Sodium flame emission determination

In the flame emission determination of sodium, lithium is often added as afi internal standard. The following data were obtained for solutions containing Na and 1000 ppm Li. I... 

Figure 8-13 Spreadsheet to illustrate the internal standard method for the flame emission determination of sodium.

For routine flame-emission determinations of alkali metals and alkaline earth elements, simple filter photometers often suffice. A low-temperature flame is employed to prevent excitation of most other metals. As a consequence, the spectra are simple, and interference filters can be used to isolate the desired emission lines. Flame emission was once widely used in the clinical laboratory for the determination of sodium and potassium. These methods have largely been replaced by methods using ion-selective electrodes (see Section 2 ID). 

All the cations of Group I produce a characteristic colour in a flame (lithium, red sodium, yellow potassium, violet rubidium, dark red caesium, blue). The test may be applied quantitatively by atomising an aqueous solution containing Group I cations into a flame and determining the intensities of emission over the visible spectrum with a spectrophotometer Jlame photometry). 

Description of Method. Salt substitutes, which are used in place of table salt for individuals on a low-sodium diet, contain KCI. Depending on the brand, fumaric acid, calcium hydrogen phosphate, or potassium tartrate also may be present. Typically, the concentration of sodium in a salt substitute is about 100 ppm. The concentration of sodium is easily determined by flame atomic emission. Because it is difficult to match the matrix of the standards to that of the sample, the analysis is accomplished by the method of standard additions. 

Analytical methods employed in soil chemistry include the standard quantitative methods for the analysis of gases, solutions, and solids, including colorimetric, titrimetric, gravimetric, and instrumental methods. The flame emission spectrophotometric method is widely employed for potassium, sodium, calcium, and magnesium barium, copper and other elements are determined in cation exchange studies. Occasionally arc and spark spectrographic methods are employed. 

Undoubtedly, while the direct method is more relevant, because the analyte activity in water plasma is actually measured, the reporting on blood sodium, potassium and chloride in terms of concentration in plasma is preferred by medical professionals, whatever method of measurement is used. This is justified by the fact that before ISEs had been invented, sodium, potassium and chloride were all determined by indirect methods, with flame emission spectroscopy (FES) for Na+ and K+, and coulometry for Cl. ... 

International Standard Organization. 1993. Water quality. Determination of sodium and potassium. Part 3 Determination of sodium and potassium by flame emission spectrometry. ISO 9964-3. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

A 250 mL sample of each solution from the polyethylene bottle was filtered through a Millipore filter (0.45 urn pore size). The concentrations of chloride, nitrate and sulfate ions in the filtrate were determined by ion chromatography using a YEW IC 100 of Yokogawa Hokushin Electric Co. Ltd. The concentrations of sodium and potassium were determined by flame emission spectrometry and concentrations of calcium and magnesium by atomic absorption spectrometry using a Hitachi 170-50 Atomic Absorption Spectrophotometer. An aliquot of each filtrate was used for the determination of Sr by ICP emission spectrometry after adding nitric acid (0.1 N), detailed analytical conditions of which are reported elsewhere (3). 

Much of the following discussion is in the nature of a preliminary report. Considerable research remains to be done, especially in the areas of sampling and standards. This research is currently in progress. Since this report is concerned with the methodology of the analysis, results will be reported in a separate publication. The elements determined were Ag, Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sr, and Zn. Of these, potassium, sodium, and strontium were determined by flame emission. The choice of elements determined by AAS was dictated almost entirely by the hollow cathode lamps available to the investigators. Concentrations of the elements determined ranged from about 10% to undetectable. 

Sodium, potassium, and strontium were determined by flame emission. Most AAS instruments are capable of this mode of operation. Ideally, the sample to be analyzed for sodium would be heavily spiked with calcium and potassium that for potassium, with sodium and calcium. This was not done in the current study however, potassium was added before the determination of sodium, calcium, and strontium. 

Herrmann and Lang (H3) studied various atomizers and recorded best results with a laboratory-built high pressure vaporizer. No ionization interference was seen in an air-propane flame and calibration curves were straight from 1 to 10 mg sodium per liter. Determinations were performed on serum diluted 1 20-1 200 and results agreed well with those concurrently obtained by emission flame photometry. 

