Cobalt was used as an internal standard to analyze a sample of titanium with ato

Question

Cobalt was used as an internal standard to analyze a sample of titanium with atomic absorption spectroscopy. A mixture was prepared by combining a 5.00 mL Ti solution of unknown concentration with 2.00 mL of a 11.5 g/mL solution of Co. The atomic absorbances were measured as 0.136 and 0.221 for Ti and Co, respectively. As a reference, a standard mixture containing 2.08 g Co/mL and 2.01 g Ti/mL was prepared and measured to have a signal-to-signal ratio of 1.72 Ti: 1.00 Co. Determine the concentration (moles/L) of titanium in the original unknown solution.

Explanation / Answer

Absorbance to concentration ratios for Co and Ti is given as -

F = ( A(Ti) / [Ti] ) / ( A(CO) / [Co] )

where F is response factor.

F = ( A(Ti) * [Co] ) / (A(Co) * [Ti] )

Using reference values -

F = 1.72 * 2.08 / 1 * 2.01

F = 1.78

Again we have,

A(Ti) = 0.136 , A(Co) = 0.221 , [Co] = 11.5 ug / mL , [Ti] = ?

F = 1.78

Using,

F = ( A(Ti) * [Co] ) / (A(Co) * [Ti] )

1.78 = 0.136 * 11.5 / 0.221 * [Ti]

1.78 = 1.564 / 0.221 * [Ti]

1.78 = 7.077 / [Ti]

[Ti] = 3.975 ug / mL

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