H. H. WILLARD,' K. A. VAN LENTE, and R. E. VAN ATTA Southern Illiiois University, Carbondale, Illinois
A SEARCH of
the literature failed to reveal a clearly defined, well balanced experiment in flame photometry suitable for approximately a two-hour laboratory period. The experiment herein described is intended for this purpose. The main objective is to introduce to the student the fundamental qualitative and quantitative aspects of flame photometry. I n this experiment no attempt is made to explore all of the intricacies, limitations, and applications of the instrument and method. Two groups of common, representative cations have been selected. One group (Group A) is suitable for use with a blue-sensitive phototube while the other (Group B) is suitable for use with a red-sensitive phototube. The proper phototube is installed by the instructor. An experiment involving either of these groups requires approximately two hours for completion; therefore, only one group is assigned to a pair of students. Correlation between the concentrations of the solutions used and the operating slit widths is desirable in order to give some uniformity of transmission at a given slit width. The slit width should be small enough t o show clearly the individual transmission peaks for the cations in a mixture and to give less than 100% transmission for each peak observed with the undiluted stock solutions. It should be large enough to give a reasonable range of transmission values for the quantitative working curves. Stock solutions that fulfill these requirements a t the specified slit widths are kept available. The concentrations of these solutions and the slit widths used are listed under Operating Instructions. The solutions are prepared from reagent grade chlorides and redistilled water except for that of magnesium which is prepared from the carbonate and HC1, properly diluted. Prior t o performing the experiment, the students are given adequate theoretical material in the classroom and they are furnished with the manufacturer's instruction manual for the instrument to be used. All preliminary major adjustments on the instrument are made by the instructor so that the instrument is ready for use when the students arrive. Essentially, the experiment consists of determining the transmission peak wave lengths for each cation in the assigned group by the use of the respective undiluted stock solutions; running a mixture of all of the cations in the group to determine whether shifting of peak wave lengths bccurs; the qualitative analysis of an uunknown,t; the construction of a quantitative working curve for a single cation; and the quantitative
' Present address: Department of Chemistry, University of Michigan, Ann Arbor, Michigan. 192
analysis of an "unknown" whose concentration is such that it falls within the range of the working curve. The detailed directions follow, as applied to the Beckmao Model B Spectrophotometer with Flame Attachment and using an oxy-hydrogen flame. With minor modifications the directions could be applied to other similar instruments. OPERATING INSTRUCTIONS Group A or B and the cation for quantitative analysis will be assigned. Students work in pairs. The term "water" refers. in all cases. to redistilled water ; c a G + ,Sr++, Baf+. Group A includes: ~ a + Mgt+, Group B includes: Li+, K+, Bs++. Concentrations (parts per million) of stock solutions: Xat, 10; Mg++, 2000; Cat+, 200; Srf+, 500; Bat+, 2500; Li+, 100; K +,50. Qualitative (1) See that the proper phototube is in position. (2) See that the Sensitivity Switch is on STANDBY and the Shutter Switch an SHTR; plug the voltage stabilizer (Raytheon) into the 110 V., a.c. line; turn on the Power Switch (left side of instrument). (3) Prepare a mixture oi all the cations in the assigned group. For Group A, use 5-ml. portions (pipet) of each of the 5 stock solutions. For Group B, use 5-ml. portions of each of the 3 stock solutions plus 10 ml. of water. Mix the solution. Rinse and fill separate sample beakers almost full with the stock solutions of t,he respective cations, the mixture, the "unknown," and water. Arrange . the beakers near the instrument in a known order. (4) Slowly open wide first the oxygen tank valve and then the hydrogen tank valve. Turn (olockwise) the oxygen tank regulator valve to 30 p.s.i. and the corresponding hydrogen valve to 10 p.8.i. Turn (clockwise) t,he oxygen valve on the control panel to 3 p.s.i., the corresponding hydrogen valve to 4 p.s.i., and light the burner. Increase the oxygen pressure a t the control panel to 10 p.8.i. and maintain these pressures throughout all the meaanement,s. (5) Turn the Sensitivity Switch to position 4 and leave it in this position for all measurements. ( 6 ) Set the Slit Width a t 1.0 mm. if Group A is to he run or a t 0.8 mm. for Group B and use this setting for all subsequent measurements. (7) Place the first sample beaker in the holder but away from the cadllary, and zero the Dark Current with the Shutter Ssiteh on OPEN. (8) Elevate the beaker under the capillary and slowly rotate the Wave Length Control from 350 mp to 650 mp for Groltp A, or from 650 mp to 1000 m& for Group B, and record the wave lengths of all the transmission peaks observed. If one beaker of solution is insufficient, refill as necessary. Turn the Shutter Switeh to SHTR when measurements are not being made. (9) Do steps 7 and 8 for each cation, for the mixture, and far the "unknown." In order to clean the burner. v s ~ a r i z water e far about 15 seconds each time solutions are cha&edand, a t the end of the series, vaporize a beakerful of water. (10) Note any shifting of transmission peaks in the mixture as to the individual stock and interpret the data. obtained for the "unknown " Record the number of the "unknown," wave lengths of the peaks observed and the components present in the "unknown."
