DETERMINATION OF COPPER (II) CONCENTRATION COLORIMETRIC METHOD
BY
N. D. E. SILAVA INSTITUTE
OF BIOLOGY ,
COLLEGE
OF SCIENCE
UNIVERSITY OF THE PHILIPPINES , DILIMAN, QUEZON CITY , PHILIPPINES D ATE PERFORMED: SEPTEMBER 22, 2010
ABSTRACT The experiment was done to determine the Cu (II) concentration in a sample by using a Spectrophotometer and by applying Beer’s law. A calibration curve was obtained by plotting the absorbance absorbance against concentration concentration of the standard Cu(II) solution and the concentration of the unknown solution was determined using the equation of the line of the calibration curve. Calculated concentration concentration of unknown sample yielded a 13% deviation which meant that colorimetry colorimetry yielded acceptable results. Results obtained were were in agreement with Beer’s law law stating that absorbance is directly proportional proportional to concentration. concentration.
called the Beer-Lambert law, the Bouger-Beer law or more simply, Beer’s law.[3] which is given by
INTRODUCTION
Colorimetry is defined as the method of trace analysis that involves absorption of radiations having frequencies in the visible spectrum. [1] this is done using a Spectrophotometer that transmits radiation from a source through a sample cell and into a detector which measures the absorbance of the sample. [1] The wavelength of radiation absorbed depends on the identity and amount of substance present in the sample. [2] The relationship between these factors is
-log(T)=-log(I/Io)=εbc=A Where T is the transmittance of the beam or the fraction of the incident radian power transmitted by the sample (I/Io), “ε” is the molar absorptivity and is replaced by the symbol “a” if concentration is given in grams/liter. “b” is the path length , “c” is the concentration of the sample and “A” is the absorbance. [3] [4] [5] 1
Cu2+→Cu(NH3)42+ (deep blue) [2]
In this experiment, a UV-Vis spectrophotometer was used for the analysis of the sample. First, solutions of 0 ml, 2 ml, 4 ml, 6 ml 8 ml and 10 ml were taken from a 2500 ppm working solution and placed in 6 50 ml-volumetric flasks. 10 ml ammonia solution was added to each flask and then diluted to mark. The blank solution was put in a plastic cuvette and into the spectrophotometer to minimize the error caused by the reflected light [3] then the analytical wavelength was set by using the most concentrated working standard. The calibration curve was then prepared by using the prepared solutions and plotting their Absorbance (A) vs Concentration (ppm Cu). Then, a sample of unknown concentration was obtained from the instructor and 3 absorbance readings were taken and the concentration was determined. Thus, it can be said that this experiment was done to determine the concentration of Cu (II) ions in a sample by taking the absorbance and using Beer’s law to compute the concentration.
The values of absorbance (A) for a 2500 ppm Cu (II) standard solution were then measured. Volume Std(ml) 2 4 6 8 10
Cu (II) (II) ppm Absorbance 100 0.082 200 0.17 300 0.268 400 0.366 500 0.456
Table 1. Absorbance of solution It was Absorbance was then plotted versus Concentration in parts per million (ppm) and the equation of the line was determined. It was necessary to use absorbance rather than transmittance since absorption peaks appear as deep valleys in a transmittance plot whereas an absorbance plot would produce a linear result. [3]
Figure 1. Calibration curve
Then concentration (x) was expressed in terms of (y) giving a working equation x=y+0.0148÷0.009
RESULTS AND DISCUSSION
This determined the concentration of the unknown solution given a value for absorbance (Y). (Y). It could be said that the experimental points obey Beer’s law since they lie on a straight line, giving a slope that represents the product of the molar absorptivity “ε” and the path/cell length “b”. [4 ] A 6 ml aliquot of the sample was obtained and diluted to 50 ml. 3 Absorbance readings were taken using the same UV-Vis Spectrophotometer [4]
The determined analytical wavelength (λ) was 606nm for optimum absorption. The blank solution was then used to reset the values and take into account the transmitted radiation lost due to scattering, reflection, etc.[3] Ammonia was then added to the samples since Cu is weakly colored to be measured and thus, the ammonia reacts with the copper to form a deep blue complex. 2
Tria rial 1 2 3 Average
become significant at higher concentrations [4] since the refractive index for the absorbed radiation is changed at high concentrations. concentrations. Thus Beer’s law is ideally applicable to solutions with concentrations below 1x10-2 M [3] that is why parts per million (ppm) is used to indicate the concentration of Cu(II). Also, Beer;s law applies to as solution that may contain more than one kind of absorbing substance provided that there is no interaction between them.[5] Chemical deviations upon dilution are also possible, leading to lesser actual concentration in some substances like the chromate ion.[2] Also, there are also instrumental limitations to consider: Stray radiation reaching the detector, sensitivity changes and power fluctuation. Fourth is when a band of wavelengths is used rather than monochromatic radiation [3] that causes the detector to measure average intensity instead of the average of the log intensities. Another source of error is the cuvette has fingerprints, affecting the transmitted radiation ang causing errors in the Absorbance.
