

the

salinity is judged to be

about

29700

and store

it

in a

well-stoppered polyethylene bottle.

The

resulting sample

of

sea water should be completely

free

from manganese

and

iron impurities with the loss

of

only

a

small

fraction of

its magnesium

and calcium content.

b. Procedure

If

good quality distilled water is used,

and the

acetate buffer is

made

from acetic acid

and sodium hydroxide,

rather than

sodium

acetate,

the

amount

of

manganese introduced by reagents should be very small

and

is unlikely to exceed

the

equivalent

of about

0.005 p.g-at/liter

on a

30-ml sample. There

is,

however,

a slow formation of

dye-stuff by

an

oxidation reaction not resulting from

the

presence

of

manganese.

The rate of

this reaction depends upon

variables

such

as pH and

temperature

but

is relatively independent of salinity.

For

any given batch

of

samples the manganese-independent reaction is reproducible

and a

"blank" determination with

"manganese-free"

sea water

of a

salinity

of

abelut 29%0 can be used to control

a

batch

of

samples having salinities

in the range of

25-35%0 .

For the

most precise

-work with samples

of

salinity below

about

25%,

or

with

a

very low manganese

content, the manganese-free

water should be prepared having

a

salinity within 2-3%0

of the

sample being analysed.

The

manganese-independent blank

extinction

increases appreciably

as

salinity decreases, being

about

1.5 times

as

great

in pure

water

as

it is

in

sea water

of

salinity 35700.

For

each batch

of

samples prepare

one or more

blank

solutions

(as necessary)

II.13. MANGANESE 115 by taking 30 ml of manganese-free sea water of suitable salinity through the method exactly as described in Section F, paragraphs 1-4. To prevent complications due to slight colour increases whilst measuring a large batch of samples, all samples should be measured against the blank solution rather than against distilled water. (The extinction of a blank against water in a 2-cm cell should not exceed about 0.25.) The reaction giving the manganese-independent blank may arise from periodate directly or from impurities in the periodate. Exposure of neutral periodate solutions to strdng light greatly increases the amount of dye-stuff formed by this reaction.

2. CELL-TO-CELL BLANKS

Refer, for example, to Part I1.2.I, Section G.1.

3. GENERAL PRECAUTIONS AND THE CLEANING OF POLYETHYLENE- AND GLASS-WARE

The precautions outlined in Part II.11.I, Section F.2, apply to the present method, if perhaps slightly less critically, for any iron contamination will appear as manganese if sufficiently large. All glassware, etc. should be cleaned in 70%

v/v hydrochloric acid. Be very careful that there is no cross-contamination from the manganous sulphate in Part 1.3. This can occur, for example, if BOD bottles, containing traces of oxygen reagents, are used first in a surface sample bucket before samples are drawn for manganese analysis.

H. CALIBRATION

1. STANDARD MANGANESE SOLUTION

Dissolve 0.255 g of manganese sulphate monohydrate, MnSO4 - H2O, in 1000 ml of distilled water containing one or two drops of concentrated hydrochloric acid.

This solution is stable indefinitely in the absence of evaporation.

1 ml - 1.50 µg-at Mn

Dilute for immediate use 1.0 ml of the above solution to 1000 ml with distilled water. This solution should not be kept for more than about 2 days.

1 ml - 0.00150 µg-at Mn = 0.05 µg-at Mn/liter on a 30-ml sample

2. PROCEDURE

The amount of malachite green formed from a given manganese content is a function of salinity (increasing about 40% as the salinity decreases from 35%o to 20%o and being 60% higher in distilled water). There are also slight variations in sensitivity from batch to batch of samples for reasons not fully understood. For the most precise work, therefore, each sample should be analysed in duplicate, adding 1.0 ml of dilute manganese standard to one aliquot and calculating the final manga-nese concentration as described in Section F, paragraph 6. With a large batch of samples of water having practically the same salinity and from the same location, e.g. the deep samples from a single hydrographic station, only one sample need be chosen for this standardization procedure. The increase in extinction to be expected from the 0.05 µg-at Mn/liter added for standardization purposes varies between about 0.3 and 0.6 with a 2-cm cell.

11.14. DETERMINATION OF COPPER INTRODUCTION

The diethyldithiocarbamate reagent for copper is practically specific for this metal in sea water and a simple extraction procedure, using small volumes of carbon tetrachloride and long-path-length absorptiometer cells, gives an adequate sensitivity.

Chow and Thompson (J. Marine Res., 11: 124 1952) have suggested a similar direct approach, but, mainly because of optical considerations, their method is con-siderably less sensitive than the one which follows. Both this method and the methods for manganese and soluble iron described previously determine only the non-complexed or weakly-non-complexed forms of the metal ions. A significant fraction of the "soluble" copper in the sea may be strongly bound to organic matter and not react with diethyldithiocarbamate (below).

The present method concludes this section on micronutrients without mention of several metals of significance to plant physiology, in particular molybdenum and zinc. However, there is as yet little requirement for an analysis of sea water for such elements on a routine basis. Any such work would probably form part of detailed research on specific aspects of nutrition and is out of place in the present compilation.

A. CAPABILITIES

METHOD

Range: 0.006-0.2 µg-at/liter 1. PRECISION AT THE 0.05 µG-AT/LITER LEVEL The correct value lies in the range:

Mean of n determinations -!-0.007/ni µg-at/liter.

2. LIMIT OF DETECTION

The smallest quantity of copper that can be detected with certainty by a single determination is about 0.006 µg-at/liter.

B. OUTLINE OF METHOD

The sea water is treated with sodium diethyldithiocarbamate and the yellow copper complex extracted by a small volume of carbon tetrachloride. The extinction of a 15-ml extract is measured in a 10-cm cell.

C. SPECIAL APPARATUS AND EQUIPMENT

A pear-shaped separatory funnel of 1-liter capacity with the stem cut to about 1-inch length and the end tapered somewhat. Only one funnel is required per analyst as it is inconvenient to undertake more than one analysis at a time. Before use this funnel and all other glass apparatus should be given a rinse with 50% v/v hydro-chloric acid, followed by copious quantities of distilled water, to remove any copper, iron or other metal impurities on the glass surfaces.

"Small volume" spectrophotometer cells having a path length of 10 cm but holding 10 ml or less of solution.

117

118 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS D. SAMPLING PROCEDURE AND SAMPLE STORAGE

It is mandatory that samples for copper analysis be taken with all-plastic equipment. The use of a sampling bottle with bare brass, monel metal, or other copper alloys appearing anywhere at internal or external surfaces, will invalidate results. Stainless-steel straps and clips probably give no trouble. Samples should be transferred to polyethylene bottles for storage. We have no experience concerning the stability of samples in storage but, presumably, if they are frozen to —20 C and stored in the dark there will be no deterioration. Samples should not be filtered before analysis unless a "soluble" copper result is specifically required.

E. SPECIAL REAGENTS

Note: The water used in this method is best "de-ionized" by passing ordinary distilled water through a small column of cation exchange resin (in the hydrogen form) before use.

1. CARBON TETRACHLORIDE

The analytical reagent grade chemical may contain a trace of copper and should be redistilled from all-glass equipment before use. Store the purified solvent in a dark glass bottle.

2. SODIUM DIETHYLDITHIOCARBAMATE SOLUTION (DEDTC REAGENT)

Im Dokument A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS (Seite 124-130)