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blogfast25
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Perhaps you should run some other tests on it, like pH and conductivity?
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bfesser
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<strong><a href="viewthread.php?tid=19355">Why does my Distilled water have a pH of 5.6 ???</a></strong>
<strong><a href="viewthread.php?tid=25352">Conductivity cell: DIY ideas needed!</a></strong>
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sbreheny
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I don't have the proper probe to do conductivity tests. I do have a pH meter - I will try but I'm not sure it is accurate if the ionic content is too
low (needs a minimum conductivity I think).
I do have some more information, though.
I bought a class 1 50g calibration weight. On my balance it measures as 49.9930g. The ratio of 50g/49.9930g is 1.000140. The ratio I get when I do the
same thing with my class M1 150g weight is 1.000112, so it looks like my balance is consistently reading 0.01% low. However, the ratio of the true
density of pure water to what I measure is 1.003. The difference from 1 in this case is almost 22 times the balance calibration error.
Second piece of new info: I repeated my water density experiments with a 100mL volumetric flask and with distilled water from two sources: Target and
McMaster Carr. I got 0.99468 g/mL for the McMaster Carr water and 0.99502 for the Target water. Compare this to about 0.994 (+/- 0.0005) for the
result I got before with the several 25mL flasks.
Now, I also tried measuring the density of acetone and reagent-grade 99% isopropyl alcohol. Here's where it gets interesting. For acetone, I get
0.7879 g/mL when the ideal value is 0.7910. This ratio is 1.00406. For IPA, I get 0.7806 g/mL, and the ideal is 0.7860. The ratio is 1.0069.
So, here are the ideal value/my value ratios for several scenarios:
These are all for fluid and room air between 19 and 21 deg C.
25mL flask #1, distilled water which is some mix of Target and McMaster origin: 1.0037
25 mL flask #2, same source water: 1.0045
25 mL flask #3, same source water: 1.0050
100 mL flask, Target distilled water: 1.0030
100mL flask, McMaster distilled water: 1.0033
25 mL flask, acetone: 1.0041
25 mL flask, isopropyl alcohol: 1.0069
So - it looks like something is causing me to get consistently low density readings, by roughly the same amount regardless of the fluid measured,
which suggests that it is something in my technique or equipment and not the fluids themselves. It also does not seem to be the balance. It's hard to
understand how it could be the volumetric flasks because it holds true over several of them, even between 25mL and 100mL.
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Chemosynthetic
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I am very impressed with this thread. While I have nothing to add to the latest development, I would like to note as a reminder that commercial
distilled water often has plasticizer leaching if stored for long periods of time in the jugs stores sell them in, and these types of substances are
not always removable through distillation without breaking an azeotrope. For this reason, DI water is often preferred where I work.
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sbreheny
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Quote: Originally posted by Chemosynthetic  | | I am very impressed with this thread. While I have nothing to add to the latest development, I would like to note as a reminder that commercial
distilled water often has plasticizer leaching if stored for long periods of time in the jugs stores sell them in, and these types of substances are
not always removable through distillation without breaking an azeotrope. For this reason, DI water is often preferred where I work.
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I am a stickler for accuracy and I often learn a lot by pursuing tiny unexplained error. My day job is as an electrical engineer and the same
philosophy often pays there, too. The experts on here are very good, too, at proposing possible sources of error.
I do also have DI water from McMaster Carr which I use for soldering in electronics (corrosion of the soldering iron tip is only really dependent on
ions present in the water). I believe that I have roughly measured the density of that before, too, but I can try repeating that.
[Edited on 16-2-2014 by sbreheny]
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sbreheny
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I am trying yet another volumetric measuring device, a buret. I don't have a result to report with that, yet, but I will post it when I do.
I did discover something else interesting, though. To avoid having to use heat to dry my receiving containers (like the volumetric flasks) I have been
using acetone to speed drying. I have been waiting until there is no more visible acetone (or any other droplets) and then weighing to get the empty
weight of the container. I discovered that if I do this too quickly, the mass of the acetone vapor adds a measurable amount of weight to the empty
container reading, which in turn makes the measured mass of the water smaller since the water displaces the acetone vapor. Acetone vapor at room temp
is about twice the density of air.
This is not enough to explain the full error but it is something I will keep in mind (by allowing more time for the acetone vapor to diffuse, as
confirmed by no longer being able to smell acetone on the container).
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Chemosynthetic
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I never understood why some of my former colleagues didn't "get" engineers. I admire the mindset and training. If possible, you may look into
running an inert line, such as nitrogen from a small tank such as those sold for paintball, into your flask to speed up evaporation convectively.
Reminds me of Raoult's law problems.
