Questions about How Blue is your Sport Drink?

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IMKudaimi
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Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Hello,
I'm really having a tough time with 2 things. First, I don't completely understand how to find the total concentration of dye from the light absorption. Second, what are the units for the equation, especially the molar absorption coefficient?
Thanks, IMK
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

I’m assuming that you are working on the project described here:

https://www.sciencebuddies.org/science- ... inks.shtml

To calculate the dye concentration from the light absorption you have to calibrate your spectrophotometer. This involves measuring the light absorption for solutions with known dye concentrations. These data are used to make a chart showing the straight-line relationship between the dye concentrations and the light absorption values. The chart can be used to find the dye concentration in sports drink sample for which you’ve measured the light absorption.

The units for the variables included in equation 1 are given in the background section of the project. The unit for the molar absorption coefficient is Liters/(mole X cm).

I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Thank you.
A couple of other things.
So when you find the Molar absorption coefficient, you multiple the liters of the drink by something? Or am I understanding it wrongly?
Also, so you mean for the concentration, you compare the sports drink light absorption to the other dye concentrations' (the ones you put food coloring in) light absorption? If it's the same light absorption, it's the same total concentrations?
Lastly, after subtracting the resistence of the water from the average resistance of the drink, and put that on the y-axis, do you put the concentration levels on the x-axis or something else? Or am I missing something there too/

Thanks, IMKudaimi
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

And one more thing, when measuring the path length in cm, is it just till it reaches the solution or from the resistor to the LED? And what is the x in moles x cm? I understand moles per liter for concentration but not moles x cm for the molar absorption coefficient?
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

The molar absorption coefficient is a factor in the Beer-Lambert equation (equation 1 in the project background). But you don’t calculate the coefficient’s value in the project. The calibration curve that you make according to the project procedure includes the molar absorption coefficient and path length so that you don’t need to “find” either factor.

Take a look at equation 1. The molar absorption coefficient is a constant for a given chemical (the blue dye) and light wavelength. The path length is also a constant for a given cuvette size (the container for your dye solutions). Because these two factors are constants in this experiment, the light absorption (A in equation 1) only varies if the amount of dye in the solution changes (the concentration, or c in equation 1).

Concentration refers to the number of moles of the dye per liter of solution. Let’s say that the path length is 1 cm. The light absorption has no units. So the units of the molar absorption coefficient must cancel the concentration (moles/Liter) and the path length (cm) units. Thus the coefficient has units of Liters/mole multiplied by 1/cm. This also can be written as Liters/(mole X cm). Here the “X” means “multiply by”.

The purpose of the calibration curve is to show the relationship between the concentration of the blue dye and the absorbance (resistance) of the dye solution. Once you have this relationship you can use it to find the concentration of blue dye in “unknown” solutions like the sports drinks.

The calibration curve is a graph of the dye concentration on the x-axis and the resistance on the y-axis. Your calibration curve should be a straight line. Find the resistance value you measured for a sports drink sample on the y-axis of the calibration graph. Next, move along a horizontal line from the resistance value on the y-axis to the point where the horizontal line intersects the calibration curve. Now move along a vertical line to the x-axis. The intersection at the x-axis is the concentration for your sports drink sample.

I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Thank you very much and just 2 last things:
1. Does the higher the resistance, the higher the concentration come into play here (is that true)? Does that mean the trend line will be up with the higher concentration and resistance?
2. And what if the calibration curve cannot be in a straight line because of the data you found?

Thanks, and this is probably all for now,
IMKudaimi
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

Higher concentration of dye in the solution causes higher resistance in your spectrophotometer’s photoresistor. The resistance is directly related to light absorption so higher concentration also means more light absorption. And yes, the calibration data will show an upward trend.

The calibration data should “fit” a straight line. That means that you should be able to draw in a straight line that is positioned so that your calibration data points are as close to the line as possible. It’s OK if some of your data lie above and below the line. Just try to draw in a line with the data evenly distributed around it. This is called a “best fit” line and it represents the average relationship between the independent variable (dye concentration) and the dependent variable (resistance).

