Ro g e r C . We l l s a n d Ro l l i n E. St e v e n s, U . S. Geological Survey, W ashington, D . C.
I
N M O S T rock and mineral analyses on record no mention is made of rubidium and cesium. Lithium is reported as a trace in many— but this m e a n s l i t t l e quantitatively—
though it has of course been de
t e r m i n e d occasionally by the G ooch method. T h e few re
ported results for rubidium and c e s iu m a r e questionable. A m e t h o d for all the alkalies is
needed, one that will not be too complicated and that will be adapted to the particular purpose of handling the percentages of lithium, rubidium, and cesium met with in rocks and minerals.
Previous attempts to deal with all the alkalies are noted by Hillebrand and Lundell (2) and by Noyes and Bray (4). The method of extracting rubidium and cesium chlorides from potas
sium chloride by means of hydrochloric acid and alcohol has been used by several analysts. Strecker and Diaz, however, in re
ferring to this step (8) give no details of procedure, and the ex
traction of rubidium was found to be incomplete by Moser and Ritschel (3); they, however, used relatively large quantities of the two salts in their tests. Experience shows that efforts should be directed to dealing with small quantities of lithium, rubidium, and cesium.
Attempts to devise a quantitative procedure on the scheme of Noyes and Bray gave unsatisfactory results for small quantities of rubidium and cesium, as the precipitates formed were not sufficiently insoluble. Results were correct only to a milligram or more. A complex organic compound, sodium 6-chloro-5- nitrotoluene-m-sulfonate, is suggested by Davies (I) as a precipi
tant of potassium and rubidium, the cesiu m c o m p o u n d b e in g quite soluble. The writers found, however, that the rubidium com
p o u n d w as not sufficiently in
soluble for the separation of rubid
ium from cesium.
M o re r e c e n t ly O’Leary and Papish (3) have reviewed the ana
lytical reactions of rubidium and cesium and proposed some new methods. They precipitate rubid
ium and cesium with phosphomo- lybdic acid and obtain excellent separations from potassium, al
though the procedure appears to be rather time-consuming. Sepa
ration of most of the potassium, the element generally in excess, however, seems preferable to precipitation of the minor elements first.
The methods here described afford a determination o f each member of the alkali group and are successful in dealing with the quantities of the rare alkalies found in rocks and minerals.
They presuppose that the alkali chlorides have first been ob
tained free from all other compounds. Should magnesium and calcium be present they will be found mainly with lith
ium, and sulfate will be found with sodium. The behavior of traces of borate and fluoride has n ot been determined, but these may easily be removed if known to be present. Spec
troscopic confirmation o f the presence of mere traces o f any of the alkalies should of course be obtained. A t the present time the J. Lawrence Smith method seems to be preferred for extract
ing the alkalies from silicate rocks and minerals. It has long been used almost exclusively in the U. S. Geological Survey Methods are described which afford a deter
mination o f each member o f the alkali group and are successful in dealing with the quantities o f the rare alkalies fou n d in rocks and minerals.
The procedures are relatively rapid and based chiefly on the use o f chloroplalinic acid, absolute alcohol and ether, and ammonium sulfate. The percentages o f all the alkalies fou n d in a number o f minerals are given.
440 A N A L Y T I C A L E D I T I O N Vol. 6, No. 6 and is also recommended by Washington (9), and if carefully
followed will provide alkali chlorides of sufficient purity.
Re a g e n t s a n d Ap p a r a t u s
T h e salts used for reference were of analytical purity.
Some sodium chloride of exceptional purity was available.
Potassium chloride was Baker’s analytical reagent. Cesium chloride from Eimer and Amend was found to be pure b y the spectroscope and on analysis gave an atomic weight of 132.29 for cesium. The rubidium chloride from Eimer and Amend analyzed as follows: HiO, 0.44; LiCl, 0.18; NaCl, 0.36;
K C1,4.10; CsCl, 1.26; RbCl, 93.66; apparent atom ic weight of Rb, 82.1. This salt was purified by fractional precipita
tion with chloroplatinic acid to free it from potassium, lithium, and sodium, the rubidium chloroplatinate was con
verted into chloride by removal o f platinum b y redistilled formic acid, and reprecipitated with hydrochloric acid gas and alcohol to remove any remaining cesium. A fter two such treatments the atomic weight found for the rubidium was 85.34. M ost of the experiments involving rubidium were made with the salt having the analysis first given and correc
tions were applied for the impurities if necessary. These re
sults were checked in essential particulars, however, with the purest rubidium chloride.
M ost of the reagents for the methods described are avail
able in every laboratory or can easily be procured. For the separation of cesium certain reagents are needed in particular concentrations:
A m m o n iu m S u l f a t e S o l u t i o n . Five grains of ammonium sulfate in 100 ml. of water.
A l c o h o l i c A m m o n iu m S u l f a t e . Dissolve about 1 gram of ammonium sulfate in 20 ml. of water and add slowly with stir
ring 100 ml. of 95 per cent alcohol. Remove by filtering the excess ammonium sulfate that precipitates and to the clear liquid add a few crystals of ammonium sulfate to keep it saturated.
