Mon Sep 01 11:04:13 1997

Are ml/L and mg/L Dissolved oxygen one in the same?

Basically the title says it all. If I have 10 ml/L of dissolved oxygen, does this equal 10 mg/L? Thanks Adrian

adesbara@atcon.com

No - 10mL dissolved O2 /L is about 13.3 mg dissolved O2 /L. Here's an explanation:

The concentration unit ml/L is equivalent to parts per thousand; i.e. for every 1 mL of O2, there are 1000mL = 1000mg of water. So 10 ml/L = 10 ppt dissolved O2.

But the volume and mass of gases are not equivalent. They are related by the gas laws of Charles and Boyle, which are combined in the Ideal Gas Law:

P*V = n*R*T

where

P
= pressure in atm (1 atm = 1000mb = 101.3 kPa)
V
= volume in liters
n
= moles of gas
R
= gas constant (for these units, =0.0821 L*atm/mol*K
T
= temperature in Kelvins ( T(K) = T(degC) + 273K )
Then the mass of 10 ml of dissolved O2, at 20 degC and 1 atm of pressure is

n = PV/RT = (1 atm) * (0.01 L O2) / (0.0821 L * atm / mol * K ) * ( 293 K )

= 4.16 * 10^-4 mol O2.

The molecular weight of O2 is 32.0 g/mol, so 4.16 * 10^-4 mol O2 = .013 g O2 = 13.3 mg O2.

If the water in which the O2 is dissolved is pure H2O (not sea water) then the concentration is 13.3 mg O2 / 1000 mg H2O = 13.3 ppt dissolved O2.

So 10 mL dissolved oxygen/L water = 13.3 dissolved oxygen mg/L water.

Mike Evans

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