Please check my calculations for this solution

[mrbackflush]

I am new to GC, but have much experience in LC. I am also not the best at stoichiometry.
I need to prepare a 10,000 ppm solution of 3-Pentanol. The final volume needs to be 500 mL. Here is what I came up with:
1. Calculate the mass of the solute needed: Mass of solute (mg) = (Desired ppm concentration) x (Volume of solution in mL) / 1,000,000.
(10,000 mg/L)(500 mL)/1,000,000 = 5.0 mg
Volume needed = mass of solute/(purity as decimal) (density in g/mL)
The 3-Pentanol I have is 98% pure, density = 0.815g/mL.
5.0mg/0.7987 = 6.26mL needed in 500mL.
I was taught that density must be included in the calculations when making solutions from liquid solutions. The method I was given uses weight only for making this solution from a liquid.
Thank you in advance!

[rb6banjo]

10,000 ppm is 1% (I assume your solvent is water).

5.0 g to 500 mL is 10,000 ppm.

If the reagent is 98% pure, your final concentration will be 9,800 ppm. If you want to make sure it's 10,000 ppm you need:

5.0*1.02 = 5.1 g of standard to 500 mL

Mass is very easy to measure accurately.

Generally, I think you only need the density in the calculation if you are adding some volume of analyte to some volume of solvent and you want parts-per-million at the end. You then have to convert the volume of analyte to a mass. For instance, if you are adding 5 microliters of an analyte that has a density of 0.861 g/mL and diluting it in 5 mL of methanol, I would say:

0.005 mL x 0.861 g/mL x 1,000,000 µg/g/5 mL = 861 ppm (µg/mL)

[mrbackflush]

Thanks; yes, I need ppm at the end, or mg/L. If I am starting with a reagent solution, and adding to a solvent, I will need to have the density of the reagent starting solution in the calculation. I would think that merely adding the solvent by weight into my solvent would be inaccurate.

[mrbackflush]

Thanks; yes, I need ppm at the end, or mg/L. If I am starting with a reagent solution, and adding to a solvent, I will need to have the density of the reagent starting solution in the calculation. I would think that merely adding the solvent by weight into my solvent would be inaccurate.

[Consumer Products Guy]

I need to prepare a 10,000 ppm solution of 3-Pentanol. The final volume needs to be 500 mL.

The 3-Pentanol I have is 98% pure

If me, I'd weigh 10.2 g 3-pentanol into a 100ml volumetric flask (10.2 g accounts for 98% purity of the 3-pentanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm.

[Consumer Products Guy]

I need to prepare a 10,000 ppm solution of 3-Pentanol. The final volume needs to be 500 mL.

The 3-Pentanol I have is 98% pure

If me, I'd weigh 10.2 g 3-pentanol into a 100ml volumetric flask (10.2 g accounts for 98% purity of the 3-pentanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm.

[mrbackflush]

I need to prepare a 10,000 ppm solution of 3-Pentanol. The final volume needs to be 500 mL.

The 3-Pentanol I have is 98% pure

If me, I'd weigh 10.2 g 3-pentanol into a 100ml volumetric flask (10.2 g accounts for 98% purity of the 3-pentanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm.

Thanks, but why do you recommend I abandon the density of the 3-Pentanol in the calculation?

[Consumer Products Guy]

Thanks, but why do you recommend I abandon the density of the 3-Pentanol in the calculation?

You don't need the density because you'd be WEIGHING the 3-pentanol, not adding it by volume.

Most results are reported in weight percent or weight ppm. Exception would be something like ethanol in beer/wine/spirits and hand sanitizers, or something like rubbing alcohol (2-propanol).

[mrbackflush]

Thanks, but why do you recommend I abandon the density of the 3-Pentanol in the calculation?

You don't need the density because you'd be WEIGHING the 3-pentanol, not adding it by volume.

Most results are reported in weight percent or weight ppm. Exception would be something like ethanol in beer/wine/spirits and hand sanitizers, or something like rubbing alcohol (2-propanol).

I am trying to make a solution of 3-Pentanol in Ethanol at ~25°C. I would like to use the density in the calculations...everyone seems to want to leave this out. I want to add by volume.

[Consumer Products Guy]

I want to add by volume.

If you want your final dilution to be 10,000 ppm w/v:
Then add 12.5ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm w/v.


If you want your final dilution to be 10,000 ppm v/v:
Then add 10.2ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% v/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% v/v or 10,000ppm v/v.

[mrbackflush]

I want to add by volume.

If you want your final dilution to be 10,000 ppm w/v:
Then add 12.5ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm w/v.


