WPI APOLLO 4000, Instruction Manual

Page: 54 / 74

APOLLO 4000
OXYGEN DETECTION
World Precision instruments d-6
where pH
2
o and p'H
2
o are the partial pressure of water vapor at standard
atmospheric pressure in atmospheres and in mm Hg, respectively (see Table 3 ).
For example, the pH
2
o in water- saturated air at 24 deg is 22 mm Hg (see Table 3 ).
therefore the po
2
= 2% x ( - 22/760) = 20.4 %. note that for purposes of oxygen
measurements liquid water is considered to be “water-saturated air”. the values of
water vapor pressures at different temperatures can be found in
Table 3 .
You may obtain zero % oxygen concentration by adding to your solution of several
mg na
2
s
2
o
3
per 20 ml of solution.
to present dissolved oxygen concentration in parts per million (ppm), refer to Table
1a . this table gives the solubility of oxygen in water at different temperatures at
an ambient pressure of atm. if the solution temperature is 25°c, for example, the
oxygen concentration when the probe is in water is 8.4 ppm. You do not need to
correct for the water-vapor effect for a ppm calibration since the values in table a
are obtained in “water-saturated air” at an atmospheric pressure of 760 mm Hg.
the unit ppm is equivalent to mg/l. this is illustrated as follows. the solubility of
oxygen in water at 0 deg according to the merck index is 4.889 ml per 00 ml.
using the ideal gas law we can calculate the number of moles of oxygen present in
00 ml :
PV = nrt
n = P*V/r*t
n = (0.2)*(4.889x0-3) / (0.08206)*(273)
n = 45.8 x 0
-6
moles
Where P is the partial pressure of oxygen, V is the volume of oxygen, n is the
number of moles of oxygen, r is the universal gas constant, and t is the absolute
temperature. From the number of moles of oxygen we can calculate the number of
grams of oxygen :
45.8 x 0
-6
mol * 32 g / mol
.46 x 0
-3
g
therefore there will be (.46 x 0-3 g / 0. l) 4.6 mg of oxygen per liter. since
l of water has a mass of 000 g, and there are million mg in 000 g, the
concentration in ppm shall be:
(4.6 x 0
-3
g/l) / (000 g/l) = 4.6 ppm