Dry Chemical Scale

Alan9940 · Feb 18, 2017

I currently have an electronic scale that measures accurately (afaik) down to about .2 grams. However, anything below that, like trying to measure .1 g, seems impossible to hit. I'm thinking that a scale with .001 level precision would enable me to properly measure something like .1 g? Does anyone know of a reasonable cost scale that would enable proper measurement of smaller weights like .1 g?

Thank you.

bernard_L · Feb 18, 2017

Depending on what you need to measure in small amounts, one way is to prepare a stock solution in propylene glycol (significantly less toxic than ethylene glycol). Say, e.g., 1g of substance "X" in 100ml of propylene glycol. Then 1cc of stock solution contains 0.01g of "X", and is easily measureable. Adapt the numbers to your convenience. Works for Phenidone, Benzotriazole.

Alan9940 · Feb 18, 2017

bernard,

Hmm, hadn't thought of that...thanks! Phenidone is one where I need .2 g (which my scale can barely do); the other is Potassium Bromide at .1 g. I assume the latter would work per your suggestion, too?

Rudeofus · Feb 18, 2017

There are loads of Chinese made scales offered on ebay and similar venues for very low dollar amounts. My experience with them so far is not bad: they work well, and even if they don't last forever, they are still a much better deal for casual users than professional equipment. Their construction is simple: a load cell attached to the same analog-to-digital converter that is used in electronics multimeters, and the reason I mention this is simple: the same pros/cons that different resolutions and ranges have with multimeters exists with these ebay scales:

Finer resolution (i.e. 0.001g instead of 0.01g) typically means lower top range. A scale with 0.001g resolution rarely goes beyond 20g, which is tedious if you want to mix D-76 or Dektol or fixer in quantities beyond 200ml
Scales, very much like multimeters, typically offer ten times the resolution that they offer in accuracy, i.e. a scale with 0.001g resolution on display offers roughly 0.01g absolute accuracy. You can safely assume, that a quantity of compound weighed as 0.101g is heavier than one weighed as 0.100g on the same scale, but they may actually weigh 0.105g and 0.104g, respectively, or 0.096 and 0.095 depending on scale. Many scales from the same series produced in short order my deviate into the same direction, so averaging between two scales bought at once may or may not improve absolute accuracy.
Higher number of digits would offer better resolution with high range, but measurements take noticeably longer. This can become an issue if you want to have e.g. 10.000g of some powder compound by adding/measuring/adding/measuring/... iterations.

Gerald C Koch · Feb 18, 2017

You might look at powder scales used in the reloading of cartridges. A typical one weighs to 1/10 of a grain (0.0065 gram).

Alan9940 · Feb 18, 2017

Thank you all for your replies.

I'm really warming up to the idea presented by bernard_L because my scale should be fairly accurate at 1 gram. Therefore, a follow-up question for you chemist out there (are you around PE?): If I mix either Phenidone or Potassium Bromide in propylene glycol, how long can I reasonably expect that mixture to last?

Thanks, again.

Harry Stevens · Feb 18, 2017

I checked my cheap Chinese digital scale by getting the weight from the royal mints website of the coins currently in circulation in the UK and they where all spot on.

Gerald C Koch · Feb 18, 2017

Alan9940 said:
Thank you all for your replies.

I'm really warming up to the idea presented by bernard_L because my scale should be fairly accurate at 1 gram. Therefore, a follow-up question for you chemist out there (are you around PE?): If I mix either Phenidone or Potassium Bromide in propylene glycol, how long can I reasonably expect that mixture to last?

Thanks, again.