JOURNAL OF CHEMICAL EDUCATlON
(11) Close, a t the control penel, first the hydrogen valve and then the oxygen valve. Quantilotiue (12) Assign a concentration value of 1.0 to the stock solution of the designated cation. Use one 10-ml. buret for this stock solution and another for water and prepare, in small, dry beakers or graduates, 10.0-ml. portions of solutions 0.8, 0.6, 0.4, 0.2, and 0.1 as strong as the stock solution. Thoroughly mix these solutions. Transfer each 80ltJtian to a separate sample beaker, first rinsing the beaker twice with 2-ml. portions of the solution. Include a beaker of the stock solution, a beaker of the "unknown," and a beaker of water and arrange all the beakers in a known order near the imtrurnent. (13) From the previous data, select the best transmission peak far the cation designated, and set the Wave Length Control a t this value. (14) Light the burner as directed in etep 4. (15) Adjust the Dark Current as direoted in step 7, elevate the most dilute (0.1) solution under the capillary, and read and record t,hr ner eent transmission. (16) Repeat step 15 for the solutions of conrentrations 0.2, 0.4,0.6,0.8,and 1.0. (17) Vaporize water for 15 seconds, run the "unknown," and then vaporize a beakerful of water to clean the burner. (18) First, tightly close the hydrogen tank valve and, after the flame me8 out. tightly close the oxygen tank valve. After all the gattie needies return to zero, cloae?he regulator and control panel valves (turn counterclockwise). (19) Turn the Sensitivity Switch to STANDBY, Shutter Switch to SHTR, Power Switch to OFF, and disconnect the instrument from the 110 V. line. (20) From the data obtained above, oonstruct a working curve by plotting per eent transmission as ordinate and concentration in p.p.m. as abscissa. Locate the per cent transmission of the "unknown" on the working curve and record the coneentratian in p.p.m. Record also on the graph-sheet the number of the unknown, the wsve length used, the slit width, and setting of the sensitivity switch.
DISCUSSION
Experience shows that satisfactory results are obtained with this experiment. Rarely is any component of the qualitative "unknown" missed and, when such is the case, it is usually caused by the too rapid rotation of the Wave Length Control while the "unknown" is being run. No trouble is experienced in the determination of the wave lengths of the transmission peaks with the stock solutions, slit widths, and sensitivity settings specified previously. A series of typical working curves for one peak wave length for each of the cations in both groups is shown in the accompanying figure. Similar curves are obtained for Ba++ a t 500 mp and Ca++ at 553 mp and 610 mp. When atypical curves are obtained, they are the result of definitely faulty techniques such rts errors made in the dilutions, failure to mix the solutions adequately, or a dirty burner capillary tube. The accuracy of the quantitative determination depends upon the character of the working curve, the cation involved and, inherently, upon the sensitivity of its flame spectrum. In general, an accuracy of about *ly, of the concentration of the quantitative
VOLUME 34, NO. 4, APRIL, 1957
0
0.2
0.4
0.6
0.8
1.0
concentration Working cur.Concentration 1.0 represent.: Bai+,2500 p.p.m.: K+. 50 p.p.m.: Naf 10p.p.m.: ME**, 2000p.p.m.: Lii, 100p.p.m.: SF*+,500p.p.m.: Cai+, 200 p.p.m. Slit width 0.8 mm. for Baif,K*, and Li+: 1.0 mm. for Nat. Mgf+, Srt+. Cat+. Wave length of meaalwement is indioated in pareothue..
"unknovn" can be expected with the procedure given. Such results are considered satisfactory for this experiment. Of the cations involved in this experiment, sodium is the most readily detected by means of flame photometry. If sodium is run quantitatively, corrections for the amount of it present in the water used must be made. In the qualitative analysis of Group A, using the blue-sensitive phototube, the fact cannot be disregarded that the ordinary reagent grade salts used to make up the stock solutions of the other cations may contain varying amounts of sodium salts as impurities. With Group B, using the red-sensitive phototube, the intensity of the sodium flame is not great enough to cause difficulty. Because of the sensitivity of the sodium flame a t a wave length of 589 mp, it is not advisable to use sodium as a component of a qualitative "unknown" for Group A unless enough time is available to make the necessary corrections. Sodium will be reported in an "unknown" for Group A unless reagents and water free from it are used. As an additional experiment, the quantitative determination of sodium in the water and the other reagents is interesting.
