Co(II) was used as an internal standard to analyze a sample of Ti(IV) with atomi

Question

Co(II) was used as an internal standard to analyze a sample of Ti(IV) with atomic absorption spectroscopy (AAS). A standard mixture containing 1.00 g Co/mL and 2.37 g Ti/mL measured by AAS produced a signal-to-signal ratio of 1.94 Ti: 1.00 Co. A mixture was prepared by combining 4.00 mL of a Ti(IV) solution of unknown concentration with 3.00 mL of a 14.0 g/mL solution of Co(II). The absorbance of the mixture at the Ti(IV) wavelength was 0.115 and the absorbance at the Co(II) wavelength was 0.220. Determine the concentration, in moles per liter, of Ti(IV) in the original unknown solution.

Explanation / Answer

Answer: The Beer-Lambert law is as follows.

A=Cl ; where A= absorbance, =molar extinction coefficient, C=concentration and l= path length=1

For Co(II), the formula is A1=1C1

And for Ti(IV) A2=2C2

So, A1/ A2= 1C1/ 2C2

1/1.94=1* 1/2.37* 1

1/ 1=1.22

Now, a mixture was prepared by combining 4.00 mL of a Ti(IV) solution of unknown concentration with 3.00 mL of a 14.0 g/mL solution of Co(II).

So, total volume is 7 mL.

Therefore, the concentration of Co(II) is (3*14)/7=6 g/mL.

Now, A1/ A2= 1C1/ 2C2 ; where A1=0.220, A2=0.115 and C1=6 g/mL.

So, 0.220/0.115=( 1/ 1)(6/C2)

Therefore, C2=1.22*6/1.91=3.82 g/mL.

So the concentration of the unknown 4 mL solution is =(3.82*7)/4= 6.696 g/mL.

6.696 g/mL=6.696*1000/(106*47.86) moles per L= 1.40*10-4 moles per L.

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