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unionised
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I may have missed it, but I didn't see any reference to correcting for the buoyancy of the air while making these measurements of the density of
water.
What corrections, if any, were made for that?
The error is about 0.1% if you don't correct for it.
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Chemosynthetic
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Yep. He really did his homework. I doubt latent enthalpy of evaporation is causing the error propagation either, but I just thought to mention it as
an additional potential confounding variable. Container shape (Erlenmeyer vs. beaker, per example) will affect this as well. | Quote: Originally posted by sbreheny | Thanks, chemrox. Based on a paper by NIST (Effect of Dissolved Air on the Density and Refractive Index of Water by Harvey, Kaplan, and Burnett), at 20
C this should only make 0.002 g/mL difference, which means an expected density of 0.996 - still 0.002 away which is still more than the sum of the
effects of air buoyancy and analytical balance error, although it is getting much closer so maybe I need to be more careful with my error budget.
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blogfast25
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| Quote: Originally posted by unionised | I may have missed it, but I didn't see any reference to correcting for the buoyancy of the air while making these measurements of the density of
water.
What corrections, if any, were made for that?
The error is about 0.1% if you don't correct for it. |
Never even crossed my mind. And you're right: at about 0.001 g/cm<sup>3</sup> density of air it amounts to an error of about 0.1 %. I
wonder if that's part of the reason to do it relatively, with a pycnometer, as both weight measurements would be off by the same amount due to air
buoyancy.
[Edited on 16-2-2014 by blogfast25]
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unionised
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If I had a dollar for every time someone slavishly copies down more than 3 significant figures from the balance, but forgets to account for air
density, I'd be a rich man.
Why isn't this taught in schools?
And a pyknometer doesn't solve the problem.
There's still an error in the apparent weight.
Imagine that you were trying to measure the density of mercury and rather than an atmosphere of air, you were working under water.
You would start off weighing the vessel "empty" i.e. full of water (in the same way you normally weigh it full of air).
M1
Then you would weigh it full of mercury,
M2
Then you weigh it full of water to get the reference value.
M3
You find the weight of the mercury M2-M1
and divide by the weight of water
M3-M1
But that last term is zero.
You can't divide by it.
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blogfast25
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@unionised:
So what's the applied correction? Just based on estimated volume (weighing boat + substance) of what you're weighing?
[Edited on 16-2-2014 by blogfast25]
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sbreheny
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I gave up on the buret because the repeatability I get with it is worse than with the volumetric flasks. Next thing I will try is a 250mL volumetric
flask since the relative error is less with larger flasks of the same accuracy class. I didn't do this before because it will not fit inside the draft
shield on my balance but I just remembered that I can open the top.
About pH - someone asked if I had measured the pH of my distilled water. I just did that. If I do it with just distilled water, I find the pH meter
very slow to respond and it is unclear whether the result is accurate. I get about 6.26.
So, I added several different pure salts (one at a time to separate samples of the distilled water). For each one, I added about 1g to 50mL of water.
I got the following results (and with all of these the pH meter settled almost immediately to a result):
Pure NaCl 5.45
Pure NaSO4 5.77
Pure CaCl2 5.45
With the NaCl I tried adding about 20mg, then 100mg, then 1g and it made very little (0.15) difference in the pH so I think that the salt is not
greatly affecting the actual pH of the water.
I believe that the salts help the pH meter by increasing the conductivity of the water so that you don't get a localized potential difference between
the probe and the rest of the water due to things like static charge.
So, I think the pH is very slightly acidic which is what we would expect from dissolved CO2. No additional discovery there.
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sbreheny
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OK, I think I may have the answer. I just re-did the test twice with a 25mL volumetric flask but this time, after rinsing the flask with acetone and
before I weighed it to get the tare weight, I gave it a much longer time to dry out and I made sure that there was no smell of acetone remaining. This
gave me 0.9976 g/mL the first time and 0.9969 g/mL the second time. If I then apply the air buoyancy correction of 0.0012 g/mL, I get 0.9988 g/mL the
first time and 0.9981 g/mL the second time. The water temp was about 17C where the ideal density would be 0.9988. These are well within the margin of
error for my volumetric flask (+/- 0.03 mL is 0.0012 g/mL for a 25mL flask).
So, it looks like it was due to the flask not being quite truly empty (air filled) when weighed before filling with water, plus the air buoyancy
difference. Together, the air buoyancy and acetone vapor could make as much as 0.0024 g/mL difference. Several of my readings were in the 0.993 range,
but most were 0.995. The outliers like 0.993 must have actually had a thin layer of acetone on the glass which was in equilibrium with the acetone
vapor and did not evaporate. Since the volumetric flask has such a narrow neck, it takes a long time for the acetone vapor to diffuse out. Inverting
the flask helps because of the greater density of the vapor.
