Weighing Procedure

[ajaib5]

Dear All,

Can we tare the sample weight during weighing? We r using folowing method for weighing by tare:

Keeping the butter paper on balance pan, taring it, adding the required amount of sample and then again taring it. After transfering the sample into the volumetric flask we are again placing the butter paper on balance pan to know the weight transferred and recording this weight in lab notebook.

Will it be the correct procedure for weighing? I have read somewhere that taring can introduce the error in weighing.

Please advice.

[Chris Pohl]

ajaib5,

There is nothing inherently wrong with the procedure you describe. There is, of course, an error attributable to taring as well as weighing but I would venture to guess that this error is too small to be relevant to your following analytical data. Measurement error on a good balance is in the range of 2-10 ppm (check the manual of your balance for the specifications or if it isn't in the manual check with the manufacturer for this information). Even if we assume that the taring process doubles the error, this will still be a far smaller error than any subsequent analytical measurement.

[unmgvar]

in the lab i work we perform the operation you describe for "sticky substances" all the time. then this procedure brings less errors.
we especially do it when we perform water content in karl fischer again because we cannot transfer the entire content of the plastic "holder" into the glass cup.

[MK]

Did i get this right? you can't transfer all from the paper and you want to know how much left there is?

you can do it by one taring as follows:

-place the paper and tare
-add the material and record reading W1
-transfer the material to the flask
-place the paper back and record reading W2

calculate the result W1-W2

[tom jupille]

I think MK and ajaib5 are saying the same thing, but with slightly different interpretations of the word "tare". That said, I'm not sure why taring comes into it at all. What you are describing boils down to:

1.Put the paper on the balance
2. Put the sample on the paper
3. Weigh sample + paper
4. Transfer sample to flask
5. Re-weigh paper

Subtract 3. from 5. to get weight of sample.

[unmgvar]

ajaib5 is doing as follow

putting pape on wieght
tare
putting sample+paper
tare
putting sample in flask
weighing paper (result is now negative), prints result

this way you don't need to wash the paper.

[syx]

How should we weigh hygroscopic substance? The weight tends to increase during weighing due to moist absorption.

[ajaib5]

The proceudre written by me involves double taring, which in my opinion can introduce the more error as against the single taring at the beginning only.

Moreover the USP descibes the weighing by difference as follows involving single tare.

Taring the B paper
Add materail
Record it
Transfer
rewigh butter paper

Thanks

[unmgvar]

Syx,

for very hygroscopic substance we time the weight procedure.
the analist make sure that the substance is weighted let say in 20 seconds and he make sure that he follows the same procedure for all samples. that way we minimilise the error induced due to water absorption.

even if he has finished weighing in less time he waits for the ful 20 seconds. again we follow this procedure especially for water content tests. that is the best way that we get reproducability between std test and also minimal deviation between samples.

[pawan ratra]

Hygroscopic materials can be weighed in the moisture controlled environment maintained in the Glove box.

[syx]

What about this:
1. Keep volumetric flasks those will be used without cap in the desiccator for a certain time
2. Close and take the flasks from the desiccator
3. Tare flask with the cap
4. Weight the substance directly into the flask
...

[unmgvar]

to me it looks sound for hygroscopic substance.
but you have to be sure that you do not touch the top of the neck of the flask. you risk stucking substance where the cap is.
this would be true especially for small flask where you generally also weight less. so the margin error gets bigger.
be carefull about that.

[rnelson]

When weighing substances, I think you need to be consistent with how you perform other weighings.

If you were to do a weight check, would you
-tare
-place weight on pan
-tare
-remove weight
-record negative weight?

I don't think anyone would weigh a weight this way, so why weigh a sample this way?

To me, it's an extra step that isn't necessary. I would never weigh this way unless for some reason your calibration is performed the same way.

[unmgvar]

rnelson,

there are some substances/products which are "sticky" or
"tricky" and cannot be adequately transfered after weighing to the V.F. or other recipient.
this is simply adding one more step in order to improve the accuracy for a procedure that otherwise would not be sufficient.

for your remark regard concerning calibration well in our procedure we do take it into account. when we check our scales with external standards weight once a month we don't just record the weight of the standard weight we also show that the scale correctly returns to 0.00 mg, this covers when we weight a negative result.

[mpepler]

I recommend using glass weighboats myself. I've worked with some fairly sticky substances but can always wash it from a weighboat. Just put wb on balance, tare, add sustance, record wt, wash into vf.