Identification of an Unknown Mixture

Abstract:

The purpose of this lab was to identify an unknown mixture of two compounds using powder diffraction techniques combined with scanning electron microscopy and the “PC Identify” computer software. The X-ray powder diffraction pattern of the unknown mixture was analyzed on the computer in the laboratory. A peak search-match showed possible compounds contained in the mixture for further identification. An elemental analysis was conducted using a scanning electron microscope to determine which elements were present in the unknown mixture. Information from the XRD pattern and the elemental analysis helped identify the two compounds in the unknown by comparing it with the powder diffraction files. It was determined that unknown mixture mx3 contains potassium bromate (KBrO3) and nickel oxide (NiO). The two compounds were verified using the “PC Identify” computer software.


Introduction:

Similar to the past week’s labs, this particular lab employed the use of powder diffraction techniques to identify an unknown material. In X-ray diffraction, specimens are powdered so that all orientations are uniformly represented. X-rays are electromagnetic radiation of wavelength 0.5-2.5 A, and occur in the electromagnetic spectrum between gamma-rays and ultraviolet light. In this experiment, a monochromatic source of X-rays was used. The X-ray powder diffraction technique is used to fingerprint crystalline materials and determine their structures. Each crystalline solid has a unique characteristic XRD powder pattern. Once a material is identified, X-ray crystallography can be used to determine its structure. In this experiment, an unknown mixture of two compounds was identified. The X-ray powder diffraction pattern of the unknown mixture was a superposition of each compound’s individual XRD pattern. By examining the XRD powder pattern with a peak search-match, the computer found possible compounds in the unknown mixture for further identification.

In the second part of the lab, scanning electron microscopy (SEM) was used to conduct an elemental analysis of the unknown mixture. The SEM uses electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by heating of a metallic filament. The electron beam follows a vertical path through a column of the microscope. The electron beam travels through electromagnetic lenses which focus and direct the beam down towards the sample. Once the beam hits the sample, either backscattered or secondary electrons are ejected from it. The interaction of the electrons in the SEM with the sample results in the generation of characteristic X-rays. Detectors collect the secondary and backscattered electrons, along with the characteristic X-rays, and convert them into a signal that is sent to a viewing screen producing an image. This process was used to determine the individual elements in the unknown mixture. For unknown mixture mx3 the elements detected were potassium, nickel, bromide, and oxygen.

In the third part of the experiment, the information from the elemental analysis and XRD pattern were used along with the powder diffraction files to determine the two compounds in the unknown. It was determined that unknown mixture mx3 contains potassium bromate (KBrO3) and nickel oxide (NiO). This result was verified with the “PC Identify” software.

Experimental Procedures:

Part One: Powder Diffraction

The first steps to this experiment included logging into the X-ray diffraction computer program in the laboratory, choosing an unknown mixture of two compounds, and downloading the XRD powder pattern intensity graph. The intensity graph was optimized by stripping away the Ka2 and smoothing the peaks. A chart of the d-spacings, relative intensity and scattering angle of the unknown mixture was created. D-spacing labels were applied to the graph. The graph and chart were printed and saved for later use. A peak search-match was done without restriction to find all possible compounds that might be in the mixture. These were recorded and taken to the scanning electron microscopy room.

Part Two: Scanning Electron Microscopy

The SEM was used to conduct an elemental analysis of the unknown mixture. The day this lab was conducted, Thursday October 13, 2005, the SEM was not working properly. Mr. Al Stewart ran the experiment earlier that morning and gave us his results. First a picture was taken of the sample. This picture is in the appendix and labeled Figure One: Scanning Electron Microscopy of mx3. Next, an elemental analysis of the mixture was carried out. Each compound in the mixture was analyzed by placing an “X” on it and EDEXing. On the resulting graph, the strong peaks show which elements are in each compound.

Part Three: Identification

The elemental information from part two was used with the diffraction data from part one to correctly identify the two compounds in the unknown. The diffraction data used to determine the compounds included the d-spacings, relative intensities and scattering angles. This information was compared with the powder diffraction files. In unknown mx3, the compounds were identified as potassium bromate and nickel oxide. The intensity graph from part one was re-opened. This time when trying to identify the unknown on the computer, the elements resulting from part two of the lab were set as restrictions. The “PC Identify” software verified the two compounds in the mixture. The computer also identifies which peak belongs to which compound on the XRD pattern. Powder diffraction files were used to determine which peak coincides with each compound. The crystal structure, lattice parameters, and space group for each compound were determined. Finally, the measured experimental relative intensities of each compound were compared with the theoretical data found in the powder diffraction file. An error analysis was completed on the results.

Results/ Calculations:


In part one, an unknown mixture’s XRD pattern was downloaded and analyzed. Several possible compounds resulted from the peak search-match. Among these were potassium bromate, nickel oxide and several others. The original experimental data is attached at the end of this lab.