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feacetech
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Why not use a hydrometer
[Edited on 16-2-2014 by feacetech]
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sbreheny
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First, because I don't have one. Second because once I tried this measurement and obtained results which were well beyond the limits of error which I
should have been able to attain, I wanted to keep at it until I found out what was wrong (was it my equipment? my technique?" Looks like it was my
technique but not in any of the expected ways, so I learned something!
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Chemosynthetic
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Excellent troubleshooting and results!
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jock88
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Look up specifications for water for injections. This is crazy pure stuff.
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blogfast25
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| Quote: Originally posted by sbreheny | | Since the volumetric flask has such a narrow neck, it takes a long time for the acetone vapor to diffuse out. Inverting the flask helps because of the
greater density of the vapor. |
In pro labs compressed air is often pumped through the glass work, with a rubber/glass tube.
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Zyklon-A
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| Quote: Originally posted by MrHomeScientist | An interesting distilled water-related situation I came across: I recently judged a science fair for middle and high schoolers, and one of the
projects involved measuring condutivity of different drinks to find the relative amount of electrolytes they contained. In the experiment she claimed
she got some conductivity from distilled water. If it was pure water, of course, it shouldn't conduct at all.
I haven't done this experiment myself, and it was a middle-schooler's project, so this is all hardly conclusive. Still, another interesting indication
that "pure" distilled water from the supermarket might not be quite so pure. |
I know this was posted a while ago (and is slightly off topic).
Pure water does conduct electricity actually, because of the self ionization of water.
However, I doubt that a student had the equipment necessary to measure conductivity that precisely. So it almost certainly had to do with impurity's
in the water.
[Edited on 18-2-2014 by Zyklonb]
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AJKOER
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| Quote: Originally posted by sbreheny | .....
Today I did an experiment where I took three different 25mL volumetric flasks, weighed each one three times empty, then three times when full of water
(bottom of the meniscus on the cal line), and averaged the readings for each flask and took the difference. I get the following densities for the
water in the three flasks: 0.9943, 0.9935, and 0.9930 g/mL."
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A few comments from the point of view of statistical analysis. In statistical sampling theory for ratio estimators (the density is a ratio estimate),
the common measure employed is the ratio of the means of two variate and not the average of individual ratios as the sampling variance of the latter
is much greater. It is, however, a bias estimate which can be adjusted. For an in depth discussion of various estimators and their respective standard
error see "Advances in Sampling Theory-Ratio Method of Estimation" by Hulya Cingi, Cem Kadilar at http://books.google.com/books?id=ORy83SaeWqgC&printsec=f... .
Now, in the case of your experiments, I will assume that the sample ratio (r), calculated as the sum (or average) of the respective water weights
divided by the respective volume measures, would be .9936. This statistic is, however, bias and needs to be corrected (see good discussion at
Wikipedia at on Ratio Estimator at http://en.wikipedia.org/wiki/Ratio_estimator ).
If mean of the weights and volumes employed are both greater than 10 (that is, the volume of water measured each time is over 10 cc), I would
recommend using the bias correction formula specified in Wikipedia (using a somewhat larger sample size) where the density estimate should have an
error in the order of at most 1/n cubed.
Any error observed in excess of what is expected per sampling theory should then be attributed to experimental design.
[Edited on 20-2-2014 by AJKOER]
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blogfast25
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Nope. He is measuring absolute density, not relative density. 'g/cm<sup>3</sup>' isn't a ratio, it's a quotient. Ratios are dimensionless,
see for instance 'dimensional analysis'.
[Edited on 20-2-2014 by blogfast25]
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AJKOER
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I an assuming that the water's absolute density is being ascertain by a comparison of mass to volume, with both of the latter subject to possible
error sources.
For example, even if a vessel's volume is precisely calibrated, is it properly filled per the human eye? Using multiple vessels, are all properly
calibrated or is there an equipment error distribution.
Related arguments for weight assessment using one or more scales.
For the degree of precision desired, I am not sure if one can ignore the manner in which the data is processed (like using an inefficient simple
average of individual density observations).
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macckone
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This is a most enlightening thread.
The measurement error being due to acetone remaining
in the flask is logical in hindsight but having worked in
a lab where analytical procedures were followed, I can
see where the insufficient drying and diffusion time
could lead to errors as well as the air displacement.
Now living at high altitude it makes it even more complicated
to account for air displacement.
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unionised
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| Quote: Originally posted by blogfast25 | @unionised:
So what's the applied correction? Just based on estimated volume (weighing boat + substance) of what you're weighing?
[Edited on 16-2-2014 by blogfast25] |
Google is your friend
http://metrology.burtini.ca/grav_air.html
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