Analysis of Cesium and Rubidium Salts and Metals M. C. FARQUHAR and J. A. HILL Research laboratory, American Potash & Chemical Corp., Trona, Calif.
b Flame photometry i s the best method for determining alkali-metal impurities in rubidium and cesium. However, emission intensities are enhanced b y factors of four to five b y the major component. Furthermore, enhancement factors vary 20 to 30% (depending on concentration). This severe enhancement i s overcome b y the simulated standards method, and determinations of minor components are accurate to within &2,8y0 of the amount present, at the 95% confidence level. Assays for major component alkalies are accomplished b y conversion to sulfates and correcting the weight of sulfates for determined impurities. These determinations a r e accurate to within &O.l6yo of the amount present at the 95y0confidence level.
Flame photometry provides a quick and accurate method for determining these inpuiities. However, the easily ionized major component (cesium or rubidium) represses the ionization equilibrium of the alkali impurities and enhances their flame response to a marked extent, as shown in Table I. Compensation must be made for enhancement. Otherwise, determinations of alkali impurities will be subject to large positive errors, particularly in the cases of the heavier alkali elements. However, accurate results are obtained by the use of simulated standards, which were described by Dean (S), in which standards and samples contain very nearly identical alkali metal ion concentrations. Interelement effects, including enhancement, are therefore similar for all solutions, samples, and standards alike. Other compensating methods have been proposed, such as the standard addition method (4). However, difficulties result from the influence of concentration on enhancement. The standard addition method is dependent on constant enhancement (or depres-
of cesium and rubidium compounds has increased markedly in the past few years, and metals and salts of approximately 99% purity are now commercially available. Extraneous alkalies comprise the chief impurities. SE
Table I.
Enhancement Caused b y Cesium and Rubidium
Emission Intensity (b) in (a) in 4000 P.P.RI. Water CsCl solution
Enhancement Factor, (b/a)
Alkali (as chloride) RbCl
Concentration, P.P.M. 5 20
2 6
8 18
$4.0 +3.0
KC1
5 20
7 19
20 46
$2.9 $2.4
XaC1
5 20
26 63
35 80
$1.3 $1.3
5
5 13
6 14
+1.2 +1.1
LiCl
20
Emission Intensity (b) in (a) in 4000 P.P.M. Water RbCl solution 5 20
1 2
5 8
$5.0 $4.0
5 20
7 19
22 48
$3.1 +2.5
NaCl
5 20
26 63
34
77
+1.3 $1.2
LiCl
5 20
5 13
5 14
+1.0 +1.1
CsCl KC1
222
ANALYTICAL CHEMISTRY
sion) over an extended concentration range. However, as shown in Table I, enhancement of the alkalies varies 20 to 30y0 Kith a fourfold change in concentration. EXPERIMENTAL
Apparatus a n d Reagents. A Beckman D E spectrophotometer, equipped with a Model 23,700 D U power supply and a Model 9200 flame attachment, is used with a mediumbore oxygen-acetylene burner. Certain instrument settings have been found optimum from the standpoint of precision. These are shown in Table 11, together with optimum maximum concentrations of alkali elements. Standard alkali metal solutions are prepared from chloride salts of the highest obtainable purity. I t has been found convenient to prepare stock solutions for each alkali in three concentrations (expressed in terms of the elements) as follows: 10.0000, 1.0000, and 0.1000 grams per liter. The stronger solutions are prepared by weighing the appropriate amount of pure alkali chloride, dissolving in water, and diluting to 1000 ml. in a volumetric flask. The more dilute solutions are prepared by volumetric dilution of the strong (10.000 grams per liter) solutions. All stock solutions are stored in polyethylene bottles. For convenience, these dispense through three-way stopcock burets. Pure cesium and rubidium salts are not available commercially, but may be prepared from technical grade carbonates. High purity cesium chloride is prepared by the method of Baxter and Harrington (2) in which cesium is separated from the other alkalies by repeated recrystallizations of CsN03. -4 method for the purification of rubidium was reported by Archibald, Hooley, and Phillips (1) in which rubidium was separated from lithium, sodium, and potassium by recrystallizations of RbClJ. The rubidium chloroiodide mas converted to the acid tartrate, which was recrystallized several times to remove cesium. Table I11 shows analyses of typical salts prepared by the recrystallization methods. Procedure. FOR
FLaME
SAMPLE PREPAR.4TION PHOTOMETRY. Salts.
Weigh a 1.0000-gram sample of the salt and transfer t o a 250-ml. volumetric flask. Dissolve the sample in distilled water, acidify with concentrated hydrochloric acid to a few drops in excess of the alcoholic methyl
end point. Volatilize the organics on a steam bath, transfer the aqueous metal chloride solution to a 250-ml. volumetric flask, dilute to volume, and mis thoroughly. Use aliquots for analysis. Flame Photometric Procedure for Alkali Impurities. As has been mentioned earlier, it is necessary t o make standard comparison solutions which correspond closely t o t h e composition of the sample solution. Before proceeding t o the actual determinations, i t is first necessary t o provide such standards. T o accomplish this, set t h e spectrophotometer according t o the data in Table 11, and burn the
Table II. Optimum Conditions for Flame Determinations Maximum Concentration, Wavelength, Element Grams/Liter RIIp Phototube Slit, Mm. cs 0.1 852 Red 0.1 Rb 0.1 780 Red 0.1 K 0.02 7iO Red 0.1 Na 0.016 589 Blue 0.04-0.1 Li 0.040 67 1 Red 0.1 Other settings: sensitivity, full clockwise to two turns countercloclmise; Resistor, 10,000 megohms; oxygen pressure, 10 p.s.i.; acetylene pressure, 5 p s i Wavelength settings vary with individual instruments and should be set to give maximum readings for each element.
Tabie 111.
Typical Cesium and Rubidium Standard Salts Cesium Rubidium Chloride Chloride 99.95% CsCla 99.90% RbCl" 0.005 RbCl 0.062 CsCl 0.003 KCl 0.043 KCI 0 005 NaCl 0.003 KaC1
