Hi,

I have a random question for everyone. Any thoughts on this matter would be appreciated. I'm taking 10X, 20X, 50X and 50X serial dilutions of a certain substance. Why is it that when I take all these different dilutions the concentrations I get from each dilution differ sometimes up to and over 10%? Besides human error, which I tried to decrease by taking each dilution 3 times, what would be the reason for such differences in concentration?

Thanks.

Human error (that is imprecise dilutions) are likely to be the major factor (three replicates is next to nothing for most experiments), in addition the measurments might be unsuited for the whole range of dilutions. Maybe the 10X dilutions are measure more precise, wheras very weak concentrations are not.

IIRC, another possible source of error is that for some solvent/solute combo's there are volume changes involved. I other words 1 volume of one solution + 1 volume of another doesn't always equal 2 volumes.

IIRC, another possible source of error is that for some solvent/solute combo's there are volume changes involved. I other words 1 volume of one solution + 1 volume of another doesn't always equal 2 volumes.

Extremely possible. Dilutions go by molarity, which is related to molecular mass, not volume. Volume of chemicals change at different temperatures, and different types of chemicals take up different amounts of space based on mass (gold is dense while NaCl is not).

[edit] Just reread the bolded. 1 volume + 1 volume pretty much always = 2 volumes. Concentrationwise, it won't.

These people don't know what they're talking about. Your problem is simple: You forget to carry the 2. Any other questions?

One example of non linear volume addition which I am personally familiar with is the addition of methanol and nitromethane as a fuel for high performance piston engines. One gallon of methanol added to one gallon of nitro definitely results in a different volume than two gallons. It is often mixed by weight when the resulting concentration is considered critical. :cool:

It is fairly common for volume changes to occur when mixing disimilar liquids. Chem class was a long time ago, but I think even ethanol and water show the effect.

Inaccuressy of your measuring equipment?

IIRC, another possible source of error is that for some solvent/solute combo's there are volume changes involved. I other words 1 volume of one solution + 1 volume of another doesn't always equal 2 volumes.

Spot on! Here's the explanantion;

http://chemistry.about.com/library/weekly/blmiscible.htm