How to Plot a Standard Addition Curve.

In summary, to determine the concentration of cadmium in seawater, a standard addition calibration plot must be constructed using Excel. This can be done by calculating the initial amount of cadmium in the solution and dividing it by the final volume of the sample. Then, using the data points of absorbance and concentration, a linear regression can be performed to find the slope and intercept values. From there, the concentration of cadmium in seawater can be calculated by plugging in the absorbance value into the equation and solving for concentration.
  • #1
oxshannon
14
0

Homework Statement



The concentration of cadmium in seawater can be determind spectrophotometrically. To six 50.0mL seawater samples, various volumes of a 10.0ppm cadmium solution were added and all were taken through the analytical procedure with the following results:

Amount Cd Added (mL):
0
10
25
40
60
75

Absorbance:
0.230
0.272
0.340
0.416
0.507
0.568

Construct a standard addition calibration plot using EXCEL and calculate [Cd] in seawater in ppm (µg)/mL.\

I honestly have no idea where to start, so any help will be appreciated.
 
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  • #2
Calculate concentrations, plot absorbance vs concentration. Not a rocket science :wink:
 
  • #3
Borek said:
Calculate concentrations, plot absorbance vs concentration. Not a rocket science :wink:

Any chance you'd like to elaborate on that? The stuff in my notes doesn't correspond to what you just said, there's a ton of stuff about dilution factors and what not..

http://science.widener.edu/svb/mathcad/pdf_docs/std_addition.pdf"
 
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  • #4
Calculating concentrations is just about dilution. Amount of substance in the sample doesn't change (principle of mass conservation), what changes is the volume. Calculate initial amount of Cd in the solution that was added, divide that by final volume (50 mL of sea water + volume of Cd solution) - and you have a concentration. This will give you a series of data points - absorbance (given) vs concentration (calculated).
 
  • #5
Borek said:
Calculating concentrations is just about dilution. Amount of substance in the sample doesn't change (principle of mass conservation), what changes is the volume. Calculate initial amount of Cd in the solution that was added, divide that by final volume (50 mL of sea water + volume of Cd solution) - and you have a concentration. This will give you a series of data points - absorbance (given) vs concentration (calculated).

Right, ok so once I get the data points, I can plot this in Excel. But then how do I determine the concentration of Cd in seawater? Because wouldn't that be found using the equation

slope of the line/(molar absorptivity * cell length)?Also.. are you taking into consideration that this is 6 different samples? NOt one sample that the sample keeps getting added to? I'm sorry I'm just really really really confused..
 
  • #6
oxshannon said:
Right, ok so once I get the data points, I can plot this in Excel. But then how do I determine the concentration of Cd in seawater? Because wouldn't that be found using the equation

slope of the line/(molar absorptivity * cell length)?

Once you have data points, you can easily determine the slope. Note that if you have a calibration curve you don't have to worry about molar absorptivity nor cell length - all you need are coefficients in the equation absorbance = k*C + intercept (hopefully intercept will equal 0, making it even simpler).

Also.. are you taking into consideration that this is 6 different samples? NOt one sample that the sample keeps getting added to? I'm sorry I'm just really really really confused..

Yes. Just calculate concentration of Cd for each sample separately. Same procedure each time, just different numbers.

Obviously, when you add 0 mL of Cd concentration in the sample is zero. Show how you calculate concentration in the sample where 10 mL of Cd solution is added to sea water.
 
  • #7
Borek said:
Once you have data points, you can easily determine the slope. Note that if you have a calibration curve you don't have to worry about molar absorptivity nor cell length - all you need are coefficients in the equation absorbance = k*C + intercept (hopefully intercept will equal 0, making it even simpler).



Yes. Just calculate concentration of Cd for each sample separately. Same procedure each time, just different numbers.

Obviously, when you add 0 mL of Cd concentration in the sample is zero. Show how you calculate concentration in the sample where 10 mL of Cd solution is added to sea water.


So then is this answer not right? http://www.cramster.com/answers-sep-11/chemistry/standard-addition-plot-concentration-cadmium-seawater-determind-sp_1491656.aspx?rec=0"

I really am 100% lost and my book isn't helping at all. Ugh.
 
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  • #8
I can't see the answer without registering, and I am not going to.
 
  • #9
Borek said:
I can't see the answer without registering, and I am not going to.

oh sorry I didn't realize you had to.
Okay well here:

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  • #10
Yes, the last section about the need for εb is wrong. That is - it is correct as a part of general case, but here you don't need to know them separately. There is enough information to determine Cd concentration, assuming you will use the same cuvette, the same spectrophotometer and the same procedure.
 
  • #11
Okay great. So I just have a couple more questions...how do I go about finding the slope of the line? And since that last section is wrong, how would I determine Cd concentration?
 
  • #12
Slope for the data set - with Excel. Don't ask me why, search the help for linear regression or something similar.

After you do linear regression you will know values of slope and intercept:

absorbance = slope*concentration + intercept

(hopefully intercept value will be zero or close to zero). Solve this equation for concentration - and all you will have to do will be to just plug absorbance into the formula.
 
  • #13
Borek said:
Slope for the data set - with Excel. Don't ask me why, search the help for linear regression or something similar.

After you do linear regression you will know values of slope and intercept:

absorbance = slope*concentration + intercept

(hopefully intercept value will be zero or close to zero). Solve this equation for concentration - and all you will have to do will be to just plug absorbance into the formula.

Coolio. X intercept or Y intercept
 
  • #14
y = a*x + b, so y=b for x=0. y is absorbance, x is concentration.
 
  • #15
Borek said:
y = a*x + b, so y=b for x=0. y is absorbance, x is concentration.

Okay so is this correct?

y = 0.0558x + 0.1946
absorbance = 0.0558x * concentration + 0.1946

absorbance - 0.1946 = concentration
0.0558

So then I plug the value 0.230 for absorbance into this formula, and solve, getting 0.6344 ppm?
 
  • #16
That would be a correct approach, no idea if these numbers are correct.
 

1. What is a standard addition curve?

A standard addition curve is a graphical representation of the relationship between the concentration of a known analyte and its measured signal. It is used to determine the concentration of an unknown sample by comparing its measured signal to the curve.

2. How do I prepare a standard addition curve?

To prepare a standard addition curve, you will need to create a series of standard solutions with known concentrations of the analyte. These solutions should cover a range of concentrations that includes the expected concentration of the unknown sample. Then, measure the signal of each standard solution and plot it against its known concentration to create the curve.

3. What equipment do I need to plot a standard addition curve?

You will need a spectrophotometer or other instrument capable of measuring the signal of the analyte, as well as cuvettes or other containers to hold the solutions. You may also need a micropipette or other measuring device to accurately prepare the standard solutions.

4. What are the advantages of using a standard addition curve?

A standard addition curve is useful because it accounts for any matrix effects or interferences that may affect the measurement of the analyte. It also allows for more accurate determination of the concentration of the unknown sample compared to a simple calibration curve.

5. Are there any limitations to using a standard addition curve?

One limitation of a standard addition curve is that it can be time-consuming and requires multiple measurements to prepare the curve. It may also be affected by errors in preparing the standard solutions or in measuring the signal. Additionally, the curve may not be accurate if there are non-linear effects present in the system.

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