Gningue, I.

Gérard, Robert

Morin, P.

Oudot, Claude

Horizon / Pleins textes

.. 
J 
i 
I 
t 
a 
, 
Llmnol. Oceanogr,. 33(1). 1988, 146-150 
(D 1988, by the American Society of Limnologyend Oceanography, Inc. 
Precise shipboard determination of dissolved oxygen 
(Winkler procedure) for productivity studies 
with a commercial system' 
Abstracf-An automatic system of dissolved 
oxygen titration based on a potentiometric end- 
point detector is described. The equipment is a 
simple commercial apparatus that can be used 
by a relatively unskilled operator. The C.V. of 
the oxygen measurement is 0.1%. The titration 
is quick (3-5 min) and suitable for field studies 
of phytoplankton production and related sub- 
jects. 
Historically, oxygen has been a widely 
studied gas (Kester 1975) because it is of 
great interest to physicists studying the cir- 
culation of water masses in the deep ocean 
as well as to biologists examining oxygen 
variations due to biological processes. Since 
the introduction of the Winkler technique 
in 1888, modifications have been made to 
improve the precision and to automate the 
original method (Carritt and Carpenter 
1966; Rual and Voituriez 1969; Williams 
and Jenkinson 1982). When electrochemi- 
cal devices appeared, they had the advan- 
tage of providing a simple, fast method of 
measuring dissolved oxygen. These oxygen 
sensors-called oxygen electrodes or oxygen 
probes (membrane-covered polarographic 
detectors)- became successful because they 
allow direct measurement of dissolved oxy- 
gen in the environment without altering it 
chemically and also allow continuous mon- 
itoring of oxygen variations in space and in 
time. For these reasons CTD systems are 
often being fitted with such oxygen detec- 
tors. Despite much effort to overcome the 
main failure of the detecting system-the 
lack of precision (the common YS1 oxy- 
meter has a C.V. of lolo) and accuracy (Cza- 
plewski and Parker 1973)-oxygen elec- 
trodes generally compare poorly with 
conventional Winkler titration (Atwood et 
al. 1977). Recently Langdon (1984) im- 
I 
proved the reproducibility and long-term 
stability of such a sensor, but the precision 
remains poorer than that given by the most 
sensitive Winkler titration procedures. 
The well-known Winklcr procedure is 
founded on quantitative oxidation o f  Mn(1l) 
into Mn(II1) in alkaline solution followed 
by oxidation of iodide by Mn(II1) in acid- 
ified solution. The released iodine is titrated 
with a standard solution of sodium thio- 
sulfate. 1mprovemc.nts aimed at reducing 
the errors of the Wickler method are the 
choice of reagent conc2ntrations (Carritt and 
Carpenter 1966; Rual and Voituriez 1969) 
that limit the loss of iodine by volatization 
and replacement of visual colorimetry with 
an electrochemical or optical detector 
(Broenkow and Cline 1969; Talling 1973; 
Hartwig and Michael 1978; Williams and 
Jenkinson 1982) to determine the end-point 
of iodine-thiosulfate titration. Contrary to 
Strickland and Parsons (1 968), the poten- 
tiometric, amperometric, or photometric 
detection of the end-point improves the pre- 
cision of titration. Above all, it allows au- 
tomation of the method, which means a gain 
of time and relief for the analyst subjected 
to an increasing number of routine ship- 
board measurements. Strickland and Par- 
sons (1968) estimated a C.V. of 0.5% with 
the classical starch end-point, while Wil- 
liams and Jenkinson (1 982) claimed a C.V. 
in the range 0.03-0.1°/o on a routine basis 
with a pbotometric system controlled by 
microprocessor. According to Williams and 
Jenkinson (1 982), photometric end-point 
detection seems to be the most precise: 
0.010/0 or better. With an unmodified com- 
mercial apparatus (Metrohm) based on a 
potentiometric end-point detector, we get a 
C.V. of the titration of 0.1%. It is not a 
sophisticated system and is precise enough 
to enable the estimation of photosynthetic 
was acquired from PIREN/ production Of Oxygen in tropical areas 
(ATP Cycle du Carbone, 1982). (Oudot 1984). -~ -_._ 
-... ...I Fonds Documentaire DRSTOM I t , I l > r t  
;..YI y -  - \  