An example chart that shows a “best fit” line for a data set can be viewed at this link:

http://serc.carleton.edu/mathyouneed/gr ... stfit.html


I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Thanks you, and one very last thing:
When you write your calibration curve, do you label whenever you hit it while moving horizontally, do you label the drink? Also are there any labels for the solutions with known concentrations on the calibration curve?
Thank you very much, IMKudaimi
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

I’m not sure what you mean by “label”. When you make an x-y chart the usual practice is to plot some kind of marker or symbol for each data point. If this is the type of label you are asking about, I think you should mark your calibration data this way.

You could also put text identifiers next to some or all of the data points. I think that text labels next to your “unknown” sports drink data could be a good way to display your results. But you may find that too much text on your chart looks “cluttered”. Another option is to use different symbols to plot the data for the “unknown” drinks and include a legend on the chart to identify the various symbols. Or you could make a separate chart to display the results for “unknown” drinks. For example, a bar chart showing the dye concentrations you found might work well.

I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Thank you,
I've run into a problem. Each time I put my dye samples with known concentrations, all of them say 0.07. I know that's impossible. What could I have done wrong?
IMKudaimi
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

I’m sorry that you are having a problem with your experiment. Some items you might check are as follows.

1. Is your multimeter set up to read resistance? You might want to refer to the link below for some details on how to do this.

https://www.sciencebuddies.org/science- ... resistance

2. Does your photoresistor respond to changes in light? You can check this by measuring the resistance is a darkened room (or with the photoresistor covered). The resistance should be very high – in the mega ohm range. Now measure the resistance with the lights on. The resistance should be much lower.

3. Are all the wires on your breadboard correctly placed and securely connected? You might want to re-check each of your connections.

I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Hello,
I bought a new multimeter since the other one didn't work and for some reason all the sample solutions say 0.3. I was told that the multimeter is working as long as it does not display a 0. But is it really possible for all the solutions to have the same resistance? And if so, how can I make my calibration curve?
Thanks, IMKudaimi
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hi IMKudaimi,

Your calibration solutions should not have the same resistance readings. Rather, higher dye concentrations should give greater resistance readings.

Is your multimeter set to read resistance? If so, you may need to adjust the range of the meter. There’s more detailed information on setting up the multimeter in the tutorial (see the link in my previous post) and in steps 4 through 10 in part 1 of the project procedure.

Are you making resistance measurements for your solutions with the spectrometer circuit covered by a cardboard box to block ambient light? If not, “stray” light from the room may be interfering with your resistance measurements (see step 6 in part 1 and step 15 in part 2 of the procedure).

Does your photoresistor change resistance when the amount of light changes? Steps 4 through 10 in part 1 of the procedure describe one way to test the photoresistor. The resistance should be high when the photoresistor is in the dark and low when the photoresistor is in the light.

I hope this helps. Please post again if you have more questions.

A. Norman
IMKudaimi
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Joined: Mon Dec 05, 2016 9:21 pm
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Re: Questions about How Blue is your Sport Drink?

Post by IMKudaimi »

Thanks, I was finally able to get it to work.
But there's one thing left: If I turn to 2000 ohms, then what decimal place is it? Is 794 actually 7.94?
Thanks, Idrees
norman40
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Re: Questions about How Blue is your Sport Drink?

Post by norman40 »

Hello Idrees,

Glad to hear that your experiment is working now.

When you have your multimeter set to the 2000 ohms range a resistance of 794 ohms will display as 0.794 kilo ohms. In other words, you move the displayed decimal one place to the right for each zero in the range to get the resistance value in ohms.

The following link shows some examples of resistance readings with different range settings.

https://learn.sparkfun.com/tutorials/ho ... resistance

Good luck with your project. Please post again if you have more questions.

A. Norman
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