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 2.24 g Co/mL and 1.26 g Ti/mL measured by AAS produced a signal-to-signal ratio of 2.50 Ti: 1.00 Co. A mixture was prepared by combining 5.00 mL of a Ti(IV) solution of unknown concentration with 4.00 mL of a 12.8 g/mL solution of Co(II). The absorbance of the mixture at the Ti(IV) wavelength was 0.145 and the absorbance at the Co(II) wavelength was 0.221. Determine the concentration, in moles per liter, of Ti(IV) in the original unknown solution.

Explanation / Answer

According to Beer's law,

Absorbance=C*e*l ,where C=concentration of species, e=absorptivity ,l=path length of light

given,

As the signal to signal ratio is fixed for a given mixture,the ratio of concentration can thus be calculated

For the unknown mixture,

AAS signal-to-signal ratio of Ti:Co=0.145/0.221=0.656/1.00

concentration of Co =[Co]=4.00 ml* 12.8 g /mL=51.2g

we can write,[Ti]unknown/[Co]known=5* [Ti]unknown g/51.2g

Absorbance ratio of Ti :Co=0.145/0.221=0.656/1.00

So, 5* [Ti]unknown g/51.2ug =0.656/1.00

or,[Ti]unknown g=0.656*51.2 ug/5.00 =6.707 ug

total volume of solution=4ml+5ml=9ml

[Ti]unknown=6.707ug/9ml=6.707*10^-6g/0.009L=0.745*10^-3g/ml=(0.745 *10^-3g/47.867g/mol) per L=0.0155*10^-3moles/L= 1.55*10^-5 mol/L   

[molar mass of Ti=47.867g/mol]

[Ti]unknown=1.55*10^-5 mol/L

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