Notes 
Fig. 1. The Metrohm titration system. Above left- 
digital 605 pH meter provided with a combined Pt 
electrode to measure the redox potential of the solution 
to be titrated; middle left-6 14 Impulsomat, to regu- 
late the titration; below-625 Dosigraph to record the 
titration curve; center-655 Multidosimat automatic 
burette with a digital display of the delivered thiosul- 
fate volume; right-649 magnetic stirrer to mix in thio- 
sulfate added to the iodine solution in the titration 
vessel. 
because the classical 14C method for esti- 
mating primary production is suspected of 
underestimating gross production (Shulen- 
berger and Reid 1981; Jenkins 1982), and 
high-precision oxygen determination is a 
powerful tool to estimate primary produc- 
tion (Oudot in prep.). 
Titration of the iodine solution is per- 
formed with a Metrohm titration system 
consisting of the five major components 
shown in Fig. 1. Seawater is sampled into 
a 125-ml dark bottle. One-half milliliter of 
manganese-sulfate reagent and 0.5 ml of al- 
kaline-iodine solution are added to the sam- 
ple, and the stopper of the bottle is replaced, 
taking care not to trap air under it. The 
bottle is shaken vigorously and then allowed 
to stand until the manganese hydroxide 
precipitate has settled (at least 2 h). Then 
the stopper of the bottle is removed and 0.5 
ml of concentrated sulfuric acid is added 
immediately. The bottle is restoppered with 
no air trapped inside and shaken so that all 
the precipitate dissolves. The iodine solu- 
tion is titrated by transferring 50 ml of the 
acidified sample with a Kimax automatic- 
zeroing pipette (precision of 0.0 1%) into the 
titration vessel, which is provided with a 
147 
600 11.74 
lo 11 21 31 
Thiosulfate Volume m l  
Fig. 2. Diagram ofthe output ofthe 625 Dosigraph: 
titration curve of iodine solution by thiosulfate solu- 
tion for determining the equivalence point (E.P.) of the 
titration. 
small magnetic stirring bar. The vessel is 
closed with a plastic cover that supports the 
Pt electrode and the pipette tip delivering 
the thiosulfate. The measuring electrode is 
a combined massive platinum electrode 
(P.N. 6.041 5.000) for which the reference 
system is Ag/AgC1/3 M KCI. Calibration of 
the thiosulfate solution is with a standard 
solution of potassium iodate freshly pre- 
pared in the laboratory and checked against 
a CSK standard solution prepared by the 
Sagami Chemical Research Center (Japan). 
The major component of the system is 
undoubtedly the 6 14 Impulsomat which 
regulates the addition of thiosulfate accord- 
ing to the difference between the potential 
of the solution measured by the 605 pH 
meter and the predetermined potential at 
the end of iodide-thiosulfate reaction. This 
latter potential is previously deduced from 
the recorder trace of the titration curve of 
an iodine solution by a thiosulfate solution 
(E.P. on Fig. 2). After a continuous initial 
addition of reagent, the amount depending 
on the quantity of iodine contained in the 
sample, the burette delivers automatic pulses 
of decreasing volume of thiosulfate as the 
end-point is approached. With a 5-ml (in- 
terchangeable) unit connected to the 655 
Multidosimat burette, the last additions of 
reagent volume are quantities of 1 1. When 
148 Notes 
- 
40 
C .- 
E ?  
E" 2 0  
.- 
I - -  
- 
O 
3.939 
3.938 
3.939 
3.937 
3.941 
O I' 21 31 4 1  51 
T h i o s u l f a t e  Volume ml  
Fig. 3. Diagram of the output of the 625 Dosigraph: titrations of five replicates of a seawater sample. 
the Impulsomat recognizes that the end- 
point is reached exactly, a pilot lamp lights. 
It indicates the end of the titration and al- 
lows reading of the final volume of added 
thiosulfate before automatic refilling of the 
burette for the next sample titration. It takes 
3-5 min for a single analysis-the exact time 
depending on thc initial oxygen concentra- 
tion of the sample. 
The 625 Dosigraph has two functions. One 
is to record the delivered thiosulfate volume 
as a function of time (Fig. 3) and to check 
the dynamics of the titration of an unknown 
sample (duration of the pulses and delay 
between the pulses) and so to judge the ap- 
