Chromatography Forum

Hello, first post here but I've browsed around before.

My question relates to the USP definition of what minimum weight means on a balance. I have been under the impression that it means the net weight (weight of the analyte) is the weight the minimum weight applies to. So, if you have a weigh boat that weighs 60 grams but your analyte (say, sodium chloride) weighs 20 grams and the minimum weight of the balance is 50 grams, it is not an acceptable measurement.

I'm asking because some people I've been talking to are under the impression that the minimum weight of the balance is applied to the total weight of boat and analyte (gross weight). In the USP is states explicitly that this isn't the case, but it seems there is no convincing them. Basically, they are saying that as long as your weighing vessel is above the minimum weight of the balance then you can weigh out basically any mass of analyte via weight by difference. So all they are looking at is the weight that is actually read off of the balance that is recorded, not the weight of the analyte.

Can anyone please shed some light on this? Thank you!

My question relates to the USP ....

Can anyone please shed some light on this?

Sorry, I am unable to decipher pretty much everything from the USP.

Mercifully I do not have to worry about what the USP says, but it is a fallacy to claim that pre-loading a balance with a boat somehow increases its ability to accurately measure a further small addition of weight in the form of a sample etc.

The minimum weight specification for a balance is the smallest difference in weights that it can measure with acceptable accuracy. Modern electronic balances are extremely linear and so the measured difference between 0g and 0+xg is the same as the difference between 100g and 100+x g, or in other words if you can't weigh it by putting it straight on the pan, then you can't weigh it in a boat.

There is a loooong thread in the archives somewhere.

Peter