Department of Chemistry University of Kentucky CHE 226 – Analytical Chemistry Laboratory 24 Na Atomic Emission EXPERIMENT 4 Determination of Sodium by Flame Atomic-Emission Spectroscopy USE ONLY DEIONIZED WATER (NOT DISTILLED WATER!) THROUGHOUT THE ENTIRE EXPERIMENT Distilled water actually has too much sodium in it. Clean all glassware and rinse thoroughly with deionized water both before and after use. There is sufficient sodium in tap water and even in distilled water to invalidate your results. Remember also to rinse out your plastic wash bottle several times and then fill it with deionized water. SAFETY WARNING CAUTION – Although the natural-gas/air flame is rather small, it has such a high temperature that contact of the flesh with even the outer edge of the flame will instantly produce a third-degree burn. Hands should be kept completely out of the “chimney” or burner housing whenever the flame is burning. NEVER put your hand above the burner housing. UNKNOWN Submit a clean, labeled 100-mL volumetric flask to the instructor so that your unknown sodium solution can be issued. Your name, section number, and your locker number should be written legibly on this flask. The flask does not need to be dry on the inside, but needs to have been rinsed with deionized water thoroughly after it was washed. The flask must be turned in at least 1 lab period before you plan to do the experiment so that the Teaching Assistants will have time to prepare the unknown. Each student will have his or her own unknown to analyze even if working in pairs. BACKGROUND Flame photometry, now more properly called flame atomic emission spectrometry or “flame photometry” is a relatively old instrumental analysis method. Its origins date back to Bunsen’s flame-color tests for the qualitative identification of select metallic elements. Probably the most common example of the atomic emission effect is fireworks for 4th of July celebrations and other events. As an analytical method, atomic emission is a fast, simple, and sensitive method for the determination of trace metal ions in solution. Because of the very narrow (ca. 0.01 nm) and characteristic emission lines from the gas-phase atoms in the flame plasma, the method is relatively free of interferences from other elements. Typical precision and accuracy for analysisDepartment of Chemistry University of Kentucky CHE 226 – Analytical Chemistry Laboratory 25 Na Atomic Emission of dilute aqueous solutions with no major interferences present are about ±1-5% relative. Detection limits can be quite low. “Good” elements typically have detection limits between about 1 ng/mL and 1 µg/mL. The method is suitable for many metallic elements, especially for those metals that are easily excited to higher energy levels at the relatively cool temperatures of some flames – Li, Na, K, Rb, Cs, Ca, Cu, Sr, and Ba. Metalloids and nonmetals generally do not produce isolated neutral atoms in a flame, but mostly as polyatomic radicals and ions. Therefore, nonmetallic elements are not suitable for determination by flame emission spectroscopy, except for a very few and under very specialized conditions. Flame photometry is a highly empirical, rather than an absolute, method of analysis such as gravimetry. That is, you must calibrate the method carefully and frequently. Many different experimental variables affect the intensity of light emitted from the flame and that finding its way to the detector. Therefore, careful and frequent calibration is required for good results. INSTRUMENTATION Buck Scientific Flame Photometer, Model PFP7 The PFP7 Flame Photometer is a low-temperature (air/natural gas) flame atomic emission photometer designed for the routine determination of sodium and potassium in aqueous solutions, two very important physiological elements. The “normal” adult ranges for Na+ and K+ in plasma are 136-145 mM and 3.5-5.0 mM, respectively. These levels correspond to about 3200 and 170 µg/mL. Plasma is typically diluted 100- to 200-fold prior to analysis. Additional filters are available for this instrument for lithium, calcium, and barium. The low-temperature flame (about 1700 οC as compared to oxygen/acetylene at 3100 ο) generates strong emission only from the most easily excited elements. Wavelength isolation is by use of a simple narrow-bandpass interference filter that is designed to transmit only the intense, characteristic sodium-doublet lines at about 589.0 and 589.6 nm. [Separate filters must be used to transmit the calcium line at 442.7 nm or the two potassium lines at 766.5 and 769.9 nm.] The detector is a relatively inexpensive, sturdy p-i-n photodiode. This solid-state device has an intrinsic (non-doped) layer sandwiched between the usual p- and n-doped layers that are in any diode – thus the origin of the appellation p-i-n. This arrangement gives the detector greater sensitivity and faster operating speed than standard photodiodes. [http://www.rp-photonics.com/p_i_n_photodiodes.html] The instrument is called a “single-channel” photometer because it can determine only one element at a time and has a single direct-reading output. The filter must be changed and the instrument recalibrated for a different element. The instrument uses a capillary aspirator to inject the sample into a mixing chamber containing a PTFE spray-impact bead and several PTFE baffles that serve to mix the fuel, oxidant, and sample droplets. This combination generates a sample mist of only the smallest droplets to enter the burner; most of the sample aspirated goes down the drain. Sample solution consumption is 2-6 mL/min.Department of Chemistry University of Kentucky CHE 226 – Analytical Chemistry Laboratory 26 Na Atomic Emission The manufacturer claims the limits of detection for the instrument are 0.2, 0.2, 0.25, 15, and 30 µg/mL for Na, K, Li, Ca, and Ba, respectively. The reproducibility is said to be better than 1% relative standard deviation for 20 consecutive samples using 10 ppm Na set to read 50.0 on the meter. EQUIPMENT NEEDED • Wash bottle(s) rinsed several times and then filled with deionized water • One 500-mL volumetric flask, from your locker • Assorted volumetric and/or graduated transfer pipets (provided for you in the locker designated for this experiment. • Five 100-mL volumetric flasks for the standards (experiment locker) • Eight to ten small