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Sulaiman
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[*] posted on 18-7-2019 at 03:14
Mass and Weight


Part 1, Cheap calibration weights

As I can't do any chemistry for a while I've diverted my attention to Mass, Weight and related topics.
I intend to make a few videos on the subject as I've now got different weight sets (mainly F1 and M1) and various scales/balances.

The reason for this post is to inform members of a possibly overlooked source of cheap precision weights
... second-hand scales/balances.

- internal calibration digital scales have at least one precision weight

- some older two-pan scales have internally a complete sequence of precision weights

- possibly the best source is old single-pan scales where weight is dialed in,
these scales work by effectively removing weight from the weighing arm to operate at constant nett weight,
which maintains the same sensitivity over their weighing range.
These scales are usually old and unwanted - cheap on eBay,
yet they contain a complete sequence of highly accurate weights.

e.g. one that I am bidding on at the moment is an Oertling 200g x 0.1mg balance,
which contains weights to cover the 1mg to 200g range, (using a different method to get the 0.1mg resolution)
presumably of sufficient accuracy to match the 0.1mg resolution of the scales.

These weight sets, being internal, will almost certainly have never been handled or cleaned, so should be in near original condition.

This document covers the subject of weights. Attachment: R111-1-e04.pdf (1.1MB)
This file has been downloaded 174 times




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Pumukli
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[*] posted on 18-7-2019 at 05:18


Good subject, Sulaiman!

I'd urge all our members if they want to do chemistry on a bit more serious level that acquire a decent scale! You may be positively surprised how new avenues may open up for you to explore!

For years my most sensitive and accurate scale was a cheap, small, chinese one, with the limits of 20g/0.5 mg.
Even with this one I could start doing some analytical work because the resolution made it possible to measure with a few percent accuracy e.g. KHCO3 - as primary standard for standardizing 0.1 M HCl solution and KH((IO)3)2 as primary standard for standardizing Na2S2O3.

Of course, if you have standardized acid then you can make and standardize a base (NaOH solution) against this.

Similarly, if you have a standard iodate (or bromate) solution it makes standard thiosulfate possible and it opens up a route to other redox titrations too.

Nowadays disodium-EDTA-dihydrate is cheap and can be bought online. It can be dried at 80 C and measured exactly to make standard EDTA solutions. It makes complexometry approachable to us! (You still need indicators, but I've seen them sold continuously on e.g. Ebay.)

You will need a burette and some pipettes too, not to mention the chemicals themselves, but for me these things were easier came across than a decent scale! (Your mileage may vary.)

A few months ago a nice 220g/0.1 mg balance appeared on a local second hand website and I could buy it.
(I was the only bidder.) :)
Since then I repeatedly wonder how I could live without it in the previous years!?


Sorry Sulaiman for the almost hijacking the thread, but I can not overemphasize the importance of meaningful measurements in our hobby.

I'm curious what you would teach us about those precision weights and balances and what/how to do with them!

Edit: well, the website was not second hand. The items on the site were. I could not translate it properly. :)

[Edited on 18-7-2019 by Pumukli]
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Sulaiman
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[*] posted on 18-7-2019 at 08:38


The F1 weights (a new 1mg to 1kg set, plus a used 2kg) are to check linearity and repeatability of my 100g x 1mg scales, and to verify my M1 weights (too many ... don't tell my wife).
All other measurements will be done using M1 class weights as my other scales have less significant digits or increments.

I intend to investigate most of the parameters
(zero drift, linearity, sensitivity, creep, calibration...)
of my digital scales
(100g x 0.001g, 300g x 0.01g, 2000g x 0.1g and kitchen 2000g x 1g)
and balances
(Triple beam balance, and possibly 200g x 1mg with 0.1mg resolution)

Then do some sums to estimate the reliability of future measurements.

Next will be bouyancy related issues.




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CharlieA
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[*] posted on 18-7-2019 at 16:31


Sulaiman: ambitious and very worthwhile project. I remember how pleased I was when I learned to use double-pan balance, and impressed with the precision of results that I had never dreamt of achieving. I will admit that the pursuit of significant figures can be obsessive, especially if one confuses significant figures with the number of decimal places. I guess it all boils down to whether one wants to work with large quantities and relatively crude measuring devices, or small quantities and much better (and expensive) measuring devices.
I think you will enjoy your little project, and will learn a lot. Thank you for the reference to standard weights. Downloading it was easy, now to read (and understand) it...