proach to the end-point. The other function, 
the main one, is to trace the redox titration 
curve of iodine by thiosulfate (Fig. 2) in 
ordcr to dclcrminc tlic cquivalcnce point at 
which tlic potential will bc chosen as the 
rcl'crcncc valuc ol' 11ic 1 nipulsomat. I n  this 
latter mode (titration curve recording), the 
Impulsomat ensures a continuous scanning 
of the reference potential (a decrease in the 
present case) as a function of time for com- 
parison with the potential (measured by the 
pH meter) of an iodine solution similar to 
the one being titrated. First, the reference 
value is set as the measured value of the 
solution potential (initial value is 476 mV 
on Fig. 2). When the titration is started, the 
scanning reveals a gap between the reference 
potential and the measured potential that is 
converted into pulsed additions of thiosul- 
fate, constraining the measured potential to 
approach the reference potential. At the be- 
ginning of the titration (Fig. 2), the potential 
of the solution does not change very much 
for each addition of reagent, and the slope 
of the titration curve (potential as a function 
of reagent volume) is low. As the equiva- 
lence point nears, the reagent volume to be . 
added so that the measured value of poten- 
tial reaches the reference value becomes 
smaller and smaller; the slope oftthe curve 
becomes sharper and sharper, then the sign 
of the first derivative of the slope changes. 
This inflection point determines the value 
of the reference potential to be fixed on the 
Impulsomat as the end-point of titration. 
As to precision, it is important to distin- 
guish precision of titration of iodine by thio- 
sulfate solution from precision of the de- 
termination of the dissolved oxygen 
concentration in the sample. The former 
precision is estimated at the time the thio- 
sulfate solution is calibrated. In a flask filled 
with 250 ml of seawater, 1 ml of concen- 
trated sulfuric acid, 1 ml of alkaline iodide 
solution, and 1 ml of manganous sulfate 
solution are added, with thorough mixing. 
Finally 50 ml of this mixed solution is with- 
drawn into the titration vessel and a certain 
volume of the standard iodate solution is 
put in the vessel to liberate a known quan- 
tity of iodine. Table 1 shows the results of 
various tests of replicate titrations of iodine 
liberated by a similar volume of iodate so- 
h 
I 
'1 i. 
I 
,- 
, F  
' Notes 
.. 
149 
Table I. Replicate titrations of iodine liberated by 1-10 IL of standard potassium iodate solution. the 
concentrations of the solutions are: 0.1592 g KIO, liter-' and 2.77 g Na,S,O,, 5 H,O liter-); 10 ml of the 
standard KIO, solution liberate the same quantity of iodine as 50 ml of seawater containing about 5 ml O, 
liter-). 
KIO, volume (ml) 
I 2 3 4 
Na2S20, volume (ml) 0.409 0.815 1.230 1.639 
0.41 1 0.812 1.228 1.64 1 
0.4 13 0.810 1.227 1.64 I 
0.408 0.813 1.230 1.637 
0.409 0.812 1.225 1.639 
Mean (ml) 0.410 0.812 1.228 1.639 
SD (ml) 0.0020 0.00 I8 0.0021 0.00 1 7 
C.V. (Yo) 0.49 0.25 0.16 0.10 
5 10 
2.024 
2.027 
2.025 
2.022 
2.025 
2.025 
0.00 18 
0.09 
3.941 
3.937 
3.939 
3.938 
3.939 
3.939 
0.00 15 
0.04 
lution (varying from 1 to 10 ml). For com- 
parison, the iodine quantity liberated by 10 
ml of iodate solution is the same as is lib- 
erated by 50 ml of seawater containing about 
5 ml of dissolved oxygen per liter. The stan- 
dard deviation of the titration is relatively 
constant (O.Po2 ml) whatever the thiosulfate 
volume delivered by the burette. The C.V. 
of the iodine titration decreases as the io- 
date volume increases: for the range cor- 
responding to the most common dissolved 
oxygen concentrations in the upper layers 
of the tropical ocean (5-2 ml O2 liter-'), the 
C.V. is in the range 0.04-0.1%. 
The latter precision-the overall analyt- 
ical precision of the determination of dis- 
solved oxygen-is estimated by carrying out 
replicate analyses of single batches of sea- 
water sampled with a 5-liter Niskin bottle. 
Table 2 shows the results for five replicates 
of three seawater samples. The SD of the 
titration (0.004 ml) is higher than the pre- 
ceding one (0.002 ml) because it includes 
