[CHEKAL]

Having calculated the concentration of a solution using precise mass and precise volume of solvent it calculates to say 4.65M concentration.

However using beers law and UV absorbance with correct molar extinction coefficient i get 5.01M concentration.

What could I put down as the reasoning behind these different values? Could one factor be sample purity? I used a compound direct from sigma-aldrich so it couldn't have that much of an effect on it.

[sjb]

Having calculated the concentration of a solution using precise mass and precise volume of solvent it calculates to say 4.65M concentration.

However using beers law and UV absorbance with correct molar extinction coefficient i get 5.01M concentration.

What could I put down as the reasoning behind these different values? Could one factor be sample purity? I used a compound direct from sigma-aldrich so it couldn't have that much of an effect on it.

Samples from Aldrich aren't necessarily 100% pure. But, even taking that into account, what assumptions does the Beer-Lambert law make?

[Arkcon]

One measurement, or the other, or both, is in error.  Which one?  How exactly did you get a "precise" mass and volume?  Do you mean "precise" in an analytical sense? : http://en.wikipedia.org/wiki/Accuracy_and_precision  Are you certain you've performed all tasks carefully and properly?  Are your instruments in calibration?  It could be your fault, or there could be something very wrong with your chemical or your instruments.  We tend to take things for granted sometimes, until a wide deviation like yours show up.  Then we have to check everything.

[CHEKAL]

beers law assumes monochromatic light is used when actually the spectrometer uses a very narrow band of wavelengths.Light scattering could also have an affect.

by precision i weighed out a mass of compound to 4 d.p and used micropipettes for 1ml solvent. Everything noted was performed correctly and the instrument is fine. Im sure this is in relation to beer's law and absorptivity issues. I just wonder why differences can occur between made-up concentration and beer's law concentration with all sample being fully dissolved.

[Borek]

Google for Beers law limitations.

[Babcock_Hall]

To take just one possible problem, if there is any turbidity present, it will affect the measurement.

[Arkcon]

by precision i weighed out a mass of compound to 4 d.p and used micropipettes for 1ml solvent. Everything noted was performed correctly and the instrument is fine.

Also, please carefully review the Wikipedia page I linked to, and/or check an analytical chemistry textbook.  The definition you give above is not the definition of precision.  Precision means reproducibility -- that you've performed each dilution repeatedly, and each absorbance reading multiple times, and the results don't deviate from each other to whatever statistical standard you care (or are required) to apply.

What you want is accuracy, and you've noticed, by comparing two methods, that you don't have accuracy.  Something is inaccurate -- your analytical method or system, or your calculation and dilution system, or the label on the jar. 

Report to me exactly, what numbers did you get?  Notice:  I ask for exactly, I don't ask for precisely -- the word 'precisely' might work grammatically, but that's not the way analytical chemists talk about their work.

[Arkcon]

Also, check the bottle label -- what purity is specified.  And how was the purity determined?  Can you replicate that analytical method to double check?

[CHEKAL]

Sample: 0.0012g menadione (6.97x10-6 mol) dissolved in 1.5 ml = 0.00465 M

Extinction coefficient= 2540M-1cm-1

Results: absorbance at 333 nm = 1.242, 0.1cm path length works out as 0.00489 M

also im wondering why it increased further to 0.00507 when degassed? is this because carbon dioxide absorbs in that region?

[Borek]

Sample: 0.0012g menadione (6.97x10-6 mol) dissolved in 1.5 ml = 0.00465 M

Initially you mentioned concentrations in the 5M range, now you are three orders of magnitude down.

0.0012g usually means anything between 0.00115g and 0.00125g - its almost 9% difference.

Converting 0.0012 g to 0.00465 M you have miraculously multiplied number of significant digits - that alone should be a signal for you that something is wrong.

[Arkcon]

Results: absorbance at 333 nm = 1.242, 0.1cm path length works out as 0.00489 M

All spectrophotometers are different, and you can check the manual.  However, as a general rule, we don't use data from a sample greater than 1.0 aoAU (that's stands for any-old-Absorbance Unit, which is the output of a spectrophotometer, unless you've calibrated it with a standard.)  Briefly, your sample is too concentrated, and you may have lost linearity.  Dilute this very sample by one-half, and you will see, you will not get 50.000% signal, but instead more.  Maybe significantly more.  Dilute again, and you will see a more proportionate signal, until you go so low, you lose linearity again.

[DrCMS]

also im wondering why it increased further to 0.00507 when degassed? is this because carbon dioxide absorbs in that region?

how about that you removed some solvent and made it more concentrated during the degassing.

All in all you have not worked to high precision or stayed within the calibration limits of your instruments, you've got just two data points with different concentration values and you assume it's a limitation of Lambert's law or Sigma Aldrich selling you bad material? The errors are yours alone.

[JGK]

Sample: 0.0012g menadione (6.97x10-6 mol) dissolved in 1.5 ml = 0.00465 M

Extinction coefficient= 2540M-1cm-1

Results: absorbance at 333 nm = 1.242, 0.1cm path length works out as 0.00489 M

also im wondering why it increased further to 0.00507 when degassed? is this because carbon dioxide absorbs in that region?

If you are weighing material on a 4 dp balance, say to an accuracy of ± 0.5%, the minimum amount you should be weighing is 0.05g (50 mg). In order weigh between with any degree of accuracy you need to be using a 6 dp microbalance.

On a 4 dp balance a displayed measurement of 0.0012g can actually be any where between 0.00115 and 0.00124g (before rounding for display), a possible 0.00009 g spread. This equates to ~7.5% of the amount weighed.

5.01/4.65= 1.08. An +8% deviation, which is entirely possible if you factor in possible errors from weighing and the micro-pipettors alone, without even factoring in spectrophotometer errors.