Other
Lab report on spectophotometry
- Course
- Institution
This document is a lab report on spectometry, the experiment was done in a first-year lab for the module 'Chemistry for Biologists'.
[Show more]
Seller
Follow
151317
Reviews received
Content preview
Experiment 3: SpectrophotometryAnne Versluijs, CHU11B01 23/24, 12/10/23
Introduction
This experiment consists of two parts. The aim of part one of the experiment is to confirm the
Beer-Lambert law for KMnO4. The aim of the second part of the experiment is to estimate the
equilibrium constant for the formation of Fe(NSC)2+ from Fe3+ and NCS-.
Part 1: the Beer-Lambert law for KMNO4
Absorption of radiation can stimulate a transition from a low-energy state to one of higher
energy. A photo spectrometer can be used to monitor the absorption of light passing through
a sample at a certain wavelength (Atkins & De Paula, 2018).
According to the Beer-Lambert law absorption of radiation of monochromatic light is directly
proportional to the path length, the concentration of absorbing species, and the extinction
coefficient (“Handbook of UV Degradation and Stabilization,” 2015).
𝐼0
𝐴 = log10 = 𝜀𝑐𝑙
𝐼
The logarithm of the intensity of transmitted light (I) divided by the intensity of incident light
(I0) is the absorbance A. In this formula 𝜀 is the extinction coefficient, c the concentration of
the sample, and l the thickness of the sample.
Part 2: the equilibrium constant for the formation of Fe(NSC)2+
The equilibrium of a reversible chemical reaction is the state in which the forward and
reverse processes occur at equal rates (Flowers et al., 2019). The equilibrium constant for
the reaction between ferric (aq) and thiocyanate ions (aq) is given by:
[Fe(NSC)2+ ]
𝐾𝑐 =
[𝐹𝑒 3+ ][𝑁𝐶𝑆 − ]
Fe3+(aq) + NSC- (aq) FeNCS2+(aq). The reaction has very fast forward and reverse
reaction rates, meaning that an equilibrium is reached quickly (Lahti et al., 1999).
The equilibrium concentration can be determined from the measured absorbance of the
solution using a spectrophotometer (University of Colorado, n.d.). This is made possible
because Fe(NSC)2+ absorbs wavelengths that correspond with blue and green light (458
nm), making for a solution deep red in colour. Fe3+ and NCS- ions do not absorb visible light.
Experimental procedure
Part 1
The materials used were 4 test tubes, 6 cuvettes, a 10 mL and 5mL pipette, and a UV-VIS
spectrophotometer.
This method is based on the method in the ‘Junior Fresh Practical Chemistry for Life and
Health Sciences’ lab manual.
- 4 clean test tubes were prepared and labelled 1 to 4. 10mL of 4*10-4 M KMNO4 was
added into test tube 1 using a pipette.
- 5mL of test tube 1 was pipetted into test tube 2 and 5mL distilled water was added.
- 5mL of test tube 2 was pipetted into test tube 3 and 5mL distilled water was added.
- 5mL of test tube 3 was pipetted into test tube 4 and 5mL distilled water was added.
, - A baseline was run through the spectrophotometer using distilled water in a range of
500-550 nm, with the solutions in test tubes 1 to 4. The absorbances of test tubes 1
to 4 were read at 524 nm.
- Two samples with an unknown concentration of KMnO4 were run through the
spectrophotometer. The absorbance was read at 524 nm.
Part 2
The materials used were 5 test tubes, a 50mL volumetric flask, 4 25mL volumetric flasks, a
25mL, 10mL and 5mL pipette, 6 cuvettes and a UV-VIS spectrophotometer.
This method is based on the method in the ‘Junior Fresh Practical Chemistry for Life and
Health Sciences’ lab manual.
- 5 clean test tubes were labelled ‘a’ to ‘e’. Into each 5mL of a 0.0004 M NaNCS
solution was pipetted.
- To prepare solution A 25mL of a 0.8 M Fe(NO3)3 was pipetted into the 50mL
volumetric flask. 25mL of 2 M HNO3 was added afterwards. The mixture turned light
yellow-orange. 5 mL of solution A was pipetted into test tube ‘a’. The colour of the
solution turned deep red.
- Solution B was made by pipetting 10mL of solution A into a 25mL volumetric flask
and adding 15mL 1M HNO3. 5 mL of solution B was pipetted into test tube ‘b’.
- Solution C was made by pipetting 10mL of solution B into a 25mL volumetric flask
and adding 15mL 1M HNO3. 5 mL of solution C was pipetted into test tube ‘c’.
- Solution D was made by pipetting 10mL of solution C into a 25mL volumetric flask
and adding 15mL 1M HNO3. 5 mL of solution D was pipetted into test tube ‘d’.
- Solution E was made by pipetting 10mL of solution D into a 25mL volumetric flask
and adding 15mL 1M HNO3. 5 mL of solution E was pipetted into test tube ‘e’.
- A baseline was run through the spectrophotometer using distilled water in a range of
450 – 500 nm, with the solutions in test tubes ‘a’ to ‘e’. The absorbances of test tubes
‘a’ to ‘e’ were read at 458 nm.
Results
Part 1
Table 1 Spectrometer results KMnO4 at 524 nm
[KMnO4] (10^-4 mol/dm^3) Absorbance
0,5 0,121
1,0 0,247
2,0 0,494
4,0 0,998
The absorbances measured by the spectrophotometer were collected in a plot using Excel. A
trendline was calculated by the computer program. The formula was added in the lower right
corner.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller 151317. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $5.34. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53022 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now