SAMPLING PROCEDURE AND SAMPLE STORAGE

Samples

must

be warmed to laboratory temperature before measurement

and

this should occur with

a minimum

exchange

of

atmospheric

and

dissolved carbon dioxide.

In no

circumstances

must the

delay

in

measuring

the pH

exceed about 2 hr.

The

sample should be taken from reversing bottles immediately after

the

oxygen sample by filling

a

50-ml wide-mouth (27 mm

or more in

diameter) polyethylene bottle to

the

brim

and

closing it at

once

with

a

tight-fitting screw

cap. Store in the

dark at low temperature until just before

the

analysis is commenced.

If

thin

probe-type

electrodes

are

available, which may be inserted

in an

opening

29

30 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS

of

12-15 mm diam,

the

samples may be collected

in a

30-

or

^50-ml polyethylene narrow-mouth screw-capped bottles, filled to

the brim.

E. SPECIAL REAGENTS REQUIRED STANDARD BUFFER

0.025

M

KH2PO4 + 0.025 tvt Na,HPO,

(pH

6.87 at 20-25 C).

Dissolve

34.0

g of potassium

dihydrogen

phosphate

(monobasic), KH 2PO4

and

35.5

g of

anhydrous disodium hydrogen

phosphate

(dibasic), Na21-1PO4,

in

distilled water

and

make

the volume

to 1000 ml

in a

measuring flask.

Store in a

tightly stoppered glass bottle.

Dilute 100 ml of this solution to 1000 ml with distilled water for use.

The

dilute

solution

should be stored

in

polyethylene. It is

stable for a

few weeks

if

evaporation is prevented

and

is best preserved by

a

few

drops of chloroform.

It should be discarded

if

bacterial growth become

apparent.

F. EXPERIMENTAL

Measure

the

temperature

of the

buffer

solution

(see

Sect.

E) to

the

nearest 1 C, adjusting

it, if

necessary, so that

the

temperature

lies

between 20

and

25

C.

Set the

temperature compensator

of the pH

meter to this temperature

and the pH

to read 6.87, then standardize

the instrument

according to

the

maker's

instructions,

allowing 5

min for the

electrodes to reach equilibrium.

The pH of the

buffer is

constant in the

temperature

range

20-25 C

and a

standardization at

pH

6.87 will be

effective for the pH range

7-9. With

a

satisfactory meter

and

electrodes

the

standardization need be repeated only

once a

day.

Place the

samples

in a

thermostatically controlled water

bath

until they attain

a

temperature within ±- 3 C

of the

temperature at which

the pH

meter was standardized.

(If the

temperatures

of

samples

and

buffer differ by

more

than 3 C, errors become appreciable

as the

temperature compensator

on the pH

meter does not effect

a

complete temperature

correction.)

If

the electrodes

have

just been immersed

in

buffer they should be washed with water

and

gently dried with

a

little paper

tissue,

otherwise

the

electrodes

are

trans-ferred from

one solution

to another without rinsing

or

wiping. Remove

the cap

from

the

sample bottle

and

immerse

the

glass

and calomel

electrodes well down into

the

bottle. Measure

the pH and

temperature

of the

sample after between 3

and

5

min immersion,

adjusting

the

temperature compensator

of the

meter to

the solution

temperature just before

a final

reading is taken.

The solution

should be gently swirled

once or

twice to

ass:st the

electrodes

in

reaching equilibrium.

Note the pH and the

temperature of

the solution,

pHm

and

t., respectively.

If the

sample was initially at

a

temperature

of

t C when taken at

a

depth d meters, its

correct pH,

in situ, is given by

the expression:

pHs = pH. — a (t — t.)

neglecting

pressure

effects.

The latter are

rarely significant

but the pH

at depth d meters, prld, is related to

the pH

at

the surface,

pHs, by

the expression:

1.4.1. EXPERIMENTAL QUANTITIES 31

pHd = pH,, - Rd

Values for the coefficients a and ,l3 are given in Part VI, Tables III and IV. The temperature correction should always be applied before making the calculations described later. The pressure variation is negligible for samples taken above about 500 m.

2. TOTAL ALKALINITY DETERMINATION A. CAPABILITIES

Range: 0.5-2.8 milliequivalents per liter PRECISION

The correct value lies in the range:

Mean of n determinations -!-0.022/nk milliequivalents total alkalinity per liter.

