Salinity Corrections for Dissolved Oxygen Measurements Barbara S. Pijanowski
National Oceanographic Instrumentation Center, National Oceanic and Atmospheric Administration, Washington, D.C. 20852
Presented is a convenient method of compensating for the effects of salinity on the data acquired from dissolved oxygen meters. It is pointed out that the basic design of most of these instruments does not include provision for salinity compensation or, a t best, uses a fixed value. The need for a general method of compensation is emphasized, particularly when such meters are to be used in estuarine studies. The correction technique is based on earlier work by Gilbert et al. and is presented in a form suitable for development by computer for specific instruments. Illustrations demonstrating the methodology include curves for a meter intended basically for use in freshwater as well as for another which employs a fixed value of salinity compensation. The correction factors are dimensionless and can be applied to correct dissolved oxygen values in units of either ppm or ml/l. Dissolved oxygen is one of the few water quality parameters that can be measured automatically and in situ. The most common type of sensor is the Clarke-type polarographic electrode combination. This sensor consists of two electrodes with a potential between them of about 0.8 Vdc, surrounded by an electrolyte and separated from the medium being measured by a semipermeable membrane. Oxygen diffuses through the membrane and is reduced a t the cathode causing a current to flow which is proportional to the amount of dissolved oxygen present. Dissolved oxygen is usually reported in units of concentration such as ppm or ml O 4 m l Hp0.Membrane sensors however, respond to the partial pressure of oxygen in solution rather than to concentration, and partial pressure is related to concentrat ion by temperature and density, or salinity if seawater is the solution being measured. Most commercially available dissolved oxygen instruments employ automatic temperature compensation so that the effect of temperature 1s automatically accounted for when the output of the instrument is obtained. Very few instruments have such compensation, either automatic or manual, for the effect of salinity. If the dissolved oxygen sensor is used in freshwater this i s of little concern; however, if it is used in the ocean or estuaries, sizable errors will result by ignoring salinity corrections. In an effort to simplify salinity corrections, correction factors were derived from the work of Gilbert et al. (1967) on saturation values of dissolved oxygen in seawater as functions of temperature and salinity.
SI = salinity a t which the dissolved oxygen instrument is calibrated S:! = salinity of sample being measured D , = dissolved oxygen value as measured by a temperature compensated instrument a t temperature T, calibrated a t salinity SI D , = corrected dissolved oxygen value for a salinity Sp a t the same temperature, 7' DO1 = amount of dissolved oxygen in a saturated solution a t SI, T DO2 = amount of dissolved oxygen in a saturated solution a t Sz, T
If
DO1 and DO2 are calculated from the relationship presented by Gilbert et al. (1967): n
DOl
m-I
A,,Cll'T'
= 1-0 1-0
DO,
n
m-1
i=O
J-0
A,,CL,'TI
=
where
Sl
C1l
=
CLr =
1.8065 s*
1.8065
The 55 A , , constants for n = m = 9 have been tabulated (Gilbert et al., 1967).
I
5
10
15 20 TEMPERATURE I T
25
Figure 1. Correction factor as function
30
35
of temperature
Volume 7 , Number 10, October 1973
957
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130 CALLRRATION SALINITY = 3 4 3ppt
125
I20
Q
P
115
u L 4 2110
105
100
0 95
090
5 TEMPERATURE IT1
ENVIRONMENTAL SALINITY
Figure 2.
Correction factor as function of salinity
Figure 3. Correction factor as function of
temperature
In most cases, S1 = 0 because the instrument is calibrated for fresh water use. If the temperature and salinity of the sample being measured are known, the P i factor can be determined from Figure 1 or 2 or from the following equation obtained by a repetitive regression analysis of DOz/DOI f o r SI = 0.
--
A = 0.9987 B = 0.1455 C = 1.255 X In some cases the instrument may be calibrated for S1 equal to a salinity other than zero. The Beckman MINOS DOM for example, is calibrated for a salinity of 34.3Oh. Similar Pij curves have been generated for this case and are shown in Figures 3 and 4. In general, a set of curves or tables can be generated for any calibration salinity using the above relationships. Because the factor is dimensionless, it can be applied to correct dissolved oxygen values in units of either ppm or ml/L
0 95.
I
I
-
Acknowledgment
The author wishes to acknowledge the assistance of John A. Pijanowski, Engineering Development Laboratory, National Oceanic and Atmospheric Administration, Rockville, Md., in conducting the computer analysis by which the curves and Pij factors presented herein were generated. 8
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Environmental Science & Technology
LiteratureC i t e d Gilbert, W., Pawley, W., Park K., Oceanograph. Soc. Jup., 23, ( 5 ) 252-5 (1967), Received for reuiew April 24, 1973. Accepted July 27, 1973