Potassium bromide has limited solubility in propylene glycol ~15 g/l. Phenidone oxidizes easily in solution. Therefore I would discourage making solutions. It would really be better just to get a good scale.

avb · Feb 18, 2017

I like analytical balances used in university chem labs, research, etc. (I don't own one personally but would like to!)
The typical model can weigh up to ~160 g to 0.0001 g (to be safe a precision of +/- 0.001 g).
If you search "Mettler AE" in eBay there are many available at various prices.
The Mettler AE163 is great!
Use weighing paper or weigh dishes and a spatula and it would be perfect!

mshchem · Feb 19, 2017

There are tons of nice old Ohaus balances (scales) on Ebay. Look for a Dial-o-gram 310. Two beams and a torsion dial. It will weigh to 10 milligrams (0.01 grams ) . You can find very nice weight sets too. I have several balances from an Ohaus Solution balance (20 kg capacity ) great for making up straight hypo. And my Dad's beautiful Ainsworth twin pan analytical balance that weighs to 0.0001 grams.
A Harvard trip pan (Ohaus ) and a set of weights up to 500 grams is what I use 90% of the time. Absolutely fool proof design.

I'm forced to use one of the wondrous Chinese junk electronic scale at work. Absolute crap.
If you are really nuts and want a old single pan mechanical analytical balance from the 60's and 70's. Mettler or Sartorius. I bet I've weighted 250,000 1.0xxxg samples on these over the years. Make sure the weights aren't corroded or damaged.
Mettler-Toledo merged. There are actually some half way nice electronic units available for 400 to 500.bucks

One last point, Gerald mentioned a powder scale for reloading, I have an old Redding powder scale, you can find these used for 30 bucks, in grains not grams easy to convert and very very accurate . This would be the cheapest way to accurately weight less than gram quantities
Best Regards Mike

removed account4 · Feb 19, 2017

hello alanh
i am not sure what brand scale you have
but i bought an inexpensive ( it was IKD 10-14 years ago )
from http://balances.com
not sure where it is made but it hasn't failed me
and it works just fine.

Wayne · Feb 19, 2017

Gerald C Koch said:
Phenidone oxidizes easily in solution. Therefore I would discourage making solutions.

I've not heard this before. I've used phenidone % solutions in glycol that are years old and they seem to work, and I know other people have too. Which doesn't mean phenidone doesnt oxidize in solutions, but I have to wonder, how quickly?

RalphLambrecht · Feb 19, 2017

Alan9940 said:
I currently have an electronic scale that measures accurately (afaik) down to about .2 grams. However, anything below that, like trying to measure .1 g, seems impossible to hit. I'm thinking that a scale with .001 level precision would enable me to properly measure something like .1 g? Does anyone know of a reasonable cost scale that would enable proper measurement of smaller weights like .1 g?

Thank you.

chose an electronic scale with aax of 10g.they usually measure in 0.001g increments and are very accurate.You can test them with a calibration weight or by knowing that a letter-size piece of paper dry weights about 5g.

bernard_L · Feb 19, 2017

the other is Potassium Bromide at .1 g. I assume the latter would work per your suggestion, too?

As pointed out by G.C.Koch:

Potassium bromide has limited solubility in propylene glycol ~15 g/l.

On the other hand, Potassium bromide does not risk (AFAIK) oxidation, so dissolving in water should be OK.
Regarding Phenidone, you have contrasting statements from G.C.Koch and Wayne; your decision.

Wayne · Feb 19, 2017

It should be noted that Wayne is not a chemist (I got a hard-earned C in both General Chem and Organic Chem, 99% of which I've forgotten) and Gerry is. I think of the statements as complementary, rather than contrasting.

Alan9940 · Feb 19, 2017

Thanks, again, to everyone for your help and suggestions.

After giving it some thought, I've decided to try another electronic scale--an AWS 100 which, per a YouTube video I watched, appears to be accurate down to somewhere below 100 mg. It wouldn't register 30 or 50mg, but I don't need anything below 100mg so I'm hoping for the best. At $15 with a calibration weight, I can't argue with the price!