Pumukli: Great points you made about being able to do good and very interesting work in quantitative analysis. I think the qualitative and quantitative analytical courses were real confidence builders to confirm my laboratory skills (aka wet chemistry). Granted today's instrumental methods are truly great. But, my little basement laboratory only supports wet chemistry and no instrumental analyses.
li
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Sulaiman
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[*] posted on 14-10-2019 at 16:30


I am uploading to YouTube some un-edited videos of me testing some generic (cheap Chinese via eBay) calibration weights,
I hope to edit these videos and add a sound track one day to put on my 'nice' channel,
but for now, you may find the videos a little informative.

Weighing generic calibration weights, 1g to 100g from a 10mg to 100g kit
https://www.youtube.com/watch?v=kms21glg83M

Checking repeatability
https://www.youtube.com/watch?v=Dz9gxvIRM9I

Checking a 20g weight and a kit of 5, 10 20, 20 and 50g calibration weights
https://www.youtube.com/watch?v=lctwj58ZfMs&feature=yout...

A quick linearity check using two 20g weights
https://www.youtube.com/watch?v=nF9dF6S4lXE

The dark patch in the middle of the weighing pan is a drop of superglue that holds a thin plastic sheet to the pan
- so that I do not scratch my calibration weights (E2 class 50g, 100g and 200g plus others of lower class)

The specifications for the balance are;
220g maximum, 0.1mg resolution, 0.5mg repeatability, 0.5mg linearity
so if the balance seems more accurate than that, it is just luck.
This balance has a few features that do not work (i.r. for Cal and Tare, selectable sensitivity/weighing time) and I'm currently in a dispute with the manufacturer.
For now I can say ... do not buy from Want Balance Instrument Co.

This balance is incredibly sensitive to ambient temperature changes.

[Edited on 15-10-2019 by Sulaiman]




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[*] posted on 15-10-2019 at 12:08


Interesting videos! Any chance you might investigate whether skin oil changes then result? I handle my calibration weight (200 g F1) bare-handed because I don't think it changes anything at the milligram scale. But I've seen people use cotton gloves like yours, so maybe it does?
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[*] posted on 15-10-2019 at 16:24


In college, we handled our weights with ivory (or plastic?)-tipped forceps. Aside from possibly changing the "weight of the weights", you don't want any skin oil to corrode the weights or maybe trap dirt/dust, and thus significantly change the weight. 1 milligram is not much to add to a weight, and that is 1 part per thousand at the 1 gram level.
But practically speaking for the typical home chemist, if you deal in quantities of 1-9g and have a balance that has a precision and accuracy of 0.01 g, you are working with 3 significant figures or about 0.1%, and this is not to be sneezed at.
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Sulaiman
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[*] posted on 15-10-2019 at 16:28


The tolerance of a 200g F1 weight is 1mg so I doubt that fingerprints will cause out of tolerance calibrations
(I tried to weigh my fingerprints - unmeasurable with this balance)
a drop of sweat would definitely cause the weight to be out of tolerance
BUT
over time your fingerprints may cause the stainless steel to corrode.

Using gloves (or the supplied plastic tweezers) to handle cheap Chinese weights is also good practice,
because in my experience, the crappy steel is easily corroded through the crappy plating.

Cotton gloves do not stop humidity from my skin occasionally passing through the cotton and condensing on the weights.

The balance and three E2 weights (50, 100 and 200g) are my new toys,
so I bought the cotton gloves before they arrived because that was recommended,
I will probably be using disposable rubber gloves after this as E2 weights are not cheap, and should have a 25 year service life.
___________________________________________________________
I'm currently making a video of Mass vs Weight (this will need narration as there are so many variables)
then I plan to do some density measurement demonstrations.
When I am reunited with my chemistry equipment I'll do some videos on the accuracy of my other scales.
(a triple beam balance, and cheap Chinese digital 2kg x 0.1g, 300g x 0.01g, 100g x 0.001g)
Eventually I may actually get to use the balance for some chemistry :P




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[*] posted on 16-10-2019 at 04:11


The techniques in that .pdf are fascinating. It amazes me how much science is behind something so deceptively simple. I've used 20°C distilled water with class A (ASTM E969) volumetric pipettes to determine the linearity of cheap digital scales. You can verify 1g to ±6mg and 10g to the nearest ±20mg this way, which I find accurate enough for the majority of amateur chemistry, giving 1.2% and 0.4% possible measurement deviation, respectively. This falls between M2 and M3 classes for standard weights, and a set class A pipettes can be significantly cheaper than weights and are also useful for other chemistry applications.

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[*] posted on 16-10-2019 at 05:56


The good news is that when you use a scale of given accuracy you can have much better accuracy in your experiments, as long as you use the same scale for all weighing in the experiment.

Frequently, a certain error is specified (e.g. +/- 0.5% of full scale), but the linearity usually is much better (e.g. 0.05% of full scale if you have 4 digits of readout).

