Composition Measurement
CHEG 2810 Data Acquisition
11/17/09
Ken McFarland & Pacscal M
Abstract:
In this lab we experimented with quantitative analysis of a mixture of antifreeze and
water, starting with pure antifreeze, and washing it out of a four liter container with DI water. In
order to find the mass composition of the mixture we used a refractometer. This required creating
a calibration curve, plotting the degrees Brix of many mixtures of water and antifreeze against
their known mass composition.
The theoretical mass fraction of antifreeze in the tank was calculated at each of the times
that we took samples, using the flow rate through the tank, and the equation provided in the lab
manual. We plotted this theoretical data alongside the experimental values that we calculated
using the refractometer and its mass fraction calibration curve. In this graph we found that the
overall shape of our data curve matched that of the theoretical data, although our values for the
mass fraction of antifreeze were consistently slightly higher from about the halfway point on.
Introduction:
Knowing the chemical composition of a compound to be used is one of the most
fundamental requirements of any chemical process that involves that compound. It can tell you
the concentration of any useful components of the composition, their purity, the identity of the
impurities, if any, and many other useful pieces of information. Finding the exact chemical
composition of a given compound is frequently an expensive, time consuming process. For this
reason, shortcuts are often used when they are available.
In this lab, we used a refractometer to find the degrees Brix of a solution of antifreeze and
water. Degrees Brix is a measurement of grams of sucrose sugar dissolved per 100 grams of
solution. A solution of 25 degrees Brix has 25 grams of sugar per 100 grams of solution, or in
other words, a 100 gram sample of the solution contains 25 grams of sugar and 75 grams of
water. The degrees Brix of a solution is found by a refractometer using the principal of refraction
of light. When light passes through two substances with different densities the light bends. If the
density of one of two materials is known, and the angle that the light bends, or angle or
refraction, is measured then the unknown density of the second material can be found. The
handheld refractometer that was used in this lab was calibrated to compare the density of the
unknown material, our solution, to that of pure water to find the degrees Brix of the unknown
solution.
Results and Discussion:
The mass fraction of antifreeze and the degrees Brix for the solutions we created with
known mass fractions of antifreeze are attached in the appendix in Table 1. These data were used
to create the calibration curve for the refractometer that we used to convert from degrees Brix to
mass fraction antifreeze. The calibration curve is attached in the appendix as Figure 1. The table
of data that we collected from the washout experiment is also attached as Table 2. This contains
the degrees Brix for each of our samples, and the time that the sample was taken at. These data
were converted from degrees Brix to mass fraction antifreeze, and the mass fraction water was
plotted versus time in our graph for the washout portion, which is Figure 2 in the appendix.
For the washout portion of the experiment the flow rate through the tank was kept at a
steady 315mL/minute for the entire time that we were washing out the antifreeze. We used this
figure along with the knowledge that we had 4 liters of antifreeze in our tank to begin with to
calculate the theoretical mass fraction of antifreeze in our washout tank at each of the times that
we took a sample from it, and also plotted those theoretical points on our graph in Figure 2, to
give us some idea of how close to the theoretical correct answer we had gotten for each data
point.
While both our theoretical and experimental data sets both followed the same curve, the
experimental data was over all slightly faster than that of our theoretical data curve, with a
noticeable difference starting at about the 0.5 mass fraction point, and increasing as the mass
fraction of antifreeze decreased, until the end of the experiment, where there is a difference
between the two curves of three to four minutes. This is most likely due to the level of the fluid
in the tank not remaining constant, with more water slowly adding to the tank than was being
drained by the outflow valve, so that it was not immediately obvious that there was a change in
volume, but one that ultimately had a measureable lowering effect of the concentration of
antifreeze in our tank.
Procedure:
We created a calibration curve for the refractometer first, to allow us to convert from the
degrees Brix that the refractometer gave us to the mass fraction of antifreeze in our solution. To
do this, we found the degrees Brix of a number of samples of a known mass fraction of
antifreeze, and plotted them against the degrees Brix of the sample that we got from the
refractometer. This allowed us to find the unknown mass fractions of antifreeze in our samples
for the washout section of the experiment.
In the second part of the experiment, which was the main portion of the lab, we washed
out a four liter container of antifreeze using distilled water according to the procedure in our lab
manual. To wash out a solution is to add simultaneously add water and remove solution from a
container, while holding the overall volume of solution in the container constant, slowly diluting
the solution. To do this we had to insure that the inflow of water from the (adjustable) wall
spigot was equal to the outflow from the (fixed) drain valve. To do this we first filled the tank
with water and then adjusted the flow of the water into the tank until the level of water in the
tank remained constant. Once we had equalized the flow rates, we measured the flow rate of
solution though the tank, which is used to calculate the theoretical concentration of the solution
at any time during the washout.
We drained the tank of water, and filled it with four liters of antifreeze. We opened the
drainage valve at the same time that we started to add water to the tank. For the first eight
minutes we measured the degrees Brix of a sample of the solution draining from the tank every
minute. After the first eight minutes, we measured the degrees Brix of the outflow stream every
two minutes starting at ten minutes. This was done until a total of thirty minutes had passed, and
the degrees Brix of the outflow stream was very close to that of pure water, or 0 degrees Brix.
Finally, we converted the degrees Brix of each sample into mass fraction of antifreeze, and
plotted the mass fraction versus time.
Conclusions:
Our graph of mass fraction versus time for the washout portion of the experiment (Figure
2) shows that the relationship between the two is not linear, rather it follows the exponential
equation given in the lab manual. The theoretical values for the mass fraction that we should
have gotten at each time can also be seen on the attached graph. These show that our data had the
correct shape, and approximately the right curve, however at mass fractions of antifreeze less
than approximately 0.50 there was a noticeable increase of about 0.05 mass fraction antifreeze at
every point.
Recommendations:
We recommend that for best results the tank be left running with only water for a
minimum of at least fifteen minutes without any change in the volume of water in the tank before
the water is drained and replaced with antifreeze. Ideally, the pure water portion of the washout
would be ran for significantly longer than the experiment is expected to take, however that would
not be feasible with the time constraints of this lab. Therefore, we feel that fifteen minutes should
be sufficient, as the washout portion of the experiment took only thirty minutes for us to
complete, and if it is known that the volume of the tank will not change at all in the first fifteen
minutes, is most likely will not change significantly in the second fifteen minutes.
Appendix:
Table 1: Refractometer calibration data
Degrees Brix mass fraction antifreeze
17.4
1
14.4
0.832191781
12.2
0.6
10.6
0.530567686
9
0.472762646
8
0.421875
7
0.375579598
6.2
0.337969402
5.4
0.299261084
5
0.258510638
4.2
0.248466258
4
0.216964286
3.6
0.180803571
2.2
0.144470868
1.6
0.104876996
Table 2: Washout experiment data:
Degrees Brix Time (Min) Mass fraction Antifreeze
17.4
0
1
15.4
1
0.85
14.8
2
0.82
13
3
0.72
12
4
0.66
10.6
5
0.58
10
6
0.55
9.2
7
0.5
8.6
8
0.47
7
10
0.38
5.4
12
0.29
4.8
14
0.26
4
16
0.21
3
18
0.16
2.4
20
0.13
2
22
0.1
1.8
24
0.09
1.4
26
0.07
1
28
0.05
0.6
30
0.03
Figure 1: Refractometer calibration curve
Figure 2: Washout mass fraction versus time