Table 2. Replicate determinations of dissolved 
oxygen concentrations (ml liter-)) on three different 
samples of seawater. 
Sample 
I 2 3 
1st replicate 3.223 2.526 3.193 
2nd replicate 3.227 2.519 3.185 
all the successive errors of the determina- 
tion of dissolved oxygen (sampling, for-í 
mation of the manganese hydroxyde prel 
cipitate, dissolution of this precipitate,' 
withdrawal of the iodine solution, and ti- 
tration of the iodine solution). We conclude 
that manipulation of the sample, as well as 
detection of the end-point, limits the pre- 
cision of the method. The C.V. is 0.10- 
0.12% for a dissolved oxygen concentration 
of about 3 ml literm1. 
Without presenting results of application 
of this sensitive and automatic procedure 
of oxygen deterniination (Oudot in prep.), 
we would mention only that the precision 
is good enough to allow study of diurnal 
changes in oxygen concentration in oligo- 
trophic waters. In conclusion, with the Me- 
trohm titration system, it is very easy to 
routinely make precise measurements of 
oxygen concentration by the Winkler tech- 
nique. 
Claude Oudot 
Robert Gerard 
Pascal Ilforin 
Institut Français de Recherche 
pour le Développement en 
Coopération (ORSTOM) 
B.P. 1386 
Dakar, Senegal 
3rd replicate 3.217 2.529 3.192 Ihf Gningtie 
4th replicate 3.224 2.526 3.191 
5th replicate 3.225 2.258 3.193 Centre de Recherches 
Mean 3.223 2.256 3.191 Océanograp h i ques 
SD 0.0038 0.0039 0.0033 B.P. 2241 
C.V. (Yo) o. 12 0.15 0.10 Dakar, Senegal 
150 Notes 
References 
ATWOOD, D. K., W. F. KINARD, M. J. BARCELONA,AND 
E. C. JOHNSON. 1977. Comparison of polaro- 
graphic electrode and Winkler titration determi- 
nations ofdissolved oxygen in oceanographicsam- 
ples. Deep-sea Res. 24: 3 1 1-3 13. 
UROENKOW, . M., AND J. D. CLINE. 1969. Colori- 
metric determinations of dissolved oxygen at low 
concentrations. Limnol. Oceanogr. 14: 450-454. 
CARRITT, D. E., AND J. H. CARPENTER. 1966. Com- 
parison and evaluation of currently employed 
modifications of the Winkler method for deter- 
mining dissolved oxygen in sea water. A NASCO 
report. J. Mar. Res. 24: 286-318. 
CZAPLEWSKI, R.L., AND M. PARKER. 1973. Use of a 
BOD oxygen probe for estimating primary pro- 
ductivity. Limnol. Oceanogr. 18: 152-154. 
HARTWIG, E. O., AND J. A. MICHAEL. 1978. A sen- 
sitive photoelectric Winkler titrator for respiration 
measurements. Environ. Sci. Teclinol. 12: 7 12- 
715. 
JENKINS, W. J. 1982. Oxygen utilization rates in the 
North Atlantic subtropical gyre and primary pro- 
duction in oligotrophic systems. Nature 300: 246- 
248. 
KESTER, D. R. 1975. Dissolved gases other than CO2, 
p. 498-556. I n  J. P. Riley and G. Skirrow [eds.], 
Chemical oceanography. Academic. 
LANGDON, C. 1984. Dissolved oxygen monitoring 
system using a pulsed electrode: Design, perfor- 
mance, and evaluation. Deep-sea Res. 31: 1357- 
1367. 
OUDOT, C. 1984. Echange physique d'oxygène et pro- 
duction primaire dans la couche homogène en zone 
équatoriale de l'Océan Atlantique est. Oceanogr. 
Trop. 19: 95-109. 
RUAL, P., AND B. VOITURIEZ. 1969. Modification et 
automatisation de la méthode de Winkler pour le 
dosage en mer de l'oxygène dissous. Initiations- 
Doc. Tech. ORSTOM 10. 27 p. 
SHULENBERGER, ., AND J. L. REID. I98 1. The Pacific 
shallow oxygen maximum, deep chlorophyll max- 
imum, and primary productivity reconsidered. 
Deep-sea Res. 28 901-919. 
STRICKLAND, J. D., AND T. R. PARSONS. 1968. A prac- 
tical handbook of seawater analysis. Bull. Fish. 
Res. Bd. Can. 167. 
TALLING, J. F. 1973. The application of some elec- 
trochemical methods to the measurement of pho- 
tosynthesis and respiration in fresh water. Fresh- 
water Bio1 3: 335-362. 
WILLIAMS, P.J. LEB., AND N. W. JENIUNSON. 1982. 
A transportable microprocessor-controlled precise 
Winkler titration suitable for field station and 
shipboard use. Limnol. Oceanogr. 27: 576-585. 
Submitted: 31 December 1986 
. , Accepted: I May 1987 
Revised: 2 November I987 
. 


Precise shipboard determination of dissolved oxygen (Winkler procedure) for productivity studies with a commercial system

Abstract

Similar works

Full text

Available Versions

Horizon / Pleins textes