Spectrophotometric Determination of Iron in Aqueous Solutions as a Complex of 1,10-Phenanthroline
Theo Victor Cavite
Chem 28.1
Melanie Bucsit-Carpio
September 14, 2012
Spectrophotometric Determination of Iron in Aqueous Solutions as a Complex of
1,10-Phenanthroline
Introduction
From the previous classical method of concentration determination of an unknown sample, this experiment now deals with the instrumental one. Unknown concentrations of iron in solutions were determined by measuring their corresponding absorbances through spectrophotometry.
A spectrophotometer measures the amount of light that a solution absorbs. This is done by bombarding the sample with a beam of light of known intensity. Basically, this beam is composed of photons that the sample may absorb. As a consequence, the pre-assembled light detector in the machine receives less number of photons. The transmittance (T) relates this detected light intensity (P) to the incident one (Po) by the equation
T = P / P0 (1)
It should be noted that transmittance simply dictates the fraction of light that was able to reach the detector. It is Beer-Lambert’s Law that relates this light transmittance to the absorbance(A) according to the equation
A = - log T (2) Furthermore, Beer-Lambert’s Law linearly relates absorbance with the concentration of the sample by the following equation
A = abc (3) where a is the absorptivity constant, c is the concentration, and b is the path length that the light traverses through the sample. As can be observed from equation 2, absorbance is logarithmically related with transmittance. Hence, transmittance and concentration has an exponential relationship. For convenience sake, Beer’s Law is expressed in absorbance since absorbance is linearly related with concentration. However, there are some limitations with Beer’s Law. First, this relationship only governs at solution with concentration below 0.01 M. Also, it becomes useless when the absorbing species undergo association or dissociation. Lastly, the presence of stray light deviates the measured
Chem 28.1
Melanie Bucsit-Carpio
September 14, 2012
Spectrophotometric Determination of Iron in Aqueous Solutions as a Complex of
1,10-Phenanthroline
Introduction
From the previous classical method of concentration determination of an unknown sample, this experiment now deals with the instrumental one. Unknown concentrations of iron in solutions were determined by measuring their corresponding absorbances through spectrophotometry.
A spectrophotometer measures the amount of light that a solution absorbs. This is done by bombarding the sample with a beam of light of known intensity. Basically, this beam is composed of photons that the sample may absorb. As a consequence, the pre-assembled light detector in the machine receives less number of photons. The transmittance (T) relates this detected light intensity (P) to the incident one (Po) by the equation
T = P / P0 (1)
It should be noted that transmittance simply dictates the fraction of light that was able to reach the detector. It is Beer-Lambert’s Law that relates this light transmittance to the absorbance(A) according to the equation
A = - log T (2) Furthermore, Beer-Lambert’s Law linearly relates absorbance with the concentration of the sample by the following equation
A = abc (3) where a is the absorptivity constant, c is the concentration, and b is the path length that the light traverses through the sample. As can be observed from equation 2, absorbance is logarithmically related with transmittance. Hence, transmittance and concentration has an exponential relationship. For convenience sake, Beer’s Law is expressed in absorbance since absorbance is linearly related with concentration. However, there are some limitations with Beer’s Law. First, this relationship only governs at solution with concentration below 0.01 M. Also, it becomes useless when the absorbing species undergo association or dissociation. Lastly, the presence of stray light deviates the measured
References: DBA. (2012, January 6). Water Quality 1 - Spectrophotometric Determination of Iron in Drinking Water. Retrieved September 12, 2012, from web.pdx.edu/~atkinsdb/teach/427/Expt-IronSpec.pdf Harris, Daniel C. Quantitative Chemical Analysis 7th edition. W.H. Freeman and Company, USA. 2007