ABSTRACTThe aim of this experiment is to determine the concentration of chromium (VI) in a polluted water sample which came from lakes, rivers or streams by using spectrophotometer and a set of standard solutions. The absorbance of 5 series of standard solution of chromium (VI) is determined with different concentrations which are 20, 40, 60, 80 and 100 ppm respectively by using spectrophotometer. The graph of absorbance versus concentration of chromium (VI) standard solution was plotted as a reference to determine concentration of Cr (VI) in water sample. From the results, we can observe as the concentration of Cr (VI) increase, the value of absorbance also increases. The concentration of Cr (VI) in 10 mL of water sample is found to be 130.74 ppm. This shows that concentration of Cr (VI) can be considered as toxic in the water sample because it exceeds 0.100 ppm. Therefore, the water sample is not suitable for drinking and agricultural purpose.1. INTRODUCTIONChromium is one of the micro nutrient elements which important for human health but in low concentration of less intake of chromium. However, chromium can be considered as toxic to human health when consume it excessively. The important compound in this experiment is chromium (VI). Mostly, chromium (VI) compounds are soluble in water. Chromium (VI) is very reactive and powerful oxidising agents in acidic conditions, but much less oxidising under alkaline conditions. Chromium main uses are in alloys such as stainless steel, in chrome plating and in metal ceramics. Other application of chromium (VI) is used to make bricks and linings for furnaces. Chromium in environment exists in two oxidation states which are chromium (III) and chromium (VI). The chromium (III) only toxic at high concentration while, chromium (VI) is considered as toxic and hazard even at low concentration because it is a strong oxidizer. Many countries and states strongly regulate the standard concentration of chromium (VI) in wastewater as the step to control the usage and intake of this compound. This is because the compound can give some serious effects to human and environment. Chromium (VI) can cause various health effects. The health effects of chromium (VI) compounds will be quite different from those of chromium (III) compounds. Skin contact with liquids and solids of chromium (VI) may lead to ulcerations, redness, dermatitis and swelling. Meanwhile, inhalation of chromium (VI) compounds can damage and irritate the nose, throat, lungs, stomach and lead to asthma and bronchitis. In addition, exposure to chromium (VI) can cause ulcers, kidney and liver damage. Therefore, long-term exposure to airborne chromium (VI) can have adverse effects on the immune systems and can cause cancer. Moreover, the toxicity of chromium (VI) not only affected human health but it also affects the environment and surrounding. Normally, chromium (VI) is released to the environment from factories or industries through water, air and soil. The concentration of chromium (VI) needs to follow the standard which has been introduced by the government before being discharge to the environment to reduce its effect. Chromium (VI) can alter genetic materials of organisms. Besides, the excessive of chromium (VI) in water can damage the gills of fish that swim near the point of disposal and it can cause respiratory problems, a lower ability to fight disease, birth defects, infertility and tumour formation in animals. According to the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), in 2005, they had listed some of the chemical which are toxic and contain heavy metals including chromium (VI) which has the maximum contaminant level (MCL) values as per EPA. However, some of the industries discharge their wastewaters contain heavy metals in 100 or 1000 times more in concentration than the prescribed MCL value.

Therefore, industries are required to treat the heavy metals contaminated to meet these regulatory requirements. Table 1 shows the rank of heavy metals as per CERCLA list of priority chemicals 2005 and their regulatory limits.Table 2.1: Rank of heavy metals as per CERCLA list of priority chemicals 2005 and their regulatory limits

Heavy metalsRankMaximum concentration limit (mg/L)

Arsenic (As)010.010

Lead (Pb)020.015

Mercury (Hg)030.002

Cadmium (Cd)080.005

Chromium [Cr(VI)]180.010

Zinc (Zn)745.000

Manganese (Mn)1150.050

Copper (Cu)1331.300

Selenium (Se)1470.050

Silver (Ag)2130.050

Antimony (Sb)2220.006

Iron (Fe)-0.300

2. AIMS / OBJECTIVESThe purpose of this experiment is to determine the quantity of chromium, Cr (VI) present in a polluted water sample by using a spectrophotometer and as a set of standard solutions which is 20, 40, 60, 80 and 100 mL of diluted water samples. The other objective is to determine whether the water sample is suitable for drinking or agriculture purposes in the end of this experiment.3. THEORYSpectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. It is more specific than the general term electromagnetic spectroscopy in that spectrophotometry deals with visible light, near-ultraviolet, and near-infrared, but does not cover time-resolved spectroscopic techniques. Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a photometer that can measure intensity as a function of the light source wavelength. Important features of spectrophotometers are spectral bandwidth and linear range of absorption or reflectance measurement.

