Already have an account? Log in. Jarrod Robel Lv2. Unlock all answers Get 1 free homework help answer. Ask a question. Join us on Discord. Principles of Chemistry Molecular Approach 3rd Edition, Calibrate the spectrophotometer using one, then take an absorbance reading with the other.
Make sure that the vertical line on the cuvettes is adjacent to the mark on the plastic cuvette holder in the spectrophotometer for both the calibration and all future readings. If the vertical line is in a different position during any reading, the absorance will change slightly. The second cuvette will be the one in which you place your sample. We will call this absorbance A systematic error.
The absorbance you measure is the systematic error between the two cuvettes, using the first cuvette as the blank. This absorbance will be subtracted from the readings you get using the iron samples, as follows: Using a 50 mL graduated cylinder, obtain two samples of city tap water, 50 mL each, from two different cities.
Place each sample in a clean mL beaker. Add 10 drops of 3 M H2SO4 to each. Boil each sample on a hot plate for 2 minutes. Cool the two samples in ice until they are no longer warm to the touch. Pour each in turn back into the 50 mL graduated cylinder and add enough distilled water to bring each back to 50 mL volume. Pour contents of each into the same mL beakers to mix. Be sure to mix well after the addition of each reagent, by gently inverting the flask.
This sample will be called the city reaction flask in the description of calculations below. Determine the absorbance of this solution using the previous blank solution as the reference at the wavelength of maximum absorbance determined earlier.
Repeat this section with the second boiled sample of tap water from a different city. Subtract the absorbance which represents the systematic error between the cuvettes from each of these experimental values. The calculation of the Fe concentration of the unknown can be made by a comparison method. This, however, can only be done if the system adheres to Beer's Law in the range of concentrations involved.
In the case of the iron-bipyridyl complex that range is 0. The appropriate relationship for the calculation of the Fe concentration in the Second Unknown Dilution is:. For the absorbance value of the unknown solution use the average of the three readings obtained for each sample taken. From the value obtained for [Fe] SUD , calculate the concentration of iron, in parts per million, of the original unknown Fe solution given to you by your instructor. If city water was provided for this experiment, the equation above is used to determine [Fe] for the solution whose absorbance you measured.
Since this solution was made to 50 mL but in the process you used 15 mL of reagents prepared with distilled water, [Fe] for the city water can be found through a simple modification of the equation above:. Report the following data: 1. Fe unknown number 2.
Colorimeter number 3. Wavelength of maximum absorbance 4. Average absorbance of the Standard Fe Solution 5. Concentration of the Standard Fe Solution in ppm 6. The average concentration of the Second Unknown Dilution in ppm. Use the value given in 7, above, to calculate the original concentration in ppm Fe in the solution given you by your instructor. To get to that point, you performed two dilutions: a and b 9.
Pages in your lab notebook containing the pertinent data. Measurement of the Absorbance Spectrum In order to determine the wavelength of maximum absorbance it is necessary to obtain the absorbance spectrum of the iron-bipyridyl complex. Determination of the Absorbance of the Standard Fe Solution. Analysis of the Fe Unknown Clean a mL volumetric flask, place your initials on the ground glass area and hand it to your instructor who will pipet 10 mL of unknown solution into it and who will also give you an unknown number for it.
Analysis of city tap water If samples of city tap water are supplied in this experiment, you will determine the iron concentration in two samples. Procedure The absorbance you observe may be lower than that which you observed for your known and unknown samples. Fill the flask to the mark with deionized water and mix thoroughly.
Pipet the following solutions into each of the six volumetric flasks. Fill each flask to the mark with deionized water and mix thoroughly. Allow the solutions to stand for 10 min. Mix the solutions again before measuring the absorbance. If more than half of the standard iron solution remains when you complete the experiment, you can retain it for the other experiment in this set. To determine any possible absorbance from the matrix i.
Rinse the cuvette several times with tap water followed by deionized water, fill it with deionized water, place it the holder, and blank the spectrometer. Rinse the cuvette with each solution including the zero iron concentration sample three times, and then fill it with the solution, place it in the holder, and measure the absorbance.
Repeat the measurement with a fresh aliquot of the solution no need to rinse this time. The two absorbance values should be similar, otherwise you should measure another one. If you work from lowest iron concentration to highest, you will minimize the potential for carryover. Pipet 25 mL aliquots of your cold and hot tap water and two unknown samples into four 50 mL volumetric flasks. Finish preparing the samples as described in the procedure 1. Measure the absorbance of the solutions.
0コメント