# Surface tension and concentration

#### shayan825

when we try to measure surface tension of different concentrated solutions of n-butanol with a tensiometer, we have to go from lower to higher concentration and not the other way around. what is the reason for that?

#### epenguin

Homework Helper
when we try to measure surface tension of different concentrated solutions of n-butanol with a tensiometer, we have to go from lower to higher concentration and not the other way around. what is the reason for that?
Firstly what does 'go from' mean? Describe an experimental procedure in such a fashion we will know what it means.

Then try and make a hypothesis yourself.

#### shayan825

Firstly what does 'go from' mean? Describe an experimental procedure in such a fashion we will know what it means.

Then try and make a hypothesis yourself.
We have n-butanol solutions ranging from 0.1 to 0.8 M, which we are supposed to measure their surface tensions. We have to start from the most dilute solution and go up in concentration until we reach 0.8 M. We cannot start from the most concentrated one and go to the most dilute one or just randomly pick a solution and measure the surface tension.

#### epenguin

Homework Helper
We have n-butanol solutions ranging from 0.1 to 0.8 M, which we are supposed to measure their surface tensions. We have to start from the most dilute solution and go up in concentration until we reach 0.8 M. We cannot start from the most concentrated one and go to the most dilute one or just randomly pick a solution and measure the surface tension.
If you were asked to measure surface tensions and your posts were the description of how to do it, would you have any clue what to do?

If you can describe exactly what you do maybe someone can help (unfortunately I have to go now).

#### shayan825

If you were asked to measure surface tensions and your posts were the description of how to do it, would you have any clue what to do?

If you can describe exactly what you do maybe someone can help (unfortunately I have to go now).
we measure it in increasing order by using a tensiometer that has ring.Anyone familiar with that maybe can help? thanks

#### epenguin

Homework Helper
Well I have to guess these were not experiments starting each time completely afresh, the result of a measurement should always be the same. I guess you used the same wire and changed butanol concentrations adding butanol "going up" and diluting it "going down". It has the sound of failure to attain equilibrium. Perhaps as you dilute it is difficult for the butanol that has attached to the wire to dissociate, some always sticks there and equilibrium is not attained. Perhaps even if you dip it in water between measurements. I don't know anything about this but that is an obvious possibility.

How you clean what I read is such a delicate thing I have no idea.

#### shayan825

we did it using du nouy ring method. we calibrated the instrument with water and then measured the surface tension of each solution in an increasing order. the solutions were prepared from before.

#### shayan825

Well I have to guess these were not experiments starting each time completely afresh, the result of a measurement should always be the same. I guess you used the same wire and changed butanol concentrations adding butanol "going up" and diluting it "going down". It has the sound of failure to attain equilibrium. Perhaps as you dilute it is difficult for the butanol that has attached to the wire to dissociate, some always sticks there and equilibrium is not attained. Perhaps even if you dip it in water between measurements. I don't know anything about this but that is an obvious possibility.

How you clean what I read is such a delicate thing I have no idea.
we did it using du nouy ring method. we calibrated the instrument with water and then measured the surface tension of each solution in an increasing order. the solutions were prepared from before.

#### epenguin

Homework Helper
we did it using du nouy ring method. we calibrated the instrument with water and then measured the surface tension of each solution in an increasing order. the solutions were prepared from before.
Did you measure with water in between butanol measurements?

#### shayan825

Did you measure with water in between butanol measurements?
No, we didnt.

#### 256bits

Gold Member
I bet epenguin is correct. My guess is that it is a way to eliminate the error of contamination from previous samples, and to preserve the molarity of the samples to be as close as possible to their expected value.

You do know what M means - moles/L. Butanol has a molecular weight of 74.12g/mole. Approximately a 0.1M solution has 7 grams and an 0.8M would have about 56 grams of butanol. You could calculate more exacting values.

If you wash the device or not, whatever adheres to the device is brought into the next sample.

Estimate how much residue would be left on the device after checking a sample, with no washing, and then calculate how much this residue would change the M of the next sample, if you go up or down.
See if the percentage error due to contamination is different going up from going down -is it less, or more?
Does the percentage error vary as you progress from sample to sample, or is it constant?

It probably does not make that much of a difference for your experiment, but for an exacting experiment it might be something to genuinally consider.

( Also, as a final note, a possible error could come about if you do start at the high value of M, that a drop of the high M liquid that you do not notice clings to the device and does not drop until you measure a lower value, which could just skew things up. )

"Surface tension and concentration"

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