Gravimetric Sample Prep.

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

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I have never prepared solutions gravimetrically. We're looking to do some ICP work with trace metals, so we need to avoid any glass. I just want to make sure I've got this down correctly, if someone would be willing to confirm or provide feedback/suggestions on the hypothetical sample prep below.

Using a 2% nitric acid solution as the diluent, the density was determined using a 10-mL volumetric flask. Say we find the density to be 1.008 g/mL.

Let's say we're making a three point calibration curve of arsenic. We have a 1000 ppm stock solution and want to bring that down to 1 ppm as the highest calibration point. Using C1V1 = C2V2, we determine we need 100 uL of the stock in a volume of 100-mL. So, we would tare our container on the balance, dispense the 100 uL of the stock noting the mass (let's say it's perfectly 1 g), and then dilute until we reach a mass of ~ 100.7 g (99.9 mL * 1.008 g/mL).

Is that correct?

It appears that you're expecting your final solution's density to match that of your stock... so I'm going with a "No".
On the assumption that the diluent has the same density as the stock (or that you know the densities of each of them) then you put your stock into the tared container and record the weight (100 ul is likely to weigh about 0.1 g, not 1g as you have it), then you add more or less the calculated quantity of diluent and record the weight, rather than aiming for a target weight. You then put the weights into a spreadsheet and calculate the actual final mass fraction. Repeat for the other dilutions. It is way quicker and more precise to add approximate amounts and record actual amounts than to aim for exact targets.
Peter Apps
3 posts Page 1 of 1

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