Oxygen Saturation Index

Until now, we have discussed only oxygen-saturated water, i.e., water that has dissolved the maximum amount of oxygen that it theoretically can take up. In this water the same partial pressure of oxygen prevails as in the water-saturated air above the water surface.

However, it is possible that the oxygen concentration in water can be higher or lower than its saturation level. This means that the equilibrium of the partial pressure of oxygen between air and water no longer exists. Water, not in equilibrium, may be quite stable with regard to its oxygen concentration. The water will, of course, either lose oxygen to the air or gain oxygen from the air, but these diffusion processes are extremely slow.

In order to determine the saturation condition for oxygen of these waters, the so-called oxygen saturation index s can be applied. This oxygen saturation index specifies the percentage of the actual oxygen concentration in relation to the saturation value CO2:

where s is the oxygen saturation index, C is the actual oxygen concentration, and CO2 is the saturated oxygen concentration.

Example: What is the oxygen saturation index if water at 20°C and at an atmospheric pressure of 98 kPa contains 7.12 mg/l oxygen?

First, the saturated oxygen concentration at 98 kPa must be calculated. To do this reference the Saturated Oxygen Concentration Table below.

The value of oxygen at 20°C is 9.08 mg/l:

Now, the oxygen saturation index can be calculated:

Note that the same mass concentration of oxygen in water, but at different temperatures, will result in different saturation indexes. The previous example at 35°C would result in a saturation index of 106%.

Saturated Oxygen Concentration

α in °C
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0
14.64
14.60
14.55
14.51
14.47
14.43
14.39
14.35
14.31
14.27
1
14.23
14.19
14.15
14.10
14.06
14.03
13.99
13.95
13.91
13.87
2
13.83
13.79
13.75
13.71
13.68
13.64
13.60
13.56
13.52
13.49
3
13.45
13.41
13.38
13.34
13.30
13.27
13.23
13.20
13.16
13.12
4
13.09
13.05
13.02
12.98
12.95
12.92
12.88
12.85
12.81
12.78
5
12.75
12.71
12.68
12.65
12.61
12.58
12.55
12.52
12.48
12.45
6
12.42
12.39
12.36
12.32
12.29
12.26
12.23
12.20
12.17
12.14
7
12.11
12.08
12.05
12.02
11.99
11.96
11.93
11.90
11.87
11.84
8
11.81
11.78
11.75
11.72
11.69
11.67
11.64
11.61
11.58
11.55
9
11.53
11.50
11.47
11.44
11.42
11.39
11.36
11.33
11.31
11.28
10
11.25
11.23
11.20
11.18
11.15
11.12
11.10
11.07
11.05
11.02
11
10.99
10.97
10.94
10.92
10.89
10.87
10.84
10.82
10.79
10.77
12
10.75
10.72
10.70
10.67
10.65
10.63
10.60
10.58
10.55
10.53
13
10.51
10.48
10.46
10.44
10.41
10.39
10.37
10.35
10.32
10.30
14
10.28
10.26
10.23
10.21
10.19
10.17
10.15
10.12
10.10
10.08
15
10.06
10.04
10.02
9.99
9.97
9.95
9.93
9.91
9.89
9.87
16
9.85
9.83
9.81
9.70
9.76
9.74
9.72
9.70
9.68
9.66
17
9.64
9.62
9.60
9.58
9.56
9.54
9.53
9.51
9.49
9.47
18
9.45
9.43
9.41
9.39
9.37
9.35
9.33
9.31
9.30
9.28
19
9.26
9.24
9.22
9.20
9.19
9.17
9.15
9.13
9.11
9.09
20
9.08
9.06
9.04
9.02
9.01
8.99
8.97
8.95
8.94
8.92
21
8.90
8.88
8.87
8.85
8.83
8.82
8.80
8.78
8.76
8.75
22
8.73
8.71
8.70
8.68
8.66
8.65
8.63
8.62
8.60
8.58
23
8.57
8.55
8.53
8.52
8.50
8.49
8.47
8.46
8.44
8.42
24
8.41
8.39
8.38
8.36
8.35
8.33
8.32
8.30
8.28
8.27
25
8.25
8.24
8.22
8.21
8.19
8.18
8.16
8.15
8.14
8.12
26
8.11
8.09
8.08
8.06
8.05
8.03
8.02
8.00
7.99
7.98
27
7.96
7.95
7.93
7.92
7.90
7.89
7.88
7.86
7.85
7.83
28
7.82
7.81
7.79
7.78
7.77
7.75
7.74
7.73
7.71
7.70
29
7.69
7.67
7.66
7.65
7.63
7.62
7.61
7.59
7.58
7.57
30
7.55
7.54
7.53
7.51
7.50
7.49
7.48
7.46
7.45
7.44
31
7.42
7.41
7.40
7.39
7.37
7.36
7.35
7.34
7.32
7.31
32
7.30
7.29
7.28
7.26
7.25
7.24
7.23
7.21
7.20
7.19
33
7.18
7.17
7.15
7.14
7.13
7.12
7.11
7.09
7.08
7.07
34
7.06
7.05
7.04
7.02
7.01
7.00
6.99
6.98
6.97
6.96
35
6.94
6.93
6.92
6.91
6.90
6.89
6.88
6.87
6.85
6.84
36
6.83
6.82
6.81
6.80
6.79
6.78
6.77
6.75
6.74
6.73
37
6.72
6.71
6.70
6.69
6.68
6.67
6.66
6.65
6.64
6.63
38
6.61
6.60
6.59
6.58
6.57
6.56
6.55
6.54
6.53
6.52
39
6.51
6.50
6.49
6.48
6.47
6.46
6.45
6.44
6.43
6.42
40
6.41
6.40
6.39
6.38
6.37
6.36
6.35
6.34
6.33
6.32

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Next Article - Oxygen Measurement Principles

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