Reject duplicate determinations if the pH values after acidification (see Sect. F and Note 2, Sect. G) differ by more than about 0.03.

B. OUTLINE OF METHOD

A portion (100 ml) of the seawater sample is mixed with 25 ml of exactly 0.0100 N hydrochloric acid. The pH of the resulting solution is measured. The standard acid in excess of that required to titrate the sample to the carbon dioxide inflection point is computed from a knowledge of this pH and an empirical factor.

This excess acid is then subtracted from 2.500 milliequivalents per liter (the amount initially added by 25 ml of 0.01 N acid) and the total alkalinity of the sample is thus evaluated.

C. SPECIAL APPARATUS AND EQUIPMENT

A Beckman Model G pH meter, or an instrument of similar specification, should be employed with "wide-range" (- 5 to 80 C, pH 0-11) glass electrodes and saturated calomel half-cell electrodes. Automatic reading instruments are only acceptable if used with the greatest possible precision (.±:0.025 unit or better).

Also needed are 200-ml wide-mouth screw-capped polyethylene bottles, one for each sample.

D. SAMPLING PROCEDURE AND SAMPLE STORAGE

No great problems are encountered in sampling and storage. The samples are best stored in plastic or in glass bottles that have been soaked for several days in 1%

hydrochloric acid and then rinsed thoroughly in distilled water before they are dried.

Evaporation must be prevented by tightly stoppering the containers and it is best to carry out the determinations on the same day as the samples are taken by pipetting the seawater sample directly into standard acid (see Sect. F). However, careful storage of the samples for several weeks, as for salinity (see Part I.1), is permissible.

E. SPECIAL REAGENTS REQUIRED

1. STANDARD 0.01000 N HYDROCHLORIC ACID

Prepare by standardizing hydrochloric acid of a slightly greater concentration

32 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS

and then adjusting the volume by the addition of a calculated amount of distilled water to bring the normality to precisely 0.1000 N. Dilute this solution exactly ten-fold, by a pipette and measuring flask, before use.

2. STANDARD BUFFER

0.05 M potassium hydrogen phthalate (pH 4.00 at 20-25 C).

Dissolve 10.21 g of analytical reagent quality (primary buffer-standard speci-fication) potassium hydrogen phthalate, KHC 81-1404, in distilled water and make the volume to 1000 ml in a measuring flask. Store in a glass bottle. The solution is stable almost indefinitely in the absence of evaporation. The formation of a slight turbidity introduces no error.

F. EXPERIMENTAL

Pipette 25.00 ml of standard 0.01000 N hydrochloric acid into a dry 200-ml polyethylene wide-mouth screw-cap bottle. Add from a pipette 100.0 ml of seawater sample. Stopper the bottle and mix the solutions thoroughly. Warm the solution to room temperature and measure the pH exactly as described in the preceding method, except that the phthalate buffer is used to standardize the pH meter, instead of the phosphate buffer. The pH of the phthalate buffer between 20 and 25 C is taken as 4.00.

G. CALCULATIONS

1. Find the value of a, corresponding to the measured pH value from Table V.

The pH must lie between 2.8 and 4.0. (As the pH is measured in the range 20-25 C no temperature correction is necessary.)

2. Find the value of f from Table VI, according to the salinity and pH value.

3. Calculate the total alkalinity from the equation Total alkalinity = 2.500 — (1250 a,/f)

Note 1: For chlorinity values between 12 and 18%, (S = 22-33'4) and final pH values between 3.0 and 3.9 the total alkalinity may be read directly from Table VII without incurring appreciable errors.

Note 2: If the final pH is greater than 4.0 remove the electrodes, but do not wash them, and pipette 5.00 ml of 0.01000N acid into the bottle. Mix the solution and again measure the pH. Calculate

au

^{and f} as above and substitute in the equation:

Total alkalinity = 3.000 — (1300 aH/f)

This procedure will be necessary with most ocean waters of S greater than 33% 0.

I.4 .II. CALCULATIONS AND USE OF TABLE S

Note : Most of the data in Tables IX and X are rounded off to two significant figures and are reported only for the comparatively large pH intervals of 0 .1 pH unit. For use these tables should be made the basis of graphical interpolations .