In part two, first a picture of the sample was taken. This picture is located in the appendix and labeled Figure One: Scanning Electron Microscopy of mx3. An elemental analysis was conducted and revealed that the elements in the unknown mixture were potassium, bromide, nickel and oxide. This data is also in the appendix and labeled Figure Two: Elemental Analysis of mx3. Next, each compound in the mixture was analyzed. The results from the first compound are shown in Figure Three: Compound One. The strong peaks show that this compound consists of bromide and potassium. The results from the second compound are shown in Figure Four: Compound Two. The strong peaks indicate that this compound contains nickel and oxygen.

In part three, a comparison of the d-spacings, relative intensities and scattering angles to the powder diffraction files, allowed identification of the two compounds as potassium bromate and nickel oxide. This result was verified with the “PC Identify” software in the laboratory.

Nickel oxide is rhombohedral and has theoretical lattice parameters of a = 2.9552 A and c = 7.2275 A. The space group of nickel oxide is R3m. On the mixture’s XRD pattern, NiO has peaks at (101) and (012). Potassium bromate also has a rhombohedral crystal structure. Its theoretical lattice parameters are a = 6.014 A and c = 8.156 A. The space group of potassium bromate is R3m. On the mixture’s XRD pattern, KBrO3 has peaks at (101), (012), (110), (003), (021), (202), (113), (211), (104), (122), (300), and (024). The (hkl) for each compound are labeled on the mixture’s XRD powder pattern in the appendix in Figure Five: mx3 XRD pattern with labeled (hkl).

The PDF card file for each compound is in the appendix labeled Figure Six and Figure Seven. The measured relative intensities for each compound were compared with the card file for the compound and an error analysis was conducted. The results are in the next two tables.

Table One: Comparison of Relative Intensities for Potassium Bromate

(hkl)Experimental Intensity (%)Theoretical Intensity (%)Relative Intensity % Error
(101)18.896068.52%
(012)20.1710079.83%
(110)61.447012.23%
(003)3.351066.50%
(021)1.3235.00%
(202)20.145059.72%
(113)47.478-493.38%
(211)14.0210-40.20%
(104)32.4616-102.88%
(122)63.1420-215.70%
(300)3.751268.75%
(024)2.02874.75%

Table Two: Comparison of Relative Intensities for Nickel Oxide

(hkl)Experimental Intensity (%)Theoretical IntensityRelative Intensity % Error
(101)58.17603.05%
(012)100.001000.00%

Discussion:

The information from the X-ray powder diffraction and the scanning electron microscopy allowed for us to identify the compounds in the unknown mixture. This information was verified with the “PC Identify” computer software. We were able to identify the compounds and then determine each compound’s crystal structure, (hkl) values, lattice parameters and space group and compare the relative intensities of the experimental and theoretical data.

For the nickel oxide compound, the experimental relative intensities were very similar to the theoretical ones. However, for the potassium bromate compound, the experimental relative intensities varied greatly from the theoretical data. This may have resulted from either systematic or random errors. For example, a random error may have occurred in the counting statistics. This is the probability of the event occurring and is a measurement error. The scattering angle, theta, may not have been read properly. This is another example of a random error. The differences in relative intensity may also have been caused by a systematic error. For example, the scattering angle, theta, may not have been properly set to scale. Also, the mixture might have had a low “Z” value. In this case, the X-rays will just penetrate through the thin sample.

Summary:

The purpose of this lab was achieved. Two compounds in an unknown mixture were identified using powder diffraction techniques, scanning electron microscopy and the “PC Identify” computer software. The unknown mixture’s XRD powder pattern was downloaded from the computer and analyzed. Possible compounds in the mixture resulted from a peak search-match on the XRD powder pattern. The scanning electron microscope was used to examine the unknown mixture again and identify which elements were contained in it. Comparing the d-spacings, relative intensities and scattering angles from the XRD powder pattern to the PDF powder diffraction files helped identify the two compounds in the unknown mixture, along with the elemental analysis. It was determined that the two compounds in unknown mixture mx3 were potassium bromate (KBrO3) and nickel oxide (NiO). These compounds were verified using the “PC Identify” software.

After identifying the compounds, we were able to determine each compound’s crystal structure, (hkl) values, lattice parameters and space group, and then compare the relative intensities of the experimental and theoretical data. The experimental relative intensities for the nickel oxide compound were very similar to the theoretical ones. However, for the potassium bromate compound, the experimental relative intensities varied greatly from the theoretical data, which may have been the effect of either the random or systematic errors described earlier.

Since the scanning electron microscope was not working Thursday October 13, 2005, we may want to retry part two on our own. This way we will be confident in using the SEM for future work.


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