Question

atomic absorption spectroscopy was used to analyze a sample of ground water for arsenic concentration following the method of standard additions. First,the sample was measured, and found to have an absorbance of 0.435. Next, 10 ul of a standard solution of arsenic(1000ug/ml) was added to 10ml of the sample. This solution 9which contained sample and the standard addition) was measured, and found to have an absorbance of 0.661. What was the concentration of arsenic in the sample?

Answer 1

We know that A= ebC

given A= 0..435 e= molar absorptivity coefficeint and C= concentration of sample

0.435= ebC

C= 0.435/eb

when 10ul =0.01ml , quantity in this sample = 1000ug/ml

0.01ml contains 0.01*1000ug=10ug=10*10^-5 gm

mass of 10ml sample= (10/1000)*C=0.01C, C is concentration of sample

total mass = 10*10-5+0.01C

total volume= 10ml+0.01ml= 10.01ml =10.01/1000L=0.01001L

Concentration =(10*10^-5+0.01C)/0.01001= 10*10^-5/0.01001+C*0.01/0.01001=0.00999+0.999001*C

A= 0.661= eb* (0.00999+0.999001C)

but eb= 0.435/C

0.661 =0.435/C*(0.00999+0.999001C)

0.661= 0.435*0.00999/C +0.435*0.999001

0.661= 0.435*0.0099/C + 0.4345

0.661-0.4345= 0.435*0.0099/C

0.2265= 0.435*0.0099/C

C= 0.435*0.0099/0.2265= 0.019013 g/L