sergius - 7-12-2004 at 03:02
how can we measure the dissolved oxygen in water? Is it that dissolved oxygen imparts sweet taste to water? What is that adds taste to water?
Sergius
oops...... forgot about the mineral content, which certainly has a role on taste
Sergius
Edit works too (chemoleo) 
[Edited on 7-12-2004 by chemoleo]
chemoleo - 7-12-2004 at 03:28
Yah, often, and to my great dismay, I notice that the night time glass of tap water has gone stale and horrible the next morning - I also think
it's because 1) a lot of the dissolved gasses have come out of solution and 2) the temperature is higher (RT).
How we detect whether the water is gas-rich? No idea. With CO2 in sparkling water I can see how (acidity, and bubbles evolving on the tongue), but not
with O2 in water. Maybe there are some O2 receptors that yet remain to be discovered?
neutrino - 7-12-2004 at 03:29
I’m not sure about OTC methods, but there are test kits readily available for measuring dissolved oxygen content.
S.C. Wack - 7-12-2004 at 09:12
A MnSO4 soln is added to the water. 4 Mn(OH)2 + O2 + 2 H2O -> 4 Mn(OH)3. Then a soln of NaOH, KI, and a little NaN3 (to destroy nitrite) is added,
and the Mn precips.
Mn(OH)3 + I- + 3H+ -> Mn2+ 1/2 I2 + 3H2O. The I is determined by titration with thiosulfate/starch as usual.
STP solubility: 8 mg/L.
There are also 1/2 O2 + 2 H+ + 2e -> H2O methods.
http://www.google.com/search?hl=en&lr=&q=%22clark+ce...
JohnWW - 7-12-2004 at 10:12
The Mn(II), of which there has to be an excess relative to the dissolved O2 present, usually reacts only after the NaOH/iodide solution is added which
precipitates it as hydroxide which is immediately oxidized by the dissolved O2 present to Mn(III) and/or Mn(IV). Unless the original solution is
alkaline, of course. The iodide then is oxidized by the Mn(III) and/or Mn(IV) hydrous oxides, in an amount proportional to the dissolved O2 originally
present, and is titrated against S2O3-- with starch indicator.
I have experimented with the possibility of the I2 produced (as a yellow-brown color) being capable of measurement by spectrophotometry, eliminating
the thiosulfate titration. However, possibly due to interfering substances and other colored substances present (the waters I tested were natural
waters and treated sewage, which I analysed for biochemical oxygen demand as well as dissolved oxygen), the absorbance-concentration calibration
results showed far too much of a scatter from a straight line for this to constitute a reliable analysis method.
It was some years ago that I did this work; I do not recall having used any NaN3 to eliminate nitrite, at that time. This must be a more recent
modification. There may be a significant amount of nitrite in treated sewage. However, the Google results I got from searching for any reaction of
nitrite with iodide indicate that this happens only at elevated temperatures, and otherwise nitrite only catalyses oxidation of iodide by strong
oxidants.
Of course, most dissolved oxygen measurements, especially in the field, are now done using portable dissolved oxygen meters.