Please check my calculations for this solution

[CharapitsaS]

Parts per million (ppm) is a dimensionless quantity.
When specifying the concentration of the analyzed compounds in a standard solution in ppm, it is always necessary to specify the dimension of the amount of the introduced compound.
For example, volume or mass.
The minimum uncertainty of the concentration of the analyzed compounds in the prepared standard solution can be achieved on the basis of the weighing procedure.
To obtain the dimension of the concentration of the analyzed component, for example, mg/liter, it is sufficient to multiply the concentration value expressed as ((compound mass, mg) / (standard solution mass, mg )) by the density of the prepared solution, (kg/liter).
The procedure for preparing standard solutions of volatile compounds in water-ethanol solutions is presented in Commission Regulation (EC) No 2870/2000 quite clearly, (https://eur-lex.europa.eu/eli/reg/2000/2870/oj )

Best regards,
Siarhei Charapitsa

[Consumer Products Guy]

I think some of you folks are way too anal about a straightforward task.

I think weighing volatile liquids at room temp is a worse idea, but maybe I'm wrong. What do you think?

We did this daily. We would tare the volumetric flask and use a disposable transfer pipet to dispense INTO THE BOTTOM of the volumetric flask and weigh, then immediately add some of the solvent, then mix and dilute to volume. Try it on the electronic analytical balance, weight will be stable.

Some add some of the solvent first into the volumetric flask, then proceed as above. Try it - the weight will be stable.

In the "old days" of mechanical balances, we would weigh liquids by difference in a capped disposable syringe, but by the early 1980s we got an electronic analytical balance which made weighing much faster.

[mrbackflush]

I think some of you folks are way too anal about a straightforward task.

I think weighing volatile liquids at room temp is a worse idea, but maybe I'm wrong. What do you think?

We did this daily. We would tare the volumetric flask and use a disposable transfer pipet to dispense INTO THE BOTTOM of the volumetric flask and weigh, then immediately add some of the solvent, then mix and dilute to volume. Try it on the electronic analytical balance, weight will be stable.

Some add some of the solvent first into the volumetric flask, then proceed as above. Try it - the weight will be stable.

In the "old days" of mechanical balances, we would weigh liquids by difference in a capped disposable syringe, but by the early 1980s we got an electronic analytical balance which made weighing much faster.

OK; perhaps using weight and purity, and using a calibrated balance and a volumetric flask, might be the best way.

[rb6banjo]

Trust us. It is the best way. Several of the folks who have chimed in on this have 10's of years of experience.

The only time this approach fails is when the analyte is so volatile that it boils in the flask or transfer pipette. When I have to make acetaldehyde standards, I will add some solvent to the volumetric flask, tare that flask+solvent on the balance, then add the AA to the solvent. The AA gets trapped in the solvent and the net increase in mass is the mass of the AA added so I know what it is. Even if some vaporizes during the transfer, I know how much I added. No density required...................

[mrbackflush]

Trust us. It is the best way. Several of the folks who have chimed in on this have 10's of years of experience.

The only time this approach fails is when the analyte is so volatile that it boils in the flask or transfer pipette. When I have to make acetaldehyde standards, I will add some solvent to the volumetric flask, tare that flask+solvent on the balance, then add the AA to the solvent. The AA gets trapped in the solvent and the net increase in mass is the mass of the AA added so I know what it is. Even if some vaporizes during the transfer, I know how much I added. No density required...................

Thanks...but isn't this the method of standard prep you are suggesting for all analytes? As in: adding by weight (using purity for correction) into a tared flask with solvent added to trap the volatile reagent liquid standard? And yes, one of my standards is acetaldehyde. I keep it in the freezer, and have been adding it by volume. I'll try adding by weight next time.

[CharapitsaS]

Perhaps you will find the following methodological approach useful https://doi.org/10.1007/s11694-023-01868-x

[mrbackflush]

Perhaps you will find the following methodological approach useful https://doi.org/10.1007/s11694-023-01868-x

Thank you for your suggestions!

[rb6banjo]

Trust us. It is the best way. Several of the folks who have chimed in on this have 10's of years of experience.

The only time this approach fails is when the analyte is so volatile that it boils in the flask or transfer pipette. When I have to make acetaldehyde standards, I will add some solvent to the volumetric flask, tare that flask+solvent on the balance, then add the AA to the solvent. The AA gets trapped in the solvent and the net increase in mass is the mass of the AA added so I know what it is. Even if some vaporizes during the transfer, I know how much I added. No density required...................

Thanks...but isn't this the method of standard prep you are suggesting for all analytes? As in: adding by weight (using purity for correction) into a tared flask with solvent added to trap the volatile reagent liquid standard? And yes, one of my standards is acetaldehyde. I keep it in the freezer, and have been adding it by volume. I'll try adding by weight next time.

You can do it this way with all analytes. It's just not as critical for things that don't vaporize rapidly - like your C5 alcohol. For acetaldehyde it's pretty much the only way you can do it.

[mrbackflush]

Trust us. It is the best way. Several of the folks who have chimed in on this have 10's of years of experience.

The only time this approach fails is when the analyte is so volatile that it boils in the flask or transfer pipette. When I have to make acetaldehyde standards, I will add some solvent to the volumetric flask, tare that flask+solvent on the balance, then add the AA to the solvent. The AA gets trapped in the solvent and the net increase in mass is the mass of the AA added so I know what it is. Even if some vaporizes during the transfer, I know how much I added. No density required...................

Thanks...but isn't this the method of standard prep you are suggesting for all analytes? As in: adding by weight (using purity for correction) into a tared flask with solvent added to trap the volatile reagent liquid standard? And yes, one of my standards is acetaldehyde. I keep it in the freezer, and have been adding it by volume. I'll try adding by weight next time.

You can do it this way with all analytes. It's just not as critical for things that don't vaporize rapidly - like your C5 alcohol. For acetaldehyde it's pretty much the only way you can do it.

I'll give it a whirl. Thanks!

[MichaelVW]

If there's any doubt after you've tried the above method, $25 will get you a reference standard to check against.

[Peter Apps]

As a final tip, instead of spending ages getting an exact target addition (which allows more time for things to evaporate) just get it fairly close (most easily by adding a target volume from an adjustable autopipette), record the actual weight additions, stick them in a spreadsheet, calculate your actual final concentration and then use that instead of the target concentration in your calculations.

[CharapitsaS]

When determining the quantitative content of volatile compounds (VC) in water-ethanol solutions on gas chromatographs with a flame ionization detector, the following algorithm can be used:

(Concentration of the i-th VC, [mg/l AA]) = (peak area of the i-th VC) / (peak area of ethanol) * (ethanol density = 798270, [mg/l]) * RRFi.

For example, for HP chromatographs some RRFi (Relative Response Factor, relative to ethanol) values are given below.
Compound RRFi
Acetaldehyde 1.270
Methyl acetate 1.715
Ethyl acetate 1.299
Acetal 1.685
Ethanol 1.000
Methanol 1.416
2-butanol 0.766
1-propanol 0.784
2-methyl-1-propanol 0.666
1-butanol 0.705
2-methyl-1-butanol 0.648
3-methyl-1-butanol 0.634

To represent the concentration value in [mg / l], the previously obtained value must be multiplied by the volumetric content of ethanol (v/v) in the test sample.

Best regards,
Siarhei Charapitsa