[smileyfrowned]

Hi,

For a Bromophenol blue with a pH of 4 and at room temperature, what would be the extinction coefficient? From the data collected in lab, I got a value around 600, but I'm not sure that this is right.

[Babcock_Hall]

Offhand, I would say that this value sounds low.  Can you walk us through your calculations?  Also, you may be near its pK_a value, which complicates matters somewhat.

[smileyfrowned]

Offhand, I would say that this value sounds low.  Can you walk us through your calculations?  Also, you may be near its pK_a value, which complicates matters somewhat.

Hi,
I redid the calculation and got a number 903.96. Basically, I used the formula A=ecl. Arithmetic dilution of the BPB buffer was performed in the lab. The concentration vs. absorbance was plotted and a linear relationship was found. And the concentration chosen for the calculator was 0.000268596, which has an absorbance of 0.2428 at 600nm wavelength. Do you think my calculation was correct this time? Does the number sounds about right?

[Babcock_Hall]

It would help if you provided units for each number.  If your concentration is correct, then your calculation is correct, but I am not convinced that you have the correct concentration.  Did you look at the CRC handbook for a published value?  Again, one has to be careful with respect to pH.  http://www.chemicalforums.com/index.php?topic=63804.0

Your details are scanty, and I am not sure that I follow what you did.  If you plotted absorbance versus concentration at several concentrations, then it might be preferable to use all of the data.  What were the units along the x-axis of your plot?

[wildfyr]

http://www.sigmaaldrich.com/catalog/DataSheetPage.do?brandKey=SIAL&symbol=318752

Wavelength   613 - 619 nm
Extinction Coefficient   ≥35000

Wavelength   391 - 397 nm
Extinction Coefficient   ≥8000

Wavelength   304 - 310 nm
Extinction Coefficient   ≥13000

    c = 0.005g/L; 0.1N Sodium Hydroxide

Probably L moL-1 cm-1 for units of extinction coefficient.

I bet the unit conversion for the path length is off for OP. pH of 4 is a strange one to choose between its right where the dye is undergoing the transition in acid/base chemistry. Better to choose below 3 or above 4.6 so you arent getting both the charged and uncharged species present.

[Babcock_Hall]

My guess is that the units of concentration are in error.