The absorption spectroscopy will be used in this experiment to identify low level concentrations of chromium (VI) in a lake water sample. By using absorption spectroscopy, heavy metals can be identified via absorption of wavelengths of light. The amount of light absorbed is linearly proportional to the concentration of the solution metal ions. Absorption spectroscopy operates on the measuring principle of light before and after it passes through an aqueous metal solution. The amount of light absorbed by the chemical species in the sample is equivalent to the difference in the amount of light before it enters the sample and after it exits the sample. For the purpose of light to be absorbed by chemical species, the light must be set to a specific wavelength. For every chemical species they will absorbs different wavelengths of light. Absorption spectroscopy is function to detect the wavelengths of light absorbed by a metal in solution. In this experiment, five standard solutions will be prepared, 20ppm, 40ppm, 60ppm, 80ppm, 100ppm chromium (VI) by diluting it with distilled water. The volume of dilute solutions can be calculated by using the solution dilution formula which is:

M1V1 = M2V2Where, M is molarity, V is volume, and the subscripts 1 and 2 refer to the initial and final values. For each wavelength of light passing through the spectrometer, the intensity (Io) of the light passing through the reference cell is measured. The intensity (I) of the light passing through the sample cell is also measured for that wavelength. If I is less than Io, then obviously the sample has absorbed some of the light. The relationship between absorbance (A) and the two intensities is given by the quantitative analysis using spectrophotometer which based on Beer-Lambert Law:

A =LCA = absorbance value (dimensionless, thus no units is represented)

= molar absorbance (L / mol.cm)

L = path length of the cuvette in which the sample is contained (cm)

C = concentration of the compound in solution (mol / L)Based on Beer-Lambert Law it states that absorbance value is depends on the total quantity of the absorbing compound in the light path through the cuvette. So, if we were to plot a graph of absorbance versus concentration of the compound solution a straight line passing through the origin will be obtained. The molar absorbance, , is a constant for a particular substance, therefore if the concentration of the solution is halved, so is the absorbance value. A compound with a high molar absorbance is very effective at absorbing light at the appropriate wavelength, and hence low concentrations of a compound with a high molar absorbance can be easily detected. As what has been standardized by state and federal regulatory agencies, the water sample that is more than 0.10 parts per million concentration of chromium (VI) can be concluded to be contaminated and thus it is not suitable for drinking or agricultural purposes.4. APPARATUS AND MATERIAL

Spectrophotometer

Cuvettes

Pipette

100 mL beaker

20, 40, 60, 80, 100 and 300 ppm of Cr (VI)

Blank sample

Water sample

5. PROCEDURE1. 100ml of 300 ppm standard solution of chromium (VI) has been prepared by lab instructor.2. 5 series of standard solution was prepared in the range of 20 ppm to 100 ppm which is used in the calibration of spectrophotometer.

3. The wavelength of spectrophotometer was set to 435nm which is standard wavelength for chromium.

4. The blank sample given by instructor is inserted in to the spectrophotometer to get zero adjust.

5. For 20 ppm of concentration of chromium, 3.33 ml of chromium was poured in to the cuvettes. The distilled water is added into the solution until 50ml.

6. The cuvette (Cr) was polished and cleaned before inserted into the cuvette chamber of spectrophotometer. The readings of the absorbance were recorded.7. The steps 5 to 6 were repeated using different concentration and measured volume of the solution.

8. Graph absorbance versus concentration of chromium(IV) was plotted as the standard solution

9. 10ml sample of water was inserted in to cuvette and was calibrated in the spectrophotometer. 3 readings of absorbance were recorded.

