Question

Using the standard addition method, find Cu:
Cspike = 5.00 mg/L, Y_S+U = 0.491, Y_Uonly = 0.203

Assume the final volumes are the same. Please show all work.

Answer 1

Ans. Given,

            Concentration of spike solution = 5.00 mg/L

            Absorbance of unknown sample only (Yu only) = 0.203

            Absorbance of unknown sample + spiked solution (Y u+s) = 0.491

            Now,

            Increase in reading due to spiking = (Y u+s) - (Yu only) = 0.491 – 0.203 = 0.288

Note: Absorbance of spiked solution must always be greater than that of sample alone. So, u = unknown sample; and s = spiked solution.

Its assumed that the increase in concentration due to adding spiked solution to the unknown sample is equivalent to the “Concentration of spike solution = 5.00 mg/L” in working aliquot (whose reading is recoded).

So, an increase in reading by 0.288 units is equivalent to increase in concentration by 5.00 mg/L.

That is,

            0.288 reading is equivalent to 5.00 mg/L

      Or, 1.0        -           -           -           (5.00 / 0.288) mg/L

      Or, 0.203    -           -           -           (5.00 / 0.288) x 0.203 mg/L

                                                            = 3.52 mg/L

Thus, the reading (or, Absorbance) of 0.203 for the unknown sample is equal to the concentration of 3.52 mg/L.