At 05:03 PM 2/7/2010 -0800, you wrote:
Hello Ode,
A few thoughts...
Most people don't adhere to laboratory procedures. When baking a cake, a
precisely measured cake tastes just as good as one that has been "thrown"
together. However, in chemistry things are different.
One of the biggest challenges in chemistry is to minimize cross
contamination. When you get to the point where you are measuring micro
amounts (as in uS), any trace of contamination can throw the measurement
off.
Pure water is used in industry and one of the major difficulties is knowing
how pure it actually is. Here is an informative article on this focused on
the calibration solutions provided by the meter manufacturers. As you read
this, keep in mind that the Hanna PWT meter calibration solution is rated at
84 uS, which is below their value of 100 uS.
http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Analytical%20Documents/Liq_Article_61-1511_200706.pdf
Cole-Parmer offers this note on the FAQ for conductivity testing...
"16. Is there an expiration date on the standardizing solutions?
Unopened with an unbroken seal, the solution will last one year. Opened,
but placed in an airtight container, the solution will last six months.
Opened and exposed to air, the solution will last one day.
Note: Standards below 100 uS will degrade at a faster rate than others due
to air. The conductivity of air is approximately 120 uS; any standard lower
than 120 uS will slowly rise in conductivity until it reaches a state of
equilibrium. Evaporation drastically increases conductivity values."
It seems that the calibration solution wants to rise in conductivity to 120
uS...
## 120 uS might be the saturation point of that solution?
Any more water missing and you get nonconductive crystals?
"Air" certainly doesn't conduct at 120 uS..it's WAY higher than that and
what the meter will read...in the air.
They may be referring to the formation of Carbonic Acid when CO2 and
water meet.
Just for grins... I re-calibrated the PWT to simulate a contaminated
calibration solution. My 123 uS solution was used, but I set the PWT to
read 84 uS. Here is what I observed:
My distilled water was reading 1.1 uS. With the mis-calibrated PWT, that
dropped to 0.6 uS.
I just made a batch of EIS that came in at 11.4 us, and that solution
dropped to 7.6 uS. In this case the "adjustment" factor is 1.5.
## The most important thing when using meters is to have both the meter
and the sample at the same temperature.
and don't be warming one up while using the other..like holding the sample
in your hand.
"Temperature Compensation" isn't quite what it's promoted as...if only in
inexplicable effect.
But with no difference, it doesn't matter.
Is it possible that Trem's 1.2 factor may be influenced by calibration
errors?
## Could be anything. Can't really expect a whole lot out of a meter but
a good guess.
Another question that arises is if the calibration solution can become
contaminated by CO2, can CO2 also have an effect on the newly made EIS
solution? Tom
## Sure..Silver Carbonate can form from Carbonic Acid from exposure to CO2
and make a batch go cruddy grey in the sun So can Oxygen oxidize ions
into Silver Oxide and go yellow.
Takes a while for gasses to dissolve into water in a significant manner.
A narrow neck storage bottle limits the surface area of CS exposed to air
and with a tighter seal on the cap.
ode
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