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Determination of the Iron Content of Water via Spectrophotometry

Application of the Relationship Between Light Absorption and Concentration
by

Reid Gunn

on 23 April 2010

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Transcript of Determination of the Iron Content of Water via Spectrophotometry

HOW DO WE DETERMINE THE IRON CONTENT IN OUR DRINKING WATER? SPECTROPHOTOMETRY BEER'S LAW A = εbc Beer’s law is a simple relationship between:
Absorbance (A)
Molar absorptivity (ε)
Path length (b)
Concentration (c) IF MOLAR ABSORPTIVITY AND PATH LENGTH ARE CONSTANT,
THEN THERE IS A LINEAR RELATIONSHIP BETWEEN... ABSORBANCE

CONCENTRATION AND THIS LINEAR RELATIONSHIP CAN BE USED WITH
SPECTROPHOTOMETRIC MEASUREMENTS TO CREATE
A CALIBRATION CURVE USING KNOWN STANDARDS STANDARDS ARE PREPARED USING THE SAME PROCEDURE
AS UNKNOWNS. THE ABSORBANCE OF THE UNKNOWNS
SHOULD FALL WITHIN THE RANGE OF THE STANDARDS. A RED-ORANGE COMPLEX FORMS BETWEEN IRON(II) AND ORTHOPHENANTHROLINE ORTHOPHENANTHROLINE IS A WEAK BASE THAT REACTS TO FORM PHENANTHROLINIUM ION IN ACIDIC SOLUTIONS.

A STANDARD IRON SOLUTION WAS ACIDIFIED WITH A SMALL AMOUNT OF CONCENTRATED SULFURIC ACID TO FACILITATE THE REACTION AND TREATED WITH A SODIUM ACETATE TRIHYDRATE BUFFER TO PREVENT THE PRECIPITATION OF IRON SALTS.
SOLVING THE PROBLEM *AN EXCESS OF REDUCING AGENT, HYDROXYLAMINE HYDROCHLORIDE, HELPS TO MAINTAIN THE +2 OXIDATION STATE OF IRON. THE USE OF LIGHT TO MEASURE
CHEMICAL CONCENTRATIONS A 0.0101 M STANDARD FERROUS AMMONIUM SULFATE HEXAHYDRATE SOLUTION WAS CREATED
FIVE FLASKS WERE TREATED WITH DECREASING AMOUNTS OF THE STANDARD IRON SOLUTION

IDENTICAL AMOUNTS OF OTHER REAGENTS WERE USED TO:
BUFFER THE PH
KEEP THE IRON ION AT A +2 OXIDATION STATE
FORM COLORED COMPLEX BETWEEN IRON(II) AND ORTHOPHENANTHROLINE SOLUTIONS WERE ALLOWED TO REST FOR 10 MINUTES * TO ALLOW FOR CONSTANT COLOR INTENSITY A SAMPLE FROM FLASK III WAS USED TO DETERMINE THE WAVELENGTH OF MAXIMUM ABSORBANCE *( λ = 510 nm) A CUVETTE WAS RINSED THREE TIMES WITH THE SOLUTION
THE SIDES AND BOTTOM WERE WIPED CLEAN BEFORE PLACEMENT IN THE SPECTROPHOTOMETER
* THESE MEASURES WERE TAKEN AS A PRECAUTION TO PREVENT ERRORS IN ABSORBANCE DATA FOR EACH STANDARD IRON SOLUTION, THE CONCENTRATION WAS CALCULATED AND ENTERED INTO CARY WINUV SPECTROPHOTOMETER SOFTWARE CARY WINUV SOFTWARE USED INPUT CONCENTRATIONS
AND ABSORBANCE MEASUREMENTS TO PRODUCE A
CALIBRATION CURVE DID I MENTION WE MADE TWO UNKNOWN SOLUTIONS TOO? SEEMS IMPORTANT.. FLASKS II - V ILLUSTRATE DATA POINTS FOUND ALONG THE CALIBRATION CURVE
* NOTE THE LINEAR RELATIONSHIP BETWEEN CONCENTRATION AND ABSORBANCE A = εbc LB #1 AND WELL #2 ARE THE UNKNOWN WATER SAMPLES
* OUTPUT FROM CARY WINUV SOFTWARE cORRELATION COEFFICIENT = 0.99989 THE CLOSER TO ONE THAT THE CORRELATION COEFFICIENT IS, THE MORE ACCURATE THE DATA. THIS DATA IS ACCEPTABLE. THE CONCENTRATION OF THE UNKNOWNS WERE LATER MULTIPLIED BY A DILUTION FACTOR OF 10, GIVING THE CONCENTRATIONS OF THE ORIGINAL 100 mL SOLUTIONS THE LIGHT ABSORBED BY THE COLOR COMPLEX FORMED BETWEEN IRON(II) AND ORTHOPHENANTHROLINE WAS USED TO FIND THE CONCENTRATION OF IRON IN UNKNOWN SOLUTIONS OF WATER. ONE UNKNOWN SOLUTION OF WATER WAS FOUND TO BE 21.3 mg/L AND THE OTHER WAS FOUND TO BE 8.49 mg/L. THE DIFFERENCE IN IRON LEVELS IN EACH SAMPLE CAN ATTRIBUTED TO ITS SOURCE. THE ACCURACY OF THE DATA WAS SUPPORTED BY THE THE CORRELATION COEFFICIENT OF 0.99989, WHICH SUGGESTS THAT ANY PROCEDURAL ERRORS AFFECTING THE RESULTS WERE KEPT TO AN ACCEPTABLE MINIMUM. CONCLUSION
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