Absorbance Concentration (ppm) 0.138 169.7777778 0.139 170.8888889 0.139 170.8888889 0.139 170.5185185
Table 2. Absorbance of Unknown sample
Trial Trial Concentr oncentra ation of Unk Unknow nown n Sol'n Sol'n (ppm (ppm 1 1415 2 1424 3 1424 Average 1421
Table 3. Cu (II) Concentration of Unknown Solution The calculated value of concentration of the Unknown Solution gives us a 13.68% deviation from the theoretical value of 1250 ppm, as disclosed by the instructor. Analysis of the data of the other groups was done and a pooled standard deviation value of 0.524 was obtained. This shows that the values obtained experimentally were precise.
The shape of the calibration curve often depends on the bandwidth. That was why the wavelength range needed to be scanned to determine the best analytical wavelength for analysis. At this wavelength, change in absorbance with concentration is at a maximum, thus yielding greater sensitivity and higher accuracy. Second, molar absorptivity is constant at this band.[3]
SUMMARY AND CONCLUSIONS
It could be said that Absorbance is directly proportional to the Concentration of the substance by virtue of Beer’s law and was affirmed by the experiment performed using a UV-Vis Spectrophotometer. The experiment was taken to be a success given the small value of the Pooled Standard Deviation for the experiment. It was suggested that more accurate
However, Beer’s law is also subject to limitations. First, deviations 3
results would be obtained if a quartz cuvette was used.
2500mgL×0.002L.05L=100 2500mgL×0.002L.05 L=100 ppm
REFERENCES
B. Concentration (x) from linear equation given absorbance (y)
[1] Pickering, William F. 1966. “Fundamental Principles of Chemical Analysis”. Elsevier Publishing Company.
x=y+yinterceptslopeppm Take absorbance = 0.138
[2] Klingenberg, Joseph. 1965. “Introduction Introduction to Quantitative Chemistry ”. ”. Reinhold Publishing Group
x=0.138+0.01480.0009 x=0.138+0.01480.0009 = 169.78 ppm
[3] Pecsok, Robert, et al. 1976. “Modern Method of Chemical Analysis, 2nd edition”. John Wiley & Sons Inc.
C. Average Concentration and Absorbance
(Trial1+Trial2+Trial3)/3
[4] Bauman, Robert. 1962. “ Absorption Spectroscopy ”. John Wiley & Sons Inc.
169.78+170.89+170.893=170.52ppm
[5] Skoog, Douglas. 1962 “Fundamentals of Analytical Chemistry ” Holt, Rinehart and Winston, Inc.
D. Concentration of Unkown Solution
Concentration sample×Vdiluted sol'nVolumeAliquot=ppm Take Concentration Sample = 169.78 ppm 169.78 ppm×0.05L0.006L=14 ppm×0.05L0.006L=1415ppm 15ppm
APPENDIX
E. Percent Deviation
A. Computation of Cu (II) ppm
Concentration-1250ppm1250×100%= Take Concentration = 1421 ppm
ppm Std sol'n×Lsol'n÷Ltotal=ppm
1421-12501250×100=13.68%
Take L sol’n = 2 ml 4
D. Pooled Standard Deviation
Spooled=√((i=1n1Xi-X12+j=1n2XiX22+k=1n3Xk-X32)/(n1+n2+n3+…ns))
V working std mL
Cu ppm
Absorbanc e
2
100
0.082
0.823045267+0.823045267+093=0.524
4
200
0.17
6
300
0.268
8
400
0.366
10
500
0.456
Sample Analysis
RAW DATA
Aliquot froms Stock
6
Diluted Unknown
50
Trial
Absorba nce
Concentra tion (ppm)
1
0.138
169.77777 78
2
0.139
170.88888 89
3
0.139
170.88888 89
Average
0.138666 170.51851 667 85
GROUP A Team A
6 mL
Max λ
606.0 nm
Calibration Curve Concentra tion ppm:
2500 GROUP B 5
Team B
11 ml Average
max wavelengt h
599
Calibration Curve
0.259333 289.1481481 333
GROUP C
Concentra tion ppm:
2500
V working std mL
Cu ppm
Absorbance
2
100
0.093
4
200
0.18
6
300
0.281
8
400
0.365
10
500
0.458
Team C
14 mL
Max λ
602.0 nm
Calibration Curve Concentratio 2500 n ppm:
Sample Analysis
V working std mL
Cu ppm
Absorban ce
2
100
0.085
4
200
0.183
6
300
0.273
8
400
0.372
10
500
0.457
Aliquot froms Stock
11
Diluted Unknown
50
Trial
Absorba nce
Concentrati on
Sample Analysis
1
0.26
289.8888889
14
2
0.259
288.7777778
Aliquot froms Stock
3
0.259
288.7777778
Diluted
50
6
Unknown
Trial
Absorban Concentr ce ation
1
0.32
362.11111 11
2
0.32
362.11111 11
3
0.32
362.11111 11
Average
0.32
362.11111 11
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