Gerald C Koch · Feb 19, 2017

I have tried to mix a percentage solution of phenidone before. Used both water with added bisulfite and several organic solvents and each time the solution turned pink within a few hours. The color is due to oxidation and a sure sign that this method is not practical. Phenidone is also destroyed by UV light. When attempting to measure the concentration using a UV spectrometer you could see a smoky trail where the light passed through the solution. Again the phenidone was being destroyed. Even the solid is not too stable. It should be kept in a brown glass jar. I have some that is decades old stored this way. However a sample in a clear plastic jar turned to tar in less than a year.

Anon Ymous · Feb 19, 2017

Alan9940 said:
Thanks, again, to everyone for your help and suggestions.

After giving it some thought, I've decided to try another electronic scale--an AWS 100 which, per a YouTube video I watched, appears to be accurate down to somewhere below 100 mg. It wouldn't register 30 or 50mg, but I don't need anything below 100mg so I'm hoping for the best. At $15 with a calibration weight, I can't argue with the price!

Good choice IMHO. I have an identical, but unbranded scale which I bought from ebay for about $10. I calibrated it using a glass beaker with added water. This beaker was weighed with a very accurate lab scale of the 0,1mg resolution kind. Yes, one with glass surrounding it so that air flow won't affect the readings. In the end, the only practical (tiny) difference between the two scales was only at the very low end of the scale, when weighing few tens of mg. The 100mg you mention is probably the lowest amount that can be weighed with reasonable accuracy. Anyway, if you ever need better accuracy you can spend a bit more and buy a cheap 1mg resolution scale, or use solutions of known concentration.

IMHO, obsessing over weighing accuracy doesn't really make much sense if you take into account that the chemicals you use might not be as pure as you think they are. Or if they're hygroscopic and pick up water from the atmosphere if not stored properly. Or if you're wearing something woolen and static electricity alters the readings when you're getting close enough (yes, I've seen that happening when weighing very small amounts). Or...

grahamp · Feb 19, 2017

I have one unit that offers 500g to 0.1g reported, and another that does 100g to 0.01g reported. The smaller one gets used most. The cheap electronic top pans are not too sensitive - great for handling vibration and air flow, but a bit annoying when making that last fine adjustment and you have to wait for the balance to settle

I was taught fine weighing on mechanical 4 and 5 decimal place balances in climate controlled rooms with solid slate topped benches. Then in another lab we were doing bucket chemistry where weight/volume to the nearest gram or millilitre was fine. It all depended on the constraints of the process - if the analysis was to be in parts per million or billion, we had to start at a comparable level of preision. If it was just a rock stain, the concentration did not have to be that precise - the process would not show a difference.

mshchem · Feb 19, 2017

Here's my 3 most used units. I have an analytical balance, but I don't do much it.

mshchem · Feb 19, 2017

PS, I grouped these together for a photo. The two small balances are stored in a cupboard when not in use. I have a neat lab table where I can weigh my chemicals. That big solution balance works great for making straight hypo solutions. You can look up specific gravity of solutions. Weight out the dry hypo, put a large plastic container on and tare it.
Then dump in the dry, add water until you hit your target weight.
Best Mike

Patrick Robert James · Feb 19, 2017

I have two different electronic scales, but I mainly use the one small one I have that is good up to 300g in .1 gram increments, though I wouldn't trust it to weigh just that. Solutions are the way to go as others have mentioned. KBr is so cheap just mix up a solution in water when you need only a little. For Phenidone I use Dimezone-S which is supposed to last longer in solution and mix it in Propylene Glycol as others have mentioned. I use so little of it that the solution I have currently is a few years old (maybe 5?) and I haven't noticed a single problem with it.

mrosenlof · Feb 20, 2017

In my experience, phenidone disolved in propylene glycol keeps for months at least. Maybe longer, but I tend to mix up enough that I use it up in less than a year. The key is to have no water in the solution. That's when phenidone starts to go bad.

Phenidone will also keep well dissolved in denatured alcohol.

Dry Chemical Scale

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