Sodium may be determined by atomic absorption spectrophotometry (AAS), flame emission spectrophotometry (FES), electrochemically with an Na -ISE, or spectrophoto-metrically. Of these methods, ISE methods are by far the most common. Excellent accuracy and coefficients of variation of less than 1.5% are readily achieved with modern equipment, reliable calibrators, and a good quality assurance program. Because sodium and potassium are routinely assayed together, methods for their analysis are described together later in this chapter. 

Sodium and potassium levels are difficult to analyze by titrimetric or colorimetric techniques but are among the elements most easily determined by atomic spectroscopy (2,38) (Table 2). Their analysis is important for the control of infusion and dialysis solutions, which must be carefully monitored to maintain proper electrolyte balance. Flame emission spectroscopy is the simplest and least expensive technique for this purpose, although the precision of the measurement may be improved by employing atomic absorption spectroscopy. Both methods are approved by the U.S. (39), British (40), and European (41) Pharmacopeias and are commonly utilized. Sensitivity is of no concern, due to the high concentrations in these solutions furthermore, dilution of the sample is often necessary in order to reduce the metal concentrations to the range where linear instmmental response can be achieved. Fortunately, the analysis may be carried but without additional sample preparation because other components, such as dextrose, do not interfere. 

Janssen (Jl) suggested that spectral analysis, until then used only for qualitative observations, was suitable also for quantitative work. He felt that such a development would be particularly advantageous in the case of elements like sodium which were difficult to determine by classic procedures. His suggestions bore fruit 3 years later when Champion et al. (Cl) constructed an instrument for the determination of sodium in plant ash. A solution of plant ash was introduced into the flame by means of a platinum wire and the emission intensity measured by comparing it by means of a visual photometric attachment with light from a reference constant-intensity sodium flame. This spectronatrometre was the first flame photometer and when one considers that it was capable of an accuracy of between 2 and 5 %, it is interesting that it was not for more than 70 years that the method was applied to clinical problems. 

In the early years of flame photometry, only relatively cool flames were used. We shall see below that only a small fraction of atoms of most elements is excited by flames and that the fraction excited increases as the temperature is increased. Consequently, relatively few elements have been determined routinely by flame emission spectrometry, especMly j ew of those that emit line spectra (several can exist in flames as molecular species, particularly as oxides, which emit molecular band spectra). Only the easily excited alkali metals sodium, potassium, and lithium are routinely deterniined by flame emission spectrometry in the clinical laboratory. However, with flames such as oxyacetylene and nitrous oxide-acetylene, over 60 elements can now be determined by flame emission spectrometry. This is in spite of the fact that a small fraction of excited atoms is available for emission. Good sensitivity is achieved because, as with fluorescence (Chapter 16), we are, in principle, measuring the difference between zero and a small but finite signal, and so the sensitivity is limited by the response and stability of the detector and the stability (noise level) of the flame aspiration system. 

An analyst notes that a 1-ppm solution of sodium gives a flame emission signal of 110, while the same solution containing also 20 ppm potassium gives a reading of 125. It was determined that a 20-ppm solution of potassium exhibited no blank reading. Explain the results. 

EXPERIMENT 30 FLAME EMISSION SPECTROMETRIC DETERMINATION OF SODIUM... 

The sodium in a series of cement samples was determined by flame emission spectroscopy. The flame photometer was calibrated with a series of NaCl standards that contained sodium equivalent to 0,20.0,40,0,60.0, and 80.0 pg Na20 per ml,. The instrument readings R for these solutions were 3.1.2I 5.40,9. 57.1. and 77..3. 

Furthermore, atomic absorption generally works with a larger number of atoms than do other techniques. Sodium, for instance, is one of the elements most favorably determined by flame emission yet only 1.5% of the atoms are excited at the highest obtainable flame temperatures. Most of the remaining atoms are dissociated from their compounds and unexcited, and thus available for atomic absorption. For calcium under similar circumstances, the ratio in favor of atomic absorption is much greater. For every atom that is excited and available for flame emission, at least 1,000 are dissociated and accessible to atomic absorption. 