If you want your final dilution to be 10,000 ppm v/v:
Then add 10.2ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% v/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% v/v or 10,000ppm v/v.

Thanks - so I guess you're saying adding by volume, regardless of method (volumetric pipette) is a bad idea? I think weighing volatile solutions at room temp is a worse idea, but maybe I'm wrong. I had an instructor in college tell me to use density and purity of the liquid reagent, and add by precise volume. He told me that weighing was only for solids at RT. He also said that if the density of the liquid reagent isn't factored into the equation, it's imprecise. I know people that weigh liquids anyway, and they tell me "all of these densities are close to 1.0 anyway; close enough." I disagree. What do you think?

[mrbackflush]

I want to add by volume.

If you want your final dilution to be 10,000 ppm w/v:
Then add 12.5ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% w/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% w/v or 10,000ppm w/v.


If you want your final dilution to be 10,000 ppm v/v:
Then add 10.2ml of your 3-pentanol into a 100ml volumetric flask (which can already contain some ethanol), that's 10% v/v. After mixing and diluting to volume with your solvent, I'd pipet 50.00ml into a 500ml volumetric flask, and mix and dilute to volume with your solvent. This would be 1% v/v or 10,000ppm v/v.

Thanks - I totally get what you are saying with weighing the liquid. But the density of the liquid is not automatically taken into account when weighing, correct? So I guess you're saying adding by volume, regardless of method (volumetric pipette) is a bad idea? I think weighing volatile liquids at room temp is a worse idea, but maybe I'm wrong. What do you think?

[Hollow]

But then think again, why is the error of pipettes determined by weighing?

If you weigh, then the density is not used because it connects the volume and mass. So check the units as described in the other thread (https://www.chromforum.org/viewtopic.php?t=122227)

Also the density, the volume of the liquid, is temperature dependent, while the mass is not.
Further, except if using a burette, you often don't have a volumetric pipette of the exact volume needed e.g. I've never seen a volumetric pipette of 10.2 or 5.1 ml. Do you have such one?

The point of solvent evaporation during weighing is a valid one but countermeasures can be taken (and once the flask is closed, you can note the exact mass of reagent added. something you don't know with volumetric method; you don't know how much did evaporate...)

But best if you make your own experiments: take an inexpensive solvent like ethanol or isopropanol and dispense a certain volume of your choice in a tared and closeable container and weigh back. Do it 6+ times to see how repeatable and accurate it is. Also test the same thing by direct weighing, then compare your results.

Also with your standard, I guess in the end it will be quantified against the preceeding batch of the standard. So you will see the result of your work. If allowed, prepare two batches, one with each method and compare.

[mrbackflush]

double post - please see below

[mrbackflush]

But then think again, why is the error of pipettes determined by weighing?

If you weigh, then the density is not used because it connects the volume and mass. So check the units as described in the other thread (https://www.chromforum.org/viewtopic.php?t=122227)

Also the density, the volume of the liquid, is temperature dependent, while the mass is not.
Further, except if using a burette, you often don't have a volumetric pipette of the exact volume needed e.g. I've never seen a volumetric pipette of 10.2 or 5.1 ml. Do you have such one?

The point of solvent evaporation during weighing is a valid one but countermeasures can be taken (and once the flask is closed, you can note the exact mass of reagent added. something you don't know with volumetric method; you don't know how much did evaporate...)

But best if you make your own experiments: take an inexpensive solvent like ethanol or isopropanol and dispense a certain volume of your choice in a tared and closeable container and weigh back. Do it 6+ times to see how repeatable and accurate it is. Also test the same thing by direct weighing, then compare your results.

Also with your standard, I guess in the end it will be quantified against the preceeding batch of the standard. So you will see the result of your work. If allowed, prepare two batches, one with each method and compare.

I see what you are saying. Correct me if I'm wrong: you are suggesting that the lack of available volumetric pipettes, coupled with the theoretical fluctuation in ambient temperatures, make using density and adding a reagent solution by volume inaccurate. And I also think you are saying that weighing the liquid, ignoring the density in the calculation, and only using the purity, and furthermore taking precautions to reduce evaporation during transfer, is the best way to make a standard solution from a reagent solution (such as 3-Pentanol).
I had been using a calibrated pipettor for my standard prep. I have over 20 volatile liquid compounds in my mixed standard. My reasoning there is twofold: I could make the transfers faster than weighing (reducing evaporative loss), and thus make my mixed standards much faster than weighing. Also, when weighing multiple standards into the same flask, there is a much greater chance over overshooting the weight.