Nova-Tech 2700 Series Benchtop Meters - Manual (Page 43)

39
Solubility of oxygen in water contact with water saturated air
at standard atmospheric pressure
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30 35 40 45 50
Temperature °C
S
o
lubi
lit
y
m
g/
L
DO Solubility in Water vs. Temperature
19. Dissolved Oxygen Theory
Dissolved Oxygen (DO) refers to the volume of oxygen that is contained in water.
There are two main sources of DO in water; atmosphere and photosynthesis.
Waves and tumbling water mix air into the water where oxygen readily dissolves
until saturation occurs. Oxygen is also produced by aquatic plants and algae
during photosynthesis.
The amount of DO that can be held by water depends on 3 factors:
1. TEMPERATURE:
DO increases with decreasing temperature
(colder water holds more oxygen)
2. SALINITY:
DO increases with decreasing salinity
(freshwater holds more oxygen than saltwater does)
3. ATMOSPHERIC PRESSURE:
DO decreases with decreasing atmospheric pressure
(amount of DO absorbed in water decreases as altitude increases)
Measurement Units
One measure of DO in water is parts per million (ppm) which is the number of
oxygen molecules (O
2
) per million total molecules in a sample. The “mg/L” unit is
equivalent to ppm (a liter of water weighs 1 million milligrams—one part in a
million is similar to one milligram in a liter). Calculating the % saturation is