Measuring small quantities without a balance?

I've been frustrated in my attempts to use inexpensive balances that

are sufficiently accurate/precise enough for subgran quantities. I'm

through with balances!

 

Many chemical quantities are specified in volumetric terms, such as

teaspoons, tablespoon, etc.

 

How are people achieving consistent results measuring out these

quantities? It's my understanding that most powder-based chemical

agents used in the preparation of photographic chemicals have bulk

moduli that vary up to 30% (in other words, Tuesday's teaspoon could

include 30% mass more than Wednesday's teaspoon).

 

What's the secret?

I dunno. Google? Here's one:

 

http://scientificsonline.com/product.asp_Q_pn_E_3037535

Certain chemicals, especially those with significant effects by small errors, can be mixed in 99% alcohol (benzotriazole is one) and then dosed easily into your mixtures. Personally, I use a reloading balance that is meant for firearms reloading and is good to 0.1 grain. My recipes are kept on spreadsheets and I add an extra column with the gram to grain conversion done (15.4324 grains = 1 gram). I feel that I'm getting easily reproduceable results this way and these scales are very inexpensive, especially used. Mine is an RCBS. I have a digital scale for larger volume weighing that is supposed to be good to 0.1 gram but I trust the reloading scale more for the truly small amounts.

Craig,

 

I have that same RCBS balance. In fact, I've gone through two!

 

Despite using every best practice I can find, the best precision I could find was about 0.15 g (the first was only to 0.20g!). I know that based on comparisons with a $12,000 (yup, you read that right) electronic balance that I temporarily had access to (and was operated by a technician working for a biochem company) for the comparison.

 

I can live with that imprecision when I'm using 3g/L of Metol for a solution but not when I'm using 0.25 g of Phenidone.

Weights need to be reproducable, but not absolutely precise. Every measurement every time needs to be the same and you calibrate to that.

 

I have a $12 balance from 1960 and recently aquired a balance that looks like it came from a 1905 drug store, an amazing antique. Results are the same as a factory pack of D76 with either.

 

If you have the money go for one of the analytical balances that are glass enclosed. A new car would be a better deal though. Then all you have to worry about is the potency of your chemical stock. Some oxidise, some pick up water, etc.

Ron,

 

Reproducability is the problem I have. If it were one of accuracy - I could calibrate as you suggest. But the measurements using these balances routinely vary by 0.15g or more - so I can't be at all sure what I've got regardless of what the balance may read.

Hi Al.

 

As Craig suggested, you can make up a % solution of phenidone in alcohol/glycol and measure from that with a measuring syringe for very consistent results. You could do the same for metol, hydroquinone, BZT, pyrogallol, catechol, amidol, ascorbic acid, etc. I keep % solutions of many of the developer components I use regularly. The glycol for a liter of 1% phenidone solution is far more expensive than the phenidone, but measuring 10g is more practical than measuring fractions of a gram, and if you need precision and consistency, you'll have it for a long time.

 

Jay

Thanks Jay!

 

Which glycol is that - propylene or ethylene?

I actually got better precision from Lee's cheaper gunpowder scales. They look like cheap crap compared with RCBS but the mechanism provides better precision.

Al... A couple of years back, I ran a Gage R&R for a project at work. One of the variables was in our weighing method for some components that varied from about 1 gram to 400 grams. I was surprised at the variation with some of the scales, even extremely expensive glass-encased, quarterly calibrated units. What we tended to see was that certain scales did well at repeatablilty at higher weights and some at measuring smaller amounts. We also included an older model (oil-damped) reloading beam and were quite amazed at how well it did, especially on the smaller amounts. I imagine that dynamic leveling/alignment differences from different location placements of the unit could add some variability from one session to another. Maybe I should be more suspicious of my little RCBS!

 

While we're discussing weighing issues, I might comment on a good system to consider for weighing out components. I realize this might seem obvious to many people but it wasn't to me until someone demonstrated.... Place the source vessel or temporary source (cupcake papers work well) onto your digital scale. Tare and then measure in the negative. Use your lab spoon to remove the material amount needed and transfer to the receiving vessel. You can't over-add what never came out of the source vessel. You can then tap your spoon on the receiving vessel to deal with the small amount of clinging material, too. I do lab work with upper level diamond abrasive resins/slurries, etc in my work and this is how we handle measuring very expensive components to minimize ruining an entire batch by trickling too much of something into our mixes.