1 . CALCULATION OF THE CARBONATE ALKALINITY

Required: Total alkalinity, chlorinity or salinity, initial temperature, pH, and the temperature at which the pH was measured .

a . Calculate the initial pH of the water from Table III.

b . Look up the quantity A (milliequivalents per liter) in Table VIII .

Carbonate alkalinity = total alkalinity - A (milliequivalents/liter) . 2 . CALCULATION OF TOTAL CARBON DIOXIDE CONTENT (ALL FORMS )

Required: Total alkalinity, chlorinity or salinity, initial temperature, pH, and the temperature at which the pH was measured .

a . Calculate the initial pH of the water from Table III . b . Calculate the carbonate alkalinity as in 1 above . c . Look up the factor F, in Table IX .

Total CO, content = carbonate alkalinity x Fz, (millimoles/liter) .

3 . CALCULATION OF THE PARTIAL PRESSURE AND CONCENTRATION OF CARBON DIOXID E

Note : In the equilibrium

CO2+11 ZO ~ H2CO3

for sea water, only 1% or less of the total carbon dioxide is in the form of carbdnic acid . This equilibrium is attained relatively slowly in sea water . In living tissue the enzyme carbonic anhydrase greatly accelerates the combination of water and carbon dioxide gas .

Required : Carbonate alkalinity (see 1 above), chlorinity or salinity, initial temperature, pH, and the temperature at which the pH was measured.

a . Calculate the initial pH of the water from Table III . b . Look up the factor Fp in Table X .

c. Calculate the partial pressure of carbon dioxide, (Pco

2), from the express-ion :

Pco2 - carbonate alkalinity x F,, (atmospheres) .

d . Look up the solubility of carbon dioxide in unbuffered sea water of the appropriate salinity and temperature in Table XI . If y is this value in millimoles per liter, the following holds .

Concentration of dissolved CO2 = Pco, x y(millimoles/liter) .

4 . CALCULATION OF BICARBONATE ION CONCENTRATION {HCO3] '

Carbonate alkalinity, chlorinity or salinity, initial temperature, pH, and the temperature at which the pH was measured, the dissociation constant K, where K is defined as

aH X [CO3]

34 A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS

where a, is measured by a glass electrode. Values for K as a function of temperature (in situ) and chlorinity (or salinity) are collected in Table XII.

a. Calculate the initial pH of the water from Table III.

b. Look up the corresponding a, value from Table V.

Bicarbonate concentration:

[HCO]i — = carbonate alkalinity x r

as millimoles/liter.

L a,

+

2K )

5. CALCULATION OF CARBONATE ION CONCENTRATION [CO3^{] 2—}

Reqtared: Carbonate alkalinity, chlorinity or salinity, initial temperature, pH, and the temperature at which the pH was measured, the dissociation constant K.

a. Calculate the initial pH of the water from Table III.

b. Look up the corresponding a, value in Table V.

Carbonate concentration:

[C00 2— = carbonate alkalinity X Ir K

aH + 2K as millimoles/liter.

) If the pH exceeds 8.1:

[CO3] 2— = total carbon dioxide content —

where the total carbon dioxide is evaluated as in calculation 2 above.

1.5. DIRECT DETERMINATION OF TOTAL CARBON DIOXIDE

INTRODUCTION

This procedure is a simple modification of the Van

Slyke

gas analysis method, adapted for use with a

5-ml

sample. Corrections have been evaluated assuming that

3.7%

of the carbon dioxide

redissolves

during the compression of stripped gas to a volume of

0.5 ml,

that the volume of carbon dioxide remaining in sea water under reduced pressure is as given by

Buch (see

Harvey's

The Chemistry and Fertility of Sea Water,

Cambridge University Press,

1957),

that a coefficient of expansion of

1.00384

per

C

allows for the change of both gas and mercury volumes with temperature and that the molar value of carbon dioxide is

^22.265

liters. The method is particularly suited for the measurement of the relatively small changes in the total carbon dioxide content of a seawater sample that occur as a result of biological processes. Any slight absolute error in the determination is then of minor significance.

METHOD

A. CAPABILITIES

The coefficient of variation for determinations in sea water of about

³⁰⁷⁰⁰

Im Dokument A PRACTICAL HANDBOOK OF SEAWATER ANALYSIS (Seite 41-47)