jsc
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Best oxygen getter for atmospheric oxygen tester
I am working on an simple apparatus to measure the percentage of oxygen in air. I need an oxgen getter as the consumable for the apparatus.
The usual chemistry class method is to use steel wool wetted in an acid so that it rusts rapidly. Even so it must be left for hours or overnight. The
problem with this is that the atmospheric pressure may change in the meantime and affect the reading. Also, I don't want to wait hours for an answer.
On the other hand it cannot react too quickly because it could cause premature oxgyen deprivation before the apparatus is sealed.
So, I need a getter that will consume all the oxgen in a column in about 5-10 minutes. I was thinking lime (calcium oxide) might be a good choice.
Will lime react this fast? Any better ideas?
[Edited on 14-11-2014 by jsc]
[Edited on 14-11-2014 by jsc]
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WGTR
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Maybe I'm missing something, but how would lime do the job?
I've removed oxygen from a gas mixture before. Perhaps some of this information will be useful. After collecting anode gasses from Kolbe
electrolysis in a large syringe, carbon dioxide was removed with a small amount of sodium hydroxide solution. Following this, oxygen was removed by
shaking the gas with a small amount of CuCl in HCl solution. The copper is maintained as Cu1+ by fine copper wire in the bottom of the storage
bottle.
It seemed that only a few minutes of shaking the solution in the syringe was necessary to remove the oxygen. Bubbling the gas through the solution
would probably accomplish the same thing. Just consider that some gasses are a little soluble in water. Don't use any more solution than is really
necessary.
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jsc
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The apparatus is a graduated column with a shelf on the top. The bottom is open and you put it in water. The getter is on the shelf. As the oxygen is
consumed by the getter, water rises into the column and by the height of the water you can tell how much oxygen was in the column.
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AJKOER
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Per my prior comments on SM with respect to an iron-air battery, and Fe with Aluminum and hypochlorous acid (from the action of vinegar on NaOCl), the
formations of a colloidal iron oxide and iron solution, respectively (see http://www.sciencemadness.org/talk/viewthread.php?tid=153&am... ), perhaps something useful for your oxygen tester. I would expect shaking a
previously prepared basic solution of colloidal iron or Fe(OH)2 in air could, in the presence of NaCl, with your 5-10 minute time frame, meet your
goal.
This source provides a good summary reaction diagram https://www.google.com/url?sa=t&source=web&rct=j&...
A related point, I do recall an incident involving the death (via oxygen deprivation) of a worker cleaning a recently drained iron pipe.
Apparently, iron does love oxygen, so be mindful.
[Edited on 14-11-2014 by AJKOER]
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WGTR
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Quote: Originally posted by jsc  | | The apparatus is a graduated column with a shelf on the top. The bottom is open and you put it in water. The getter is on the shelf. As the oxygen is
consumed by the getter, water rises into the column and by the height of the water you can tell how much oxygen was in the column.
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I don't know how fast CuCl solution would work in that case. The gasses may diffuse too slowly into the solution for this to work in a timely manner.
I know that every time I open the bottle, there is a sucking sound as more air rushes inside.
Are you accounting for the weight of the water as it rises in the tube? In other words, as the water gets "sucked" up the tube, above the level of
the water outside, the gas pressure inside the tube drops, skewing the results. This effect can be minimized by simply adding water outside the tube
until the two water levels are the same height.
[Edited on 11-14-2014 by WGTR]
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jsc
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| Quote: Originally posted by WGTR | | Quote: Originally posted by jsc | | The apparatus is a graduated column with a shelf on the top. The bottom is open and you put it in water. The getter is on the shelf. As the oxygen is
consumed by the getter, water rises into the column and by the height of the water you can tell how much oxygen was in the column.
|
I don't know how fast CuCl solution would work in that case. The gasses may diffuse too slowly into the solution for this to work in a timely manner.
I know that every time I open the bottle, there is a sucking sound as more air rushes inside.
Are you accounting for the weight of the water as it rises in the tube? In other words, as the water gets "sucked" up the tube, above the level of
the water outside, the gas pressure inside the tube drops, skewing the results. This effect can be minimized by simply adding water outside the tube
until the two water levels are the same height.
[Edited on 11-14-2014 by WGTR] |
People who describe this experiment in textbooks never mention this as an issue.
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Tsjerk
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Can't you check the atmospheric pressure and correct for it? Or even check the weather forecast for the actual pressures?
How btw do you check for water vapor not replacing the oxygen?
