CHEG 281
Experiment 6 --- Composition Measurement
A correct chemical composition is critical for executing chemical reactions and a prime
determinant of product quality. Full, quantitative, chemical analysis is a difficult, time
consuming, and expensive process for most chemical streams (no matter what you see on
CSI). Naturally, shortcut methods are sought, especially for process streams whose
chemical composition is supposedly constant. In this case, simplified methods are often
used to check such a stream and deviations are quickly and cheaply determined.
As you well know (I hope), light travels in a straight line. Its speed is 3*1010 cm/s in a
vacuum. In any other medium, the speed of light is less. Thus, when light travels
between a medium of one density to a medium of a different density, its speed changes.
This change in speed is manifested as a change in angle of the light beam relative to the
incoming beam. The refractive index of a substance is the ratio of the speed of light in a
vacuum to that in the substance and is determined by measuring the change in angle
between the incident and transmitted light beams. The ratio of the sine of the incident
angle measured from the normal to the interface to the sine of the refracted angle
measured from the normal to the interface is also the substance’s refractive index. The
refractive index is dependent on the wavelength of light, which is why a prism splits the
light spectrum into component colors.
The refractometer is an inexpensive analytic device used to measure a substance's
refractive index. The refractometer measures the change in angle as a light beam passes
through a transparent sample and then into a glass of known density. By measuring the
change in the angle, the density of the sample can be computed from the known density of
the glass. The density of sample is dependent upon its composition. Rather than doing
all the necessary calculations to first compute density and then composition, it is much
easier to build a calibration curve for a given refractometer. With a proper calibration
curve, a measurement of the refractive index can be related to a value of the composition
of the sample. Note that refractometers do not display the density of the sample. Rather
they display either the refractive index directly or some measure related to the refractive
index.
In this experiment, you are going to calibrate a refractometer. You will then use this
calibration curve to measure the change in concentration as a material is “washed out” of
a container. The refractometers that we use in this lab are calibrated in degrees Brix
(Brix) rather than directly in angle or index of refraction(Bonus --- what is a degree
Brix?) . You will be using antifreeze and diluting it with distilled water (use the distilled
water tap, not the building water tap). You might want to verify that the refractometer
reads 0 Brix for the distilled water.
Experiment 6 -- Composition Experiment
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CHEG 281
Calibration:
Obtain the experimental solution from the lab instructor. Once you have the concentrated
solution, extract about 30 ml and place in a small beaker. Determine the exact weight of
this extracted aliquot.
1. Measure the refractive index (using the Brix scale) for this solution. To preserve
the solution volume, return any remaining solution from the eye-dropper to the
beaker. (Operating and cleaning instruction for the refractometer are in the lab.)
2. Add about 5 ml of water to the aliquot. Record the weight of the resulting
solution and its Brix value. (The exact amount is not critical. You want to dilute
the solution enough to see the change but not so much that the change is so large
that you can not perform several dilutions.)
3. Repeat step 2 five to ten times or until the Brix reading approaches zero. (As the
solution become more dilute, you will want to increase the amount of dilution
water you add so that you can see the change in refractive index.
4. Compute the weight fraction of the chemical in each of the dilutions.
You now want to plot the Brix reading vs. the weight fraction of your compound in the
aliquot. This is the calibration curve for your solution.
Experiment 6 -- Composition Experiment
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CHEG 281
Washout Experiment:
Once you have a calibration curve for the refractometer, you ready to do the "washout"
part of the experiment.
For this piece of the experiment, you will need approximately 1 L of the antifreeze
solution you used for calibration.
Assume you have a well mixed1 vessel of volume V containing a solution of
concentration Co. At time 0 you start adding water (containing no dissolved material) at
some volume flow rate Q and also extract solution at this same flow rate. Since volumes
are approximately additive, the volume in the vessel should not change. However, the
concentration in the vessel will change. Under these ideal conditions, the concentration C
of the compound in the vessel is given by:
C  Co  e

Q *t
V
Where
V is the volume of the vessel
Q is the volume flow rate through the vessel
t is the time from t = 0
with everything in consistent units.
Set up the tank so that you can fill it with water from the tap and drain from the spigot to
the sink drain. Make sure the stirrer is securely attached and will not interfere with any
hoses. Fill the tank about 3/4 full with water, measure this volume, mark the level on the
tank, and start the stirrer. Open the drain spigot and the feed line valve until the inflow
and outflow are balanced and the level remains constant at your marked volume.
Measure either (or both) the influent or effluent flow rates. Adjust the flows until the
ratio V/Q (the residence time) is between 10 to 20 minutes. (This is a trial and error task
and will take some time and effort.) Let the system run a few minutes to ensure that the
level in the tank stays constant. Record the level and the water temperature.
Now divert the incoming flow for a short while until the level in the tank drops by
approximately 1 L. Then simultaneously add your pure antifreeze solution and re-install
the feed water line. The concentrated solution will start to wash out immediately. Take
several samples of the effluent and measure the concentration of the dissolved material
using your refractometer. Samples should be taken every 2 - 4 minutes until the
concentration in the effluent approaches zero.
Experiment 6 -- Composition Experiment
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CHEG 281
Plot your data in such a way that you can determine Co and V/Q.
Do these values correspond to the values estimated from the known conditions?
If not, give some reasons why not.
It is your responsibility to leave the experimental area in a clean, neat state.
Safety Issues
There are no major safety concerns associate with this experiment. Antifreeze is
relatively harmless, but is toxic if ingested in sufficient quantity. Do not drink any of the
solutions, in this (or any) lab.
The mixer is electric and there is always a potential hazard when electricity is used near
water. Make sure hoses are directed properly and the mixer electronics are not wetted.
Promptly clean up all spills and be careful of wet floors.
1
A well mixed vessel is one in which the composition is the same at any point in the vessel. This implies
that the exit composition is the same as the composition in the vessel. It also implies that any incoming
material is instantaneously distributed throughout the vessel.
Experiment 6 -- Composition Experiment
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