All good answers, but something is wrong with somebodys technique. If the error is .15g and you're weighing .25g of phenidone, that's more than half the amount! You could see that something was amiss. Are you swinging the balance, or letting it stop? It should swing freely in equal amounts on either side of zero. Correct technique is not to let it come to rest, unless it has so much magnetic damping that you have no choice. My big old Harvard trip balance (the kind with two big ceramic pans) is consistant to .1g, a powder balance should be consistant to a tenth of that. Look for somthing contacting that shouldn't, damaged bearings, air flow from vents, and don't breath on the thing while using it! If all that fails, using standard percentage solutions is certainly an even more accurate way to go.

Conrad,

 

Great post. Yes, I am letting it swing freely. Both balances demonstrated a behavior that when allowed to swing freely they slowly came to rest at a position that did not match the marker.

 

That sort of behavior is known as overdamping (vs. critical or underdamping) and seemingly results from mechancial friction other than magnetic dampening. The Lyman balance I used to own demonstrated a similar problem.

 

The rather expensive mechanical balances I used in college and grad school always exhibited slight underdamping, so I can only assume that lightning has managed to strike me three times with defective balances.

I use a Cent-a-gram balance (magnetic dashpot) which so far is reliable to its 0.01 gram marking. Got it for $20 in a flea market. If you can find one used they're a fine balance.

Hi Al.

 

I use propylene glycol from:

 

http://www.chemistrystore.com/propylene_glycol.htm

 

I would reiterate that making up a PG/phenidone solution increases the cost of your phenidone several times. A liter of PG costs about $12 w/shipping, while 10g of phenidone (for a 1% solution) costs about $2. It's more economical to make up a 10% solution, but then your measuring out fractions of a ml. In the end, if you demand precision, you have to pay for it, one way or another.

 

Jay

i have 2 ohous scales, one is supposedly good for 0.01g, and a triple beam balance that is somewhat less accurate.

at the time ohous technicians suggested that I do not get the models wit the spring/dial mechanism as it was less accurate than the straight beam and slider scales.

 

so the powder/reloadind scales will likely do well.

I have an old Mettler P1000 lab scale which I got from an eBay seller but I have not calibrated it yet. The spoon method works fairly well if you are consistent in your practices. I have several sets of spoons so I can measure almost anything that way. There are inexpensive electronic scales which you might consider. I weighed one ingredient with an electronic postage scale and then converted to grams.

 

One thing you might consider is mixing up a larger volume of whatever you are looking to make. This way you can worry less about a very small inaccuracy.

Hello everyone. Do a Google search on "powder scale" and enjoy the fun. Lowest price for 0.1 gram was $28. The sky is the limit! Bill

In my experience, Tuesday's teaspoon has never been as far off as 30 percent from Wednesday's, if you're spooning from the same jar of chemical. More like a few percent for most measures. So repeatability is good, even if accuracy isn't. It may matter here that my spoons are deep and narrow, which means I have less heaping.

Incidently, if a scale is not giving reproducable measurements you

need to clean the pivots, they must be very clean, if they are

chipped or broken, the scale has probably been dropped and needs to

be rebuilt, probably cost more than a new one.

Interesting comments about powder scales.

 

I might point out that if you have a powder scale that really does vary measurement by 0.15 g (which is about 2.25 gr), you'd have a scale that could produce a gun blowup when measuring very light charges of powder for some pretty standard loads -- one of the old standby combinations is 2.7 gr of a particular powder with a particular bullet, which (in a .38 Special case) is almost lost in the cavernous case volume.

 

For whatever it's worth, my Ohaus powder balance is one of the lower end ones with magnetic damping and agate bearings, single beam and a screw advanced weight for fractional grains. It has a maximum capacity of 500 grains, weighs to 0.1 gr (which is just over 6 milligrams) and has an adjustable zero; in my experience, swinging the balance and letting it redamp results in resettling in the same position, to an error of less than half the pointer width (a scribed line, at the end of the beam, less than 1 mm wide) -- corrsponding to a good bit less than .01 gr, or about 0.6 mg weighing error between measurements. In reloading, I've routinely been able to see the effect on the scale of trickling in a single granule of powder a little larger than a crystal of table salt.

 

I'd have to make four weighings for the sulfite to make up a liter of D-76, but damned if I can think of anything affordable that will weigh more precisely (repeatably) or accurately (correctly) than this little scale (which I bought, used, in 1981 for well under $20). An identical model is still available, BTW, with new price under $50.