gregxy
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Calibrate cheap PH meter
I bought a cheap PH meter on the internet. It did not come with any calibration
solutions so I thought I would try and do it with out them using the very limited
house hold chems at hand. I want to use it for hydroponics so the range of 5-9
is most important. The meter has a single adjustment pot. The manual says to
use a calibration solution with PH of 6.6 to adjust it.
Tap water tests 7.6 (reasonable)
Distilled water (in a plastic bottle that had been sitting for a while) gives 6.3,
I guess the water could have become acid from the bottle.
I made a solution of 1g vitamin C in 31 ml of H2O. = 0.18Molar, Pka= 4.10 The calculated PH is 2.4 and the meter reads 2.4
A weak solution of baking soda tests 7.6, looking on the internet, I found a formula: average of 10.3 + 6.35 Ka for bicarbonate and carbonate=8.3 I
could not find a good explanation of where this formula comes from and what it is sensitive to.
Any ideas why this value would be off? I cannot adjust the meter so that both
the vitamin C value and baking soda values are correct.
Any suggestions (other than buying the correct buffer solution)?
[Edited on 24-6-2012 by gregxy]
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unionised
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Borax and cream of tartar give quite well defined pH
"Using an excess of the salt in water, a saturated solution is created with a pH of 3.557 at 25 °C."
from
http://en.wikipedia.org/wiki/Potassium_bitartrate#Chemistry
Saturated borax is pH9.32
http://www.borax.com/pdfs/dist/Profile_Borax_Decahydrate.pdf
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blogfast25
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Quote: Originally posted by gregxy  |
A weak solution of baking soda tests 7.6, looking on the internet, I found a formula: average of 10.3 + 6.35 Ka for bicarbonate and carbonate=8.3 I
could not find a good explanation of where this formula comes from and what it is sensitive to.
Any ideas why this value would be off?
[Edited on 24-6-2012 by gregxy] |
The 10.3 + 6.35 Ka formula may be a distortion of:
pH = 14 - 1/2 pK<sub>b</sub> + 1/2 log<sub>10</sub> C<sub>b</sub>
which gives the approx. pH for a weak base with pKb = - log<sub>10</sub> K<sub>b</sub> (K<sub>b</sub> the base
constant of the base) and C<sub>b</sub> the nominal concentration of the weak base. Valid at low concentrations of the base.
[Edited on 24-6-2012 by blogfast25]
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unionised
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"The 10.3 + 6.35 Ka formula may be a distortion of:..."
Or it might be the average of the two pKa values for bicarbonate gaining or losing a proton.
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blogfast25
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In any case 'one point calibrations' will tend go give erroneous values towards both ends of the pH scale, I think.
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Swede
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Was the meter used? If so, the probe may have been dried out, contaminated, or otherwise no good. pH probes is one lab item I'd never buy used,
especially sealed probes, which is the type most often seen with cheap setups.
Distilled H2O is useless as a test. The borax is a good one.
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gregxy
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It is one of the pen type meters for aquariums. It cost about
$10 new. Since there is only one adjustment pot it can
only be calibrated at one point. Good meters calibrate at
3 points. I think for this cheap meter the idea is to buy
a buffer solution very near the PH that you want and
use the meter to verify that the aquarium matches the
buffer. PH values close to that point will be accurate but
not values further away. If I calibrate it to the baking soda
(8.3) I get a fairly good value for milk expected to be 6.7.
Borax or boric acid are good suggestions.
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