6. RESULTS Absorbance of Chromium (VI) based on it concentration:Concentration of Cr (VI) (ppm)Volume of Cr (VI)

(mL)Absorbance

203.330.089

406.670.178

6010.000.262

8013.330.356

10016.670.461

Direct absorbance measurement of Chromium (VI):Water sampleAbsorbance

123Average

10 mL of Cr (VI)0.5930.5920.5980.594

Graph of absorbance versus concentration of chromium (VI) 7. SAMPLE OF CALCULATIONS5 solutions that give a series of standard solution were calculated using the equation below to determine volume required in each different concentration of:

In this experiment, 5 concentrations of Chromium (VI) were chosen at range of 20 to 100 ppm. The concentration has been chosen are, 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm. The sample of calculation for the 20 ppm of concentration is shown below:

Determination of concentration chromium (VI) in the water sample From the graph the linear regression equation that was obtained is:

In order to determine the concentration of chromium (VI) in the water sample, the absorbance that obtained from the experiment was substitute in to the equation

8. DISCUSSIONDetermination of chromium in water sample is started with preparation of five standard solutions from 300ml of 300 ppm Cr (VI). The absorbance of these five solutions was plotted as the reference in the determination of concentration of Cr (VI) in the sample water. Based on the Beer Lambert law, the amount of light absorbed by a medium is proportional to the concentration of the absorbing material or solute present. Therefore the spectrophotometer was set to 435nm wavelength, ( for Cr (VI).From the graph, absorbance was increased as the concentration of Cr (VI) increased. According to Beer Lambert law, the plot must be in linear and pass through or near to origin. The linear regression analysis in this experiment is using excel in order to determine the linear best fit for the absorbance versus concentration data. The R2 in the graph is 0.9982, where the function of R2 is actually to show how well the regression analysis fit to absorbance-concentration data. The closer the value of R2 to 1.00, then it becomes better regression analysis.

R2= 0.9982 is not good enough to show that this regression linear analysis fit to absorbance-concentration. Data error may be occurred during measuring of the absorbance, before inserted cuvette into spectrophotometer, the surface of cuvette must be cleaned to prevent finger smudges which can affect the reading. This is because the optically transparent cell is sensitive to any finger smudges as it will avoid the light to passes through the sample.

The absorbance of the water sample was measured by spectrophotometer. The absorbance value was substitute into the linear regression equation obtained by the graph. As the result, the concentration of sample was exceeding 0.100 ppm. State and federal regulators agencies consider Cr (VI) to be toxic at concentration levels more than 0.100 ppm. Therefore this sample of water is not considered as suitable for drinking water and agriculture.9. CONCLUSIONIn the end of this experiment, the concentration of chromium (VI) in a polluted water sample can be determined. From the graph plotted, the linear regression analysis (R2) obtained was 0.9982 which approximately to 1. In sample of calculation, the concentration of chromium (VI) in 10 mL of water sample is found to be 130.74 ppm. Compared to the standard water quality for drinking water, the allowable limit of chromium (VI) concentration is 0.100 ppm. It shows that the water sample used in this experiment not suitable for drinking and agriculture because it exceeded 0.100 ppm.10. RECOMMENDATIONS1. When diluting the chromium (VI) sample, make sure that the eye position is parallel to the reading in the pipette to avoid parallax error.2. We should to detect anotherheavy metal that maycontain in sample water whichcan cause harmful to organisms such as copper (II).3. The chromium (VI) samples that have the same concentration should be testedmore thanonceinorder to get accurate result.

4. Makesure theaccurate reading, anyfinger prints, scratches or depositedon thesurface of cuvette must be avoided. This is because it can disturb the absorption of light by the aqueous heavy metal.

11. REFERENCES Engineering Chemistry Laboratory Manual (2012).

Ingle, J. D. J. & Crouch S. R. (1988). Spectrochemical Analysis. New Jersey: Prentice Hall

Lenntech, B. V. (2011). Chromium-Cr. Retrieved November 12, 2012, fromhttp://www.lenntech.com/periodic/elements/cr.htm. Anwar, J., Aslam, M. A. & Ahmad, M. (1996). Selective Determination of Cr (VI) byIndirect Atomic Absorption Spectroscopy. University o f Punjab, Pakistan.

Helmenstine, A. M. (2003). Concentration Units & Dilutions. Retrieved November 12, 2012, from http://chemistry.about.com/od/lecturenotesl3/a/concentration.htm. Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), USEPA, 2005.

12. APPENDICES EMBED Excel.Chart.8 \s

2

_1414769219.xlsChart1

0.089

0.178

0.262

0.356

0.461

Y-Values

Concentration (ppm)

Absorbance

Absorbance versus Concentation of Chromium (VI) standard solution

Sheet1

X-ValuesY-Values

200.089

400.178

600.262

800.356

1000.461

To resize chart data range, drag lower right corner of range.