Determination of Sodium by Flame Atomic-Emission Spectroscopy

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 analysis
department 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 ?g/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 ?g/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 ?c as compared to oxygen/acetylene at 3100 ?)
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 dropinglets. this combination generates a
sample mist of only the smallest dropinglets 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
?g/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 plastic containers for aspirating solutions (experiment locker)
preparation of solutions
standard sodium stock solution, 100.0 ppm
1. accurately (to 0.1 mg) weigh out by difference 0.1271 g of reagent grade nacl into a small
plastic weighing boat. it is very difficult and time consuming to weigh out exactly this
amount. get it as close as you reasonably can, record the exact mass, and correct your
concentrations accordingly. remember: never transfer chemicals inside an analytical
balance.
2. carefully transfer the salt quantitatively into a 500-ml volumetric flask. use a few squirts of
deionized water from your wash bottle on the weighing boat and the sides of the flask to
wash all of it down into the flask. [0.100 g na/l = 100 mg/l = 100 ?g/ml = 100 ppm na).
3. add about 100 ml of deionized water to the flask, swirl several times, and dissolve all of the
salt before diluting to volume with deionized water. this is critical.
sodium standard calibration solutions
1. use deionized water for the “blank”.
2. pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of the standard 100-ppm sodium solution into the
first, second, third, fourth, and fifth 100-ml volumetric flasks, respectively.
3. dilute carefully to the mark with deionized water and mix thoroughly.
unknown solution
obtain the unknown from the instructor and carefully dilute to the 100-ml mark with deionized
water. mix thoroughly.
department of chemistry university of kentucky
che 226 – analytical chemistry laboratory 27 na atomic emission
procedure
carefully follow the instructions provided you for use of the instrument and measure the
emission intensity for the blank (deionized water), each standard, and the unknown(s).
1. when you are approaching the time to begin taking emission readings, call the teaching
assistant to light the flame, stabilize the flame photometer, and instruct you in its proper and
safe use. the instrument should have been turned on and the flame lit for 15 minutes
[aspirating deionized water] to ensure stability
2. thoroughly rinse all the equipment you will use in this experiment, first with lots of
distilled water, secondly with deionized water from a rinse bottle.
3. fill the tall, 25-ml, capped polyethylene vials with the blank (deionized water), the five
standards (1, 2, 3, 4, and 5 ppm na) and the unknown solution(s) – in that order – and place
them in the plastic holder designed for them. because water dropinglets cling to the vials, their
insides will need to be pre-rinsed with small amounts of their solutions first. put a ml or two
into a vial, cap it, shake it, then shake the contents into the sink. do this at least 3 times for
each vial.
4. aspirate deionized water until the meter reading stabilizes, this may take 30-90 sec. use the
blank knob to set the meter reading to 0.00. then aspirate the highest standard (5 ppm) until
the meter reading has stabilized. use the fine sensitivity knob to set the meter reading to
5.00. [the coarse sensitivity switch should be in the correct setting and not have to be
switched.]
5. repeat the two-step calibration procedure with deionized water and the 5 ppm standard as
many times as it takes to get them both stabilized at 0.00 and 5.00, respectively.
6. aspirate the blank, the 5 standards, and the unknown(s) in that order. take three replicate
readings of each solution once the meter reading has stabilized. there will be some
“bounce” (noise) in the readings, especially at the higher concentrations.
7. for the second calibration run, place the unknown solution(s) between the two standards
whose readings bracket that of unknown(s), so that the concentrations of the solutions
aspirated now all increase monotonically. atomic emission instruments work best when
going from low to high concentrations.
8. repeat the whole process of calibration and taking triplicate readings as before at least 1 or 2
more times. the more data you have to review, the better you will be able to detect and
eliminate determinate error – inaccuracy in the final reported value.
9. instrument alert. the aspirator compartment does not drain properly sometimes.
the symptom to watch for: you are aspirating some solution and the reading starts drifting
around. you then aspirate deionized water, but there is still a drifting and significant signal
for na. if you open the little window on the flame chimney, you can see some of the
yellowish emission of na. this means that the aspirator compartment needs to be “flushed”,
that is, get it draining again. usually this can be accomplished by gently wiggling the drain
tygon tubing that is attached to the bottom of the aspirator-burner, just a little below where it
is attached. if you have continuing trouble, ask the ta for assistance.
department of chemistry university of kentucky
che 226 – analytical chemistry laboratory 28 na atomic emission
10. when completely done with the experiment, aspirate deionized water to clean out the
aspirator/burner, clean the work areas up thoroughly, and notify a ta that the instrument is
ready to be shut down.
11. thoroughly rinse all the glass and plastic ware provided you for the experiment with
deionized water. drain the water out and put the equipment back in the drawer.
hazardous waste disposal
no hazardous materials are used or generated in this experiment. all we have are dilute
solutions of ordinary table salt. empty all the solutions down a sink drain with cold water
running. thoroughly rinse all glass or plastic ware you used with deionized water.
data treatment
prepare a calibration curve by plotting the emission intensities as a function of na concentration.
determine the concentration of sodium in the unknown sample by reading the concentration of
the sample which corresponds to its emission intensity from the calibration curve.
depending on the drift in the instrument and other factors, it may be better to average all three
values for each solution and obtain one final value for the unknown, or to get three separate
values for the unknown, each using its “own” calibration curve, and average the three values.
try both to see if one approach seems to be better than the other. if the plot appears to be
reasonably linear, or at least that portion of it that includes your unknown, use the excel linest
function to do a linear-least-squares fit to the data, which will also provide you some quality
parameters for the fit.
report the “best estimate” for the average concentration of sodium in ppm (?g/ml) and the
associated standard deviation of the value.
text reference
d. a. skoog, d. m. west, f. j. holler, and s. r. crouch, analytical chemistry: an introduction,
7th ed., chapter 23, pp. 594-631.
revised march 8, 2007
copyright © by the department of chemistry, university of kentucky, 2007