ARC PECTRUM OF IRON /COPPER / BRA Aim : To determine the wavelength of prominent lines in the emission spectrum of iron/ copper/ brass. Apparatus : Constant deviation spectrometer, dc voltage source, metal rods, comparator and mercury vapor lamp. Principle : For the study of emission spectra, a sample is first excited by irradiating it with thermal or electrical energy. The radiation emitted on subsequent de-excitation is studied for both qualitative and quantitative analysis. Here we employ the method of electric arc for excitation. The de-excitation gives line spectrum, which is characteristic of the type of atoms present in the material. Line spectrum consists of discrete irregularly spaced lines and is obtained when the light emitting substance is in the atomic state. The wavelength of the prominent lines in the arc spectrum of different substances are studied by using the Hartmann s formula, C, where, C and are Hartmann s constants that must be evaluated experimentally. This is done by substituting the comparator scale readings of three known lines (in the mercury spectrum) from an arbitrary point and their wavelengths in the above equation and solving the three equations. If we could find three lines with scale readings, 2 & 3 corresponding to known wavelengths, 2 and 3 respectively, then we can find Hartmann s constants using the following relations A2 A B B 3 ( 2)( C ( 2 )( ) 2 ) C where 2 A and 3 prominent lines in the spectrum can be calculated. B 2. using the above equations, the wavelength of 3
Procedure : Constant deviation spectrometer is a convenient and fairly accurate type of spectrometer used in the visible and UV region of the electromagnetic radiation. In this spectrometer a special type of prism- a constant deviation prism is used. The portions ABC and ADE of the prism is half of a 6 prism and the portion BCD is a 45 prism. ABD 75. If PQ is the incident ray such that the refracted ray QR is parallel to AD. The ray QR strikes the face BD at an angle of 45 and is totally internally reflected along R. The ray finally emerges in the direction T. In this case, the angle of incidence is equal to the angle of emergence (θ). Thus the ray of light passes through this prism similar to a 6 prism in the minimum deviation position. The advantage of this arrangement is that the rays PQ and T are normal to each other. A constant deviation spectrometer has a collimator and a telescope fixed to rigid stand such that they are mutually perpendicular to each other. The prism P is placed on a table, which can be rotated about a vertical axis with the help of a drum D. A is a spring which allows the lever B to move when the drum D is rotated. The drum D has a graduated scale, which is calibrated and the index C shows the wavelength of different lines directly. pectrometer can be converted to a spectrograph by replacing the telescope eyepiece by a photographic plate. The objective of the telescope is replaced by a telephoto lens of suitable focal length. Glass prism is used for visible light where as a quartz prism is used for UV region of light. A fluorspar prism may be used for IR region with photosensitive devices suitable for this region. Initial adjustments of the constant deviation spectrograph are made such that we get a well defined spectrum for the mercury on the ground glass screen. The illumination of the screen is adjusted till we get equal intensity for the whole spectrum. The ground glass screen is now replaced by the photographic plate. The slit is adjusted to full height and mercury spectrum is photographed. The slit height is now reduced ( about 2/3 height ) and the mercury lamp is replaced by the metal rod, the spectrum of which is to be recorded. By applying suitable potential an arc is produced between the rods and it is photographed ( at the same position as that of mercury spectrum so that mercury spectrum overlaps arc spectrum). Then the film is
carefully taken out and developed.the developer used is Zettol. The film is dipped in diluted zettol solution. After keeping the film in zettol solution for few minutes, the film is washed with cold water and dipped in hypo solution. After a minutes, the film is again washed with water and dried by keeping it in a dry atmosphere. Using a comparator, the readings corresponding to different lines are taken. (Readings are taken such that scale reading increases with wavelength). From the readings corresponding to the mercury spectrum, the Hartmann s constants are calculated and the wavelengths for different lines in the spectrum are calculated. Then they are identified by comparing with the standard wavelengths which are known for a particular material. Result: The wavelengths of the prominent lines in the emission spectrum of.. are determined. 2 A 3 2 B 3 A2 B3 A B ( 2)( )( C ( 2 ) C 2 )
B Q P A R D T E
OBERVATION Comparator tandard Calculated tandard reading of wavelength of wavelength(nm) wavelength(nm) spectral mercury lines of lines in arc of lines in arc lines(cm) (nm)( ) spectrum spectrum (s)
Prominent lines in the emission spectrum of copper Wavelength(A o ) Wavelength(A o ) Wavelength(A o ) Wavelength(A o ) 428 482.94 53.54 5988.3 4275 499.78 5.9 6. 4378.2 493.85 52.7 68.32 445.2 4953.73 524.46 64.47 445.37 56.79 527.48 69.55 455 59.83 528.2 627.73 457.5 52.8 522.7 647.3 453.82 56.6 526.99 65.42 4539 52.28 5276.52 654.2 4555.92 55.78 5287.82 672.2 4586.95 558.9 564.39 686.86 4649.27 565.45 5682.42 688.89 465.3 567.98 578.75 624.27 4674.76 572.29 5792.8 628.46 468.99 583.99 586. 626.9 4697.49 588.2 5897.99 474.7 594.79 594.7 Prominent lines in the emission spectrum of brass Wavelength(A o ) Wavelength(A o ) Wavelength(A o ) Wavelength(A o ) 44.23 (Cu) 43.8 (Zn) 48.53 (Zn) 522.7 (Cu) 43.2 (Zn) 4378.2 (Cu) 49.6 (Zn) 5266.99 (Cu) 423.29 (cu) 4445.6(Cu) 4924.4 (Zn) 5277. (Zn) 477.76 (Cu) 448.38 (Cu) 4953.3 (Cu) 564.24 (Cu) 4242.26 (Cu) 457.59 (Cu) 52.28 (Cu) 5682.42 (Cu) 4248. (cu) 453. (Cu) 55.78 (Cu) 5777. (Zn) 4259.4 (Cu) 4586.96 (Cu) 588.99 (Cu) 5879.99 (Cu) 4275.3 (Cu) 4649.27 (Cu) 5.9 (Cu) 5988.3 (Cu) 4298.88 (Zn) 468.4 (Zn) 553.23 (Cu)