Several elements (Zn, Pb, Cuy Ni, Ca, Mg, Fe, and Mn) are determined routinely in water samples using atomic absorption spectroscopy. Sodium and potassium are determined by flame emission. The preparation of the samples the analytical methody the detection limits and the analytical precisions are presented. The analytical precision is calculated on the basis of a sizable amount of statistical data and exemplifies the effect on the analytical determination of such factors as the hollow cathode sourcey the ffamey and the detection system. The changes in precision and limit of detection with recent developments in sources and burners are discussed. A precision of 3 to 5% standard deviation is attainable with the Hetco total consumption and the Perkin-Elmer laminar flow burners. 

The samples were first run on the Jarrell-Ash instrument, with the three burner set and the same instrument was used as a flame emission spectrophotometer for the determination of sodium and potassium. A statistical summary of analytical precision for eight elements in the eight samples are shown in Table 1. Here the precision is expressed as the % standard deviation (coeflBcient of variation), which is defined as one hundred times the ratio of standard deviation to the mean concentration (9). It can be seen from this data that the last three elements, which are present in a quantity near the limit of detection, have large deviations. It is clear that these figures could be lowered if the analysis were run using higher concentrations. But our purpose was to evaluate the usefulness of the technique for routine determinations. 

The first quantitative analysis based on the flame emission technique was made by Champion, Pellet, and Grenier in 1873. They determined sodium by using two flames. One flame was concentrated with sodium chloride and the other was fed with the sample solution along a platinum wire. The determination was based on the comparison of the intensities of the flames by dimming the brighter flame with a blue glass wedge. 

Hospitals routinely determine blood calcium, potassium, and sodium to determine any changes from normal. These routine determinations are usually performed using flame emission or atomic absorption methods. 

At present, flame emission still retains advantages in approximately the same area in which it began, determination of alkali metals. Howeveq sodium and potassium are so abundant that they are readily determined by absorption in most sample types. Rubidium has been used as a tracer in studies of insect-plant relationships, taking advantage of its low natural abundance, low toxicity, and great sensitivity by flame emission. The higher red-sensitive photomultipliers such as RCA 4840 should be used beyond 600 nm. 

The procedure for the determination of the alkalis (NaaO, K2O, and Li20) is effectively identical for all classes of material in that 0.25 g of sample is decomposed with hydrofluoric acid together with dilute nitric and sulfuric acids in a platinum dish on a sand bath. The residue is dissolved in dilute nitric acid and alkalis determined directly on the solution by flame photometry or flame atomic absorption spectroscopy (FAAS) in emission mode. Cesium and aluminum sulfate buffers are added to aliquots for the flame photometric determination of sodium and potassium. 

Excitation and ionization interferences are nonspectral interferences. When a sample is aspirated into a flame, the elements in the sample may form neutral atoms, excited atoms, and ions. These species exist in a state of dynamic equilibrium that gives rise to a steady emission signal. If the samples contain different amounts of elements, the position of equilibrium may be shifted for each sample. This may affect the intensity of atomic emission. For example, if sodium is being determined in a sample that contains a large amount of potassium, the potassium atoms may collide with unexcited sodium atoms in the flame, transferring energy in the collision and exciting... 

During the past two decades, plasma spectroscopy has dominated the analytical market for atomic emission spectrometry. Consequently, we will not discuss the use of flames for atomic emission, as the industry, in large, has moved toward plasmas for this application. On some occasions, flame emission is used for analysis of sodium and potassium. Nonetheless, these elements can be easily determined by inductively coupled plasma spectrometry. 

The ratio, Nj/N0, can therefore be calculated. For the relatively easily excited alkali metal sodium, it is 9.9 x 10 6 at 2000 °K and 5.9 x 10 4 at 3000 °K this latter temperature is about the highest commonly obtained with flames used for atomic absorption or emission work. Hence, only about 1(T3 % of the sodium atoms are excited at 2000 ° and 6 x 1(F2 % at 3000°. For an element such as zinc,Nf/N0 is 5.4 x 10"10 at 3000 and so only 5 x 10"8% is excited. In spite of the small fraction excited, good sensitivities can be obtained for many elements by flame photometry if a high temperature flame is used, because the difference between zero and a small but finite number is measured. For example, seventy elements can be determined by flame photometry using the nitrous oxide-acetylene flame 1H. 

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