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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: 1stplace on April 19, 2008, 12:44:08 PM

Title: Spectrophotometric Titration curve
Post by: 1stplace on April 19, 2008, 12:44:08 PM

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg76.imageshack.us%2Fimg76%2F8127%2F68191621tv5.jpg&hash=ff93f786ea1b7c13b3f41112106e023905ef5c22)

This is the graph and the previous part to the question that I don't get.

The question is : The spectrophotometer that was used for the titration cannot properly measure differences in light intensity by less than 1%. Using the above data, calculate the minimal concentration of the complex that can be accurately measured.

I really don't know where to start. Can somebody help me?

Title: Re: Spectrophotometric Titration curve
Post by: Arkcon on April 19, 2008, 01:04:34 PM

So the chart should have error bars of 1%.  Given that, and the shape of the graph, what do you think of the chart?  What range is useful?  What do you think of the shape of the graph?

Title: Re: Spectrophotometric Titration curve
Post by: ARGOS++ on April 20, 2008, 05:45:21 PM

Dear Arkcon;

I don’t believe that the “Error-Bars” will be very helpful, because:

Quote from: 1stplace on April 19, 2008, 12:44:08 PM

  .. differences in light intensity by less than 1%.

  -  is not very linear to Absorptions, but Absorptions is “very” linear to Concentrations!
That means that the “’1.0 %’ -Error-Bars” will have a different size over de whole Absorptions scale.

Dear 1stPlace;

As the second Question asks:

Quote from: 1stplace on April 19, 2008, 12:44:08 PM

Using the above data, calculate the minimal concentration  . 

    -   it is required to solve prior the first Question, otherwise you will not have any Concentration result!

To solve the 1. Question you best “see” the Spectrometer as only a little special Indicator!

• -   So, at how much mL’s your “Indicator” has changed its color?
• -   And the remaining you can handle as a normal  “…….”-Titration: It’s Stoichiometry!

Then to solve the 2. Question you need the Mrs. “Beer and Lambert”, because you will have to translate:

• -   Your Concentration and corresp. Absorption you have read from the Graph into an of ε_total!
• -   1.0% Transmittance into its corresponding Absorption (A_1.0%).
• -   Finally you can translate your A_1.0% with the help of ε_total into a Concentration.

Mrs. “Beer and Lambert” with Graphs, Formulas, and Explanations you can find on:

"Corrected Beer-Lambert Law (http://www.p-forster.com/English/themes/Spectroscopy/BASICS/Corrected Beer's Law.htm)”.

I hope this gives you enough help to start your both Solutions.

Good Luck!
                    ARGOS⁺⁺

P.S.:  For Stoichiometry the following page may be of some help:   "Stoichiometry Problem (http://www.chemicalforums.com/index.php?topic=23025.0)”
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