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Katarina Ong
Connor Wilson
Michael Oliver
January 21 & 28, 2014

Decolorization of Crystal Violet (CV)
Experiment Background
A certain dye company needs to know the rate of decolorization of crystal violet (CV)
...
This task will be accomplished by measuring the
absorbance of varying concentrations of crystal violet in a colorimeter to construct a
Beer-Lambert calibration curve
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Using the data for concentration of CV (and the base
solutions) over time will facilitate the determination of the rate law for the reaction of CV
with NaOH
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Materials and Equipment
Equipment

Materials

3 - 1 mL Pipets

1
...
10 M NaOH (​aq)​

1 - Colorimeter

Deionized Water

3 - 3 mL Cuvettes
DataLogger Software (with associated
hardware)
Tissues (for cleaning Cuvettes and
Colorimeter)
Apparatus

1

Method
Initially, 2
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0 × 10−4 M Crystal violet was diluted with 10
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0 × 10−5 M crystal violet solution
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0 × 10−5 M Crystal Violet and 1 mL of 0
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A blank cuvette was filled with deionized water and placed within the colorimeter to
calibrate the device
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0 × 10−5 M Crystal Violet
and placed within the colorimeter, cycling through the wavelength settings to determine
the best wavelength to test absorbance in Crystal Violet (see Table 4)
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The data collected during this time was manual
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The first step in determining the rate law for the reaction of Crystal Violet and NaOH was
to establish a Beer-Lambert calibration curve
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The cuvettes were
loaded into the colorimeter individually, and the absorbance values, as seen in Table 1,
were noted for each one
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The next step in determining the rate law for the reaction of Crystal Violet and NaOH
was to collect absorbance data over time for several different mixtures of Crystal Violet
and NaOH
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Using several graduated cylinders
and 1 mL pipettes, the quantities of Crystal Violet, deionized water and NaOH specified
in Table 2 were added to 3mL cuvettes, adding the NaOH last so as not to start the
reaction prematurely
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The software was set up to record the
absorbance value every second for 6 minutes (or 600 seconds) due to the initial
observations the group made that the Crystal Violet would take several minutes to mostly
decolorize
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2

Data Collection
To select a wavelength for the colorimeter to transmit, the absorbance of a ​3mL cuvette with
2
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The 565nm
setting was used, as the device has higher precision at higher absorbances and the higher
absorbance would allow for a larger Beer-Lambert curve
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104

470

0
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917 *

635

0
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Absorbance (see Figure 1, below)
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0 × 10−5 M
Crystal Violet (mL)

Deionized
Water (mL)

Concentration (M)

Absorbance

3
...
0

2
...
917

2
...
5

1
...
796

2
...
0

1
...
686

1
...
5

1
...
592

1
...
0

6
...
408

0
...
5

3
...
197

0
...
0

0
...
000

Sample Dilution Calculation:

3

For the trials of CV reacting under basic solution, the concentrations of each were
decided using a final volume of 3
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The initial concentration of Crystal Violet was
2
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10 M
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Table 3: Composition of each trial’s Cuvette
Trial
Number

Crystal
Violet
(mL)

NaOH
(mL)
(0
...
00

1
...
00

3
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67 × 10−6
...
00

1
...
00

3
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33 × 10−5
...
00

2
...
00

3
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67 × 10−6
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0 × 10−5
M)

Concentration of
NaOH (M)

Each trial was conducted while recording the absorbance every second
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4

Data Processing
The absorbance data taken in conjunction with the known concentrations of CV were
plotted and a best-fit curve approximated in order to approximate the concentration using
absorbance data (see figure 1)
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These concentrations were then used to plot time with respect to
concentration (figure 2), the natural log of concentration (figure 2a) and the inverse of
concentration (figure 2c)
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Each graph shown uses data from Trial 1
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time had an R​2​ value that was closest to 1
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Also, as crystal violet is first order in the
rate law, one can also conclude that the sodium hydroxide is also first order
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5

Figure 2a: Actual Concentration of crystal violet versus time
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Figure 2c:​ ​Inverse concentration of crystal violet versus time
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Trial 1

Trial 2

Trial 3

Trend Slope (M​-1​s​-1​)

-0
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00369

-0
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999

0
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983

[CV] (M)

6
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33 × 10−5

6
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033

0
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067

The slope of the 1st order graph becomes the “pseudo-k” of the rate law; the average
“pseudo-k” was found to be
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Because the relation between the pseudo-k and the
concentration of NaOH is k ′ = k [OH] , one can infer that the real k is found through
k′
k = [OH]

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044
...
003
, which equals k = 0
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044
1
therefore the full rate law is R = 0
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The rate law was determined to be
R = 0
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The rate law of Crystal Violet and Hydroxide relates to the
reaction CV + OH​-​ → CVOH in that CVOH and OH​-​ are colorless in solution while CV
gives the solution its color
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To arrive at the final rate law, the concentration of CV over time
had to be determined
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To arrive at this curve, absorbance needed to be
collected for a variety of concentrations of CV
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The absorbance curve was then created through a
best fit curve, and in this case the curve was linear
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The experimental data showed that the rate of decolorization increased
when the concentration increased; rate of decolorization with respect to CV (comparing
Trials 1 and 2) and OH​-​ (comparing Trials 1 and 3) , see Table 4
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In the investigation, the concentration of the CV was kept
low enough for the colorimeter to process absorbance
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Therefore, the most appropriate was to
slow the decolorization of crystal is to lower the overall basicity of the solution
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First, the colorimeter has a tendency to stray out of the calibration, and greater
accuracy in measuring absorbance would have yielded more accurate concentration data
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To further the
colorimeter error, when the data is collected through the software (DataLogger), the
absorbance has several significant figures, but when the data is recorded by hand, only
three are shown
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” In some of the trials performed, the
sensitivity of the device itself caused aberrations in the calculated trends, knocking or
jiggling the colorimeter or cord lead to tiny blips in the data
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Another big place in the
procedure that could cause error was in measured volumes later used for concentrations
of solutions
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The devices used, including 1mL pipets and 10mL graduated cylinders, were
difficult to attain a high level of accuracy with, and any bubbles or drips from the pipet
may have affected the accuracy of the solution concentrations
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delivering the volume by buret would have been more accurate

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