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jsc
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| Quote: Originally posted by Tsjerk | Can't you check the atmospheric pressure and correct for it? Or even check the weather forecast for the actual pressures?
How btw do you check for water vapor not replacing the oxygen? |
Water vapor is an issue, because you have to account for any additional water vapor over and above that which was in the tube when the starting
measurement was made. The oxygen pct is adjusted by the vapor pressure of water proportional to the atmospheric pressure. This is usually about a 2%
adjustment.
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AJKOER
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A more (perhaps too) dramatic experiment: First, add air to an inverted long plastic bottle filled with water with its top cut off. Record its volume
and water temperature. Add H2 (free from oxygen) in excess, and record new volume and temperature. Insert wires or a spark maker (stove lighter, for
example) to detonate the gas mix in the inverted vessel in a large basin of water. The loss in volume is from:
O2 (g) + 2 H2 (g) = 2 H2O
so, 1/3 of the volume change, measured at the starting water temperature, relates to oxygen content when hydrogen is in excess. I would take the
calculated O2 volume to the starting air volume as my experimental estimate of oxygen in air without further adjustments.
Note, if experimental error is proportional to reading/interpolating total volume change, then this experiment may reduce measurement error on oxygen.
Potential problem is employing too much air so that the H2/O2 explosion ruptures the vessel in spite of its open end. Best vessel shape would be an
inverted V. The plastic vessel must also be held in position with a lid and weights, for example, so as not to become a water rocket.
One could add salt and heat the water to reduce oxygen solubility, or pre-saturate the water with air (like with an aquarium air pump). Depending on
the required precision, a correction may still be required.
[Edited] This experiment is inexpensive and one could be repeated many times. Upon repeating, one can compute the average of all the individual ratios
of the portion of oxygen in air, which is an unbiased estimate, to another statistic commonly employed in sampling theory. It is the ratio of the sum
of each experiments computed O2 volume to the sum of all the starting air volumes. The latter is popular as while it is slightly bias, it has a lower
expected sampling variability.
Adding pure oxygen in varying doses to the tested air might also be interesting. A regression on the observed ratio estimates of O2/air versus known
added oxygen amount/air could provide a statistical measure of precision. In particular, the regression's calculated standard error on the intercept
(which is the no added O2 dose, or the starting air), could provide a regression based precision range on the portion of oxygen in air.
[Edited on 23-11-2014 by AJKOER]
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IrC
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Is this supposed to be some purely chemical sensor for school work in chemistry class? If not why can't you just measure the electrical resistance of
Titanium dioxide? You can try building a narrow-band lambda sensor. Or go shop at the local auto parts store for various types of O2 sensors.
"Science is the belief in the ignorance of the experts" Richard Feynman
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blogfast25
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| Quote: Originally posted by jsc |
So, I need a getter that will consume all the oxgen in a column in about 5-10 minutes. I was thinking lime (calcium oxide) might be a good choice.
Will lime react this fast? Any better ideas?
|
How could CaO be an oxygen getter? The Ca is already oxidised to its maximum oxidation number!
| Quote: Originally posted by WGTR |
Are you accounting for the weight of the water as it rises in the tube? In other words, as the water gets "sucked" up the tube, above the level of
the water outside, the gas pressure inside the tube drops, skewing the results. This effect can be minimized by simply adding water outside the tube
until the two water levels are the same height.
|
You're right but the effect is very small: a 10.33 m water column is approximately 1 atm (at the bottom), work out how little a few cm of water column
would be! Temperature control is more important here to get correct gas volumetric readings.
One fairly quick way for an oxygen meter would be to connect two accurate gas syringes via a combustion tube with sufficient steel wool in the latter.
The combustion tube is heated by a Bunsen and the air contained in, say the left syringe, is passed over the hot iron into the second syringe, then
back to the left syringe. After a couple of times all the oxygen has been scavenged by the iron. Allow to cool and normalise back to atmospheric
pressure and read the volume of the now less full syringe.
Your projected set up relies on rapid oxidation at RT but oxygen is fairly sluggish at RT.
Also, you rely on gas diffusion for all the O2 to reach the getter and gas diffusion is not a very fast process.
A potentially quite fast getter at RT is sodium dithionite which is actually used in the determination of water dissolved oxygen. Diffusion would
still be a limiting factor though.
[Edited on 23-11-2014 by blogfast25]
[Edited on 23-11-2014 by blogfast25]
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unionised
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have you seen this sort of thing?
http://chem.sci.utsunomiya-u.ac.jp/v13n1/10_2d4_1.pdf
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