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Copy of Determination of Copper (II) Concentration by Colorimetric Method

Chem 26.1 Experiment 12

Julianne Malong

on 7 March 2011

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Transcript of Copy of Determination of Copper (II) Concentration by Colorimetric Method

Determination of Copper (II)

Concentration By Colorimetric

Method Objectives: define Beer's Law and apply it to calculations
do a graphical analysis using the least squares method
operate a spectrophotometer
be acquainted with the spectrophotometric method of analysis PROCEDURE? A. Preparation of Standard Solutions 1. Standard Stock Solution of 2500 ppm Cu (II)

2. Working Standard Solutions 9.5536 g Cu(NO ) .3 H O crystals in 1L distilled water 3 2 2 Pipette 0.00, 2.00, 4.00, 6.00, 8.00, and 10.00 mLstandard stock solution into 6 different 50-mL V-flasks. Add 10 ml concentrated ammonia solution and dilute to mark. B. Determination of Analytical Wavelength 1. Measure the absorbance of the most concentrated working standard Cu(II) solution against a reagent blank in 200 nm steps from 500-700 nm.
2. From the data obtained, make a plot of absorbance against wavelength. From this curve, determine the analytical wavelength for the analysis. the wavelength at which the solution absorbs strongly 651 nm we didn't get this but ito raw dapat sabi ni ma'am:) D. Determination of the Cu(II) Concentration of the Unknown Solution 1. Obtain an unknown sample and treat them in the same manner as the standard solutions (Part A, Step 2).
2. Take at least three absorbance readings for this solution.
3. From the equation obtained in Part C, determine the Cu(II) concentration of your unknown solution. Compare the value obtained graphically with the value determined using linear regression method. YEHEY!!! thank you for listening! by CJ and Jian Spectrometry Particularly in the visible region of the electromagnetic spectrum
Widely used in clinical chemistry and environmental laboratories because many substances can be selectively converted to a colored derivative
In spectrometry, we describe the absorption of radiation by molecules in relation to its molecular structure; make quantitative calculations using Beer’s law

Electromagnetic Radiation A form of radiant energy that is propagated as a transverse wave.
Described in terms of wavelength, frequency or wavenumber
Wave length

distance from one complete cycle
Frequency number of cycles passing a fixed point per unit time
Wave Number number of waves in a unit length or distance per cycle
reciprocal of wavelength
A very small part of the E.M. Spectrum
Wavelengths that appear as a color and is visible to the human eye
Visible spectrum NANOMETER Preferred unit for the ultraviolet and visible regions of the spectrum How do we see colors? "white" light actually contains all colors... white objects reflects most of the wavelengths that strike it, absorbing relatively few.
a colorful object such as a leaf appears green because when white light strikes it, the leaf reflects only the green wavelengths of light and absorbs the others.

A measure of the capacity of a solution to absorb radiant energy
Energy not absorbed= Energy transmitted
If Io is the intensity of light and I is the transmitted light, I/ Io is the transmittance
T=1 for a transparent solution
T=0 for a solution that absorbs all energy

Light Absorbance transmitted energy can be detected by a spectrophotometer It is more convenient to determine the amount of absorbed energy since it is in linear relation with the concentration of the absorbing species in solution.

The amount of electromagnetic radiation absorbed is related to the concentration of the analyte in solution... Beer's Law RESULTS Wavelength at maximum absorption = Concentration of working standard Cu(II) solution 651 nm y=mx+b y = A =absorbance of solution m = slope of the line = ab = absorptivity*path length
x = C = concentation of solution b = y-intercept Unknown Analysis y = 0.0007x - 0.0036
To get the concentration x, we substitute the absorbance y that we have determined from the spectrophotometer.
Trial 1:

Trial 2:

Trial 3:

Calculations average concentration Possible Sources of Error How would leaving the lid of the sample compartment open influence the results?
How about the addition of fingerprints to the cuvet?
How about the improper washing of the cuvet?
How about suspended material in the sample?
If the suspended materials slowly settled what would happen to the absorbance? Beer's Law Describes the amount of monochromatic light absorbed by a sample

“Incident radiation of radiant power Po passes through a solution of an absorbing species at concentration c and path length b, and the emergent (transmitted) radiation has radiant power P. This radiant power is quantitatively measured by spectrometric detectors.”
the science concerned with measuring the amount of “electromagnetic radiation absorbed at a particular wavelength or set of wavelengths.”
(or Absorption Spetrophotometry) Spectrophotometers UV-Vis (Ultraviolet-Visible) Spectrophotometer Setup

UV-Vis Spectrophotometer – covers the UV region starting around 200 nm , the entire visible region, and part of the near IR through at least 1000 n.

Visible region – 380-750 nm
Equations/Derivation of Beer’s Law Beer’s Law can be represented as a linear equation in the slope-intercept form where:
A = y
ab = m
c = x

Utilizing this equation, the concentration of a sample may be determined based on the absorbance measured by a spectrophotometer and substituting the values in the equation derived from a prepared calibration curve.
 1. Real Limitations: encountered only in relatively concentrated solutions (For Beer’s Law to apply, the concentration should be ≤ 10-3 M) ; refractive index may change with concentration, hence changing the molar absorptivity
2. Apparent chemical deviation: arises when an analyte dissociates, associates, or reacts with a solvent to generate a product that has a different absorption spectrum from that of the analyte Limitations of
Beer-Lambert’s Law Limitations of
Beer-Lambert’s Law 3. Apparent instrumental deviations with polychromatic radiation: law is adhered only with truly monochromatic radiation; in practice, devices can only have a band of wavelengths around a desired one. Measurements done at λmax where Beer’s Law is followed.
4. Instrumental deviations in the presence of stray radiation: absorbance measurements is usually contaminated with small amounts of stray radiation, Ps, (result of scattering phenomena of the surfaces of prisms, lenses, filters and windows) due to instrumental imperfections.
Sample Exercises 1. What is the Absorbance of a solution which absorbs ¼ of the incident light? Answer: 0.125 2. Compound X (MW 166.2) absorbs at 220 nm. A solution containing 0.298 mg of compound X in 20.0 mL ethanol gave A = 1.73 in a 1.00 cm cell. Calculate the molar absorptivity of compound X. Answer: 1.93x104 M-1cm-1 3. Phosphorus in urine may be determined spectrophotometrically by treating the urine with Mo6+ with subsequent conversion of the Mo6+ to a 12-molybdate complex. The complex has an intense absorption at 650 nm. A 5.00-mL sample of urine (taken from a 1.00-L bulk sample) was treated in this manner and then diluted to 250.00 mL. The resulting solution was read at 650 nm in 1.00 cm-wide cuvettes. Phosphate standard solutions were read prior to the analysis; results of the standard readings and sample analysis are summarized as follows: Calculate the concentration of P in the original sample in ppm. Answer: 218 ppm Instruments that are capable of measuring the absorbed radiation (visible and ultraviolet) at any wavelength

Colorimeters – can only measure the absorption of light.
1. Measure the absorbance of the solutions prepared in Part A step 2 at the analytical wavelength obtained in Part B.
2. Plot the absorbance (A) against concentration of the standard Cu(II) solution (ppm Cu) for the given series of standard solutions.
3. Determine the slope, y-intercept and the regression coefficient for the obtained calibration curve using linear regression method. Express the working equation for the determination of the concentration of the unknown in terms of these parameters. c. Preparation of the Calibration Curve
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