In absolute terms you may have a certain error, which may be quite large. But ALL of your measurements will have the same error in the same direction, so relatively between measurements on the same scale, your errors are much lower. To make things concrete, suppose your scale has a readout of 0.993 grams, and the real weight is 0.989 grams. If then you weigh another compound with the same scale and its readout is 1.200 grams, then the quotient 0.993/1.200 will be very close to the quotient of real weights. So, if you need certain molar ratios in your experiments, then with that inaccurate scale you still can get good precision molar ratios. Only the absolute values, compared with real weights are less accurate.

This holds, when your measurements use a substantial part of the full scale. If your full scale is 2.000 grams (with three digits after the decimal dot) and you measure just 0.003 grams, then of course you will have a very low accuracy.




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[*] posted on 11-11-2020 at 15:09


I'm trying to determine the exact concentration of HNO3.

I have measured it using three scales: one tabletop kitchen scale with 1g accuracy, one 0.1g accuracy and one 0.001g Homgeek. They all gave exact weight on 50g calibration weight. The liquid was measured in three different vessels: 1mL syringe, 10mL measuring cylinder and 100mL measuring cylinder. I can't assure the accuracy class of the cylinder's markings. Each were lifted and replaced three times to ensure constant tare.

The kitchen scale gave SG of 1.38. (59.5%)
The middle scale gave SG of 1.34. (52.5%)
The Homgeek gave SG of 1.419. (67.2%)

The temp of the solution was measured 8C during the measurement.

So, the concentration is supposed to be 68%. How can I determine which one of these is closest to the truth? The average of all them is 59.8%, which could be somewhat suitable as a reference since the reaction does not need precise concentration, but I'd like to take this as an exercise. Purchasing a $3k analytical scale is unfortunately currently out of my financial extent.
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[*] posted on 11-11-2020 at 17:25


The cheapest way might be to titrate the acid with a standardized solution of a strong base. The classical way to standardize the base is by titrating KHP (potassium alum phthlate) with the base.
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[*] posted on 13-1-2021 at 18:30


KHP is potassium hydrogen phthlate!
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Sulaiman
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[*] posted on 13-1-2021 at 20:53


try measuring the density of recently boiled (to remove dissolved CO2) distilled water.
(typical tap water will give results accurate to better than 1%)
do for each scale to get a feel for expected accuracy.

get a few weights of about 10g each (eg steel nut, screw etc.)
weigh each individually as accurately as you can,
then use combinations of these weights to check the linearity of your scales.

Density measurements are very temperature sensitive so allow everything to reach room temperature before starting.

I would trust a titration more than a density measurement.
(even sodium carbonate from roasted domestic sodium bicarbonate will give a fairly accurate determination)




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[*] posted on 14-1-2021 at 04:12


Same opinion as Sulaiman.

Buy a burette and titrate it. Even with low-resolution scale you will get good enough (for non-professional purposes) concentration results. Much better than messing with density tables and data interpolation.
Also, there are no tables for everything so it may be useful for many other things.
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[*] posted on 14-1-2021 at 05:23


You only need one calibration weight/ mass to verify that a balance works.
Pick one that's near the top of the range for your balance.

Check that the balance reads zero with nothing on it- if it doesn't... it isn't really a balance and you need to fix it.
Check that it gives the right answer when you put the expensive calibration test mass on it.
If needs be, adjust it so it does.

Repeat those steps until you can get the right answer at zero and at full scale (if you can't you need a new balance).

Get 9 coins or bars of chocolate or rocks or something that weigh about a tenth of the range of the balance. It doesn't matter much what they are as long as their mass is stable so no very short lived radioisotopes, nothing hygroscopic .
Label them A through I

Weigh each of them and note their apparent masses.
Then weigh combinations of them.
A+B
A+B + C
...
A+B+C+D+E+F+G+H+I


Use a spreadsheet to plot the observed masses vs the mass calculated from the masses of the individual items.

If that plot is linear then your balance is right at the top + bottom of the range, and is linear in between.
And that means it gives the right answer.

If you are feeling paranoid you can do another check
Get a drawing pin or something with a mas about 1/100 of the full scale range of teh balance.
Weigh it, and weigh it plus A
then A+B + the pin,
then A+B+C + the drawing pin.
And so on.

And again, get your trusty spreadsheet to calculate the differences between each stack of coins and the same stack with a pin on it.
Those differences should all be the same.

If you plot them against the indicated mass for the stack including the pin, you should get a horizontal straight line.
If you get a sloping line it gives you an indication of the size and direction of any non linearity.

I once had the entertainment of doing this for a balance at work.
It could resolve down to 100 ng (yes, nanograms) and nobody could supply me with calibration weights good enough to verify that it worked properly, so I had to do it the hard way.
Lots of fiddly little bits of thin wire were involved...



[Edited on 14-1-21 by unionised]
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