How to Plot a Standard Addition Curve.

[oxshannon]

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.

 

[Borek]

Calculate concentrations, plot absorbance vs concentration. Not a rocket science

 

[oxshannon]

Calculate concentrations, plot absorbance vs concentration. Not a rocket science

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"

 

[Borek]

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).

 

[oxshannon]

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..

 

[Borek]

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.

 

[oxshannon]

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.

 

[Borek]

I can't see the answer without registering, and I am not going to.

 

[oxshannon]

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:

 

[Borek]

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.

 

[oxshannon]

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?

 

[Borek]

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.

 

[oxshannon]

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

 

[Borek]

y = a*x + b, so y=b for x=0. y is absorbance, x is concentration.

 

[oxshannon]

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?

 

[Borek]

That would be a correct approach, no idea if these numbers are correct.