Making 100ppm each of heptane & octane in hexane

[Sandra7]

BTW, the prof does want us to use the balance to get accurate weight. It can measure with one decimal place.

most likely (I hope) this is 1 decimal of mg, in other words 4 decimals of grams.

Yes, mg. Sorry again for not stating the unit.

This would be an ordinary analytical balance. The precision of the balance in rough volume terms is 0.1 ul, which is probably about a hundred times better than you can get with a gas-tight syringe. But let's take it that since the prof does not want you to use the balance, then you have to do this by volumes.

Prof. wants us to use the balance. Volume calculation is just to get the ballpark of what to weigh in.

You have a 25 ml volumetric flask, what is the mass of solution that it will contain when at the mark ? (hint, the densities of hexane and the other alkanes are similar enough, and there is little enough of the solutes, that you do not need to worry about the impact of solutes on solution density). Given the known densities of each of the solutes, what volume of each of them do you need to have in the 25 ml final solution to have a mass fraction of 100 mg/Kg ?. Can you measure that volume accurately ?, if not you need an extra dilution step

Keep it simple.

Peter

Yes.

[Peter Apps]

Hi Sandra

Sorry, I mis-read the bit about the prof and the balance.

Ok, so what you do is to do the target volume calculations as I outlined in my last post, and as you make up the solutions you do before and after weighings on the volumetric flask and the syringe each time that you do an addition of anything to the flask. Keep the stopper in the flask as much as possible (see APPS, P.J. and ARCHER, M. 2010. Evaluation of the source of bias caused by losses of solvent vapour during sample preparation. Journal of Accreditation and Quality Control 15: 171–180 for why). Then use the actual weights to calculate actual mass fractions.

With a balance reading to 0.1 mg you need to be weighing as close to your 100 ul syringes full capacity as you can so as not to get readability errors.

Peter

[Sandra7]

With a balance reading to 0.1 mg you need to be weighing as close to your 100 ul syringes full capacity as you can so as not to get readability errors.
Peter

Thanks for making a point about weighing as close to 100ul syringe capacity. That explains why the professor said in the description to use at least 100ul syringe. I know that one group (2 people is a group) decided to use 50 ul syringe because the amount they were weighing was less then 50ul.

I was aiming to get as close as possible to 1000ppm which means weighing 25ul each of C7-C12 compounds. Now, with the information of using full capacity of 100ul syringe, I am tempted to weigh out 75ul each and then do dilutions to eventually get around 100ppm. Or should I get 50ul syringe
instead?

By the way, if I make the starter solution containing 75ul each of C7-C12 in 25 ml of hexane and then save it in the fridge until next Tuesday, would that be a bad idea? I am thinking this option if I don't get to analyze the sample today for one reason or another?

[Peter Apps]

Hi Sandra

The readability issue that I was thinking of is with the balance, not with the syringe. If you have a balance that reads to 0.1 mg you know only that the true weight is between 0.05 mg below and 0.05 mg above the readout weight. For that 0.1 mg range to be an insignificant source of uncertainty (for present purposes 0.1% relative uncertainty) you ideally need to be weighing more than 1000 times the range, i.e. 100 mg. Since your liquids have densities of around 0.7 g/ml, even a 100 ul syringe is a bit too small.

There IS readability issue with the syringe also - what are the smallest divisions marked on the barrel ?, 2 or 5 ul I would guess. You should always line the plunger tip up exactly with a graduation line, even if that is more or less than the exact volume that you density calculations says that you need. You do not need exactly 100 ppm of every solute, or even the same concentration for every solute as long as you know the actual volume:volume fraction or mass fraction that you actually have for each solute you can work out the FID response per gram or per mole.

Peter

[Sandra7]

Since your liquids have densities of around 0.7 g/ml, even a 100 ul syringe is a bit too small.

I see.

You do not need exactly 100 ppm of every solute, or even the same concentration for every solute as long as you know the actual volume:volume fraction or mass fraction that you actually have for each solute you can work out the FID response per gram or per mole.Peter

Yes.

I got those weighed.

[Peter Apps]

Hi Sandra

You are making things unneccessarily complicated again. Why would you want to adjust the weight that the balance gives ?? That is the weight of the heptane that you have in the bottle. And why do you still have the density of water in your calculation ? - there is no water anywhere in the analysis. You could calculate the volume of heptane based on the density, but I cannot see why you would want to do that either.

The only time that you would use density to correct a weight is when you want to correct for bouyancy effects - becuase what you are weighing displaces some air the reading on the balance is a tiny bit less than the true weight in vacuum. At the precision and accuracy that you are working at you do not need to worry about bouyancy.

Peter

[Sandra7]

Hi Sandra

You are making things unneccessarily complicated again. Why would you want to adjust the weight that the balance gives ?? That is the weight of the heptane that you have in the bottle.

Because I need to use the weight to calculate for the ppm (by mass of compound) and then do dilution.

And why do you still have the density of water in your calculation ? - there is no water anywhere in the analysis. You could calculate the volume of heptane based on the density, but I cannot see why you would want to do that either.[/quote

So was I supposed to use the weight given by the balance in calculating for ppm (by mass of compound)?

The only time that you would use density to correct a weight is when you want to correct for bouyancy effects - becuase what you are weighing displaces some air the reading on the balance is a tiny bit less than the true weight in vacuum. At the precision and accuracy that you are working at you do not need to worry about bouyancy.

Peter

[Peter Apps]

Hi Sandra

You are making things unneccessarily complicated again. Why would you want to adjust the weight that the balance gives ?? That is the weight of the heptane that you have in the bottle.

Because I need to use the weight to calculate for the ppm (by mass of compound) and then do dilution.

The readout on the balance gives the weight of what is on the pan - I can only repeat that by weighing the heptane etc you KNOW the weight of the heptane etc. The ppm as a mass fraction(this would all be much clearer if you got into the habit of using proper units) is the weight of the heptane divided by the weight of the solution x one million. There is no need to do any corrections.

And why do you still have the density of water in your calculation ? - there is no water anywhere in the analysis. You could calculate the volume of heptane based on the density, but I cannot see why you would want to do that either.[/quote

So was I supposed to use the weight given by the balance in calculating for ppm (by mass of compound)?

See above, I am at a loss as to why you think that you need to correct the weight given by the balance.

Work through the calculation using proper units of mg/Kg, not the ppm which I suspect is contributing to the confusion.

Peter

The only time that you would use density to correct a weight is when you want to correct for bouyancy effects - becuase what you are weighing displaces some air the reading on the balance is a tiny bit less than the true weight in vacuum. At the precision and accuracy that you are working at you do not need to worry about bouyancy.

Peter

[Sandra7]

The only time that you would use density to correct a weight is when you want to correct for bouyancy effects - becuase what you are weighing displaces some air the reading on the balance is a tiny bit less than the true weight in vacuum. At the precision and accuracy that you are working at you do not need to worry about bouyancy.

Peter

I am clear about calculating for the concentration. Thanks.