This solution contains about 0.54 gram in 100 ml.
W a s h S o l u t i o n . Prepare in the same way as the alcoholic ammonium sulfate, except that 0.16 gram of ammonium chloride is also added to the water solution of ammonium sulfate before addition of the alcohol.
Filtering was usually done with suction applied to a small bell jar containing a test tube or small graduate to receive the filtrate, and a filter containing a small plug of glass wool covered with asbestos or through a small sintered-glass filter
ing crucible. A milliliter-graduated pipet and a 10-ml. buret are needed. For the separation o f cesium the suction filter apparatus consisted of a bottle with bottom removed and rubber stopper carrying funnel and suction outlet tube. The bottom was ground smooth to make an air-tight connection with a greased ground-glass plate. A small wad of glass wool was placed in the bottom of the funnel and asbestos soup poured in to make a small dense filter. The filter was washed thoroughly with water and dried with alcohol before use.
The same filter may be used repeatedly. The funnel stem dipped through a perforated watch glass into the platinum dish or crucible which received the filtrate.
The “ radiator” used for evaporating off ammonium sulfate consisted of a 100-ml. porcelain crucible in which was placed a small porcelain support to hold the crucible being ignited.
H eat was applied to the bottom with a Bunsen flame.
Se p a r a t i o n o f So d i u m a n d Po t a s s i u m Gr o u p s
In the Smith method the total alkalies are generally weighed as chlorides, but the separations outlined in the present paper permit each alkali to be weighed separately, so that an initial weighing of the chlorides is merely a check. Ammonium chloride must of course be removed.
The alkali chlorides are first separated into two groups by the use of chloroplatinic acid; lithium goes with the sodium, and rubidium ami cesium go with the potassium. This separation
should be conducted with care to remove sodium and lithium as completely as possible from the potassium group. The potas
sium chloroplatinate group is then weighed. The chloroplati- nates are changed back to chlorides by precipitating the platinum with hot dilute formic acid. If the potassium chloroplatinate group is collected and weighed in a small Jena glass filtering crucible (fineness 4), the hot formic acid may be run through the filter several times, leaving the platinum on the filter. A further trace of platinum generally separates on evaporating the alkali chloride solution to dryness, so that it is better to remove the chloroplatinates to a dish and make two evaporations with formic acid before filtering off the platinum. The sodium chloroplati
nate solution is also evaporated twice to dryness with a little formic acid, and the salts are dissolved in water and filtered from the platinum.
Li t h i u m
Having obtained the chlorides b y removal of the platinum, the sodium solution is ready for the determination of lithium.
This is now done in the U. S. Geological Survey b y a slight modification o f the excellent Palkin method (6) rather than by the Gooch method requiring the use of obnoxious amyl alcohol.
P r o c e d u r e . Evaporate the solution of chlorides to dryness, preferably in a small glass-stoppered Erlenmeyer flask of about 30-ml. capacity. Dissolve in 0.4 ml. of water, warming slightly if necessary, cool, add 0.01 ml. of concentrated hydrochloric acid and 5 ml. of absolute alcohol, rotate the flask, add 15 ml. of ether, allow to stand about 15 minutes, filter through asbestos on glass wool in a small funnel with suction (or through a Jena sintered-glass filter or a weighed Gooch crucible for direct weighing of sodium chloride), wash well with a mixture of 1 part of alcohol to 4 or 5 parts of ether, finally evaporate the filtrate with a slight excess of sulfuric acid in a weighed dish, and heat to constant weight as lithium sulfate.
Palkin recommends a second treatment of the first lithium- bearing filtrate, bu t more sodium chloride is seldom obtained in a second treatment. W ith a zinnwaldite mica from Vir
ginia 1.92 per cent of lithium oxide was found by this method as compared with 1.85 found b y J. J. Fahey, of the Geological Survey, with the older G ooch amyl alcohol method.
So d i u m
Sodium is weighed as chloride, after separation of the lithium chloride, or as sulfate.
Ex t r a c t i o n o f Ru b i d i u m a n d Ce s i u m f r o m Po t a s s i u m
As already stated, Strecker and Diaz (8) give few details regarding the extraction of rubidium and cesium chloride from potassium chloride with hydrochloric acid-alcohol mix
tures. A number of experiments were therefore made on this point, with the object o f reducing the solubility of the potas
sium chloride as completely as possible. It was found that reduction o f the percentage of water was advantageous— for example, dissolving the chlorides (0.1000 gram) in only 0.4 ml.
of water, saturating with hydrochloric acid gas, and adding 10 ml. of anhydrous alcohol previously saturated with hydro
chloric acid gas gave the following results:
T a b l e I. S o l u b i l i t y o f A l k a l i C h l o r i d e s a t 25° C . KC1
-S O L U B I L IT T
R bCl CsCl
Grams Grams Grams
3 .0 3 7 .2 8 1 2 .7
0 .0 0 3 1 0 .0 2 1 0 .3 1 0 .0 0 0 6 0 .0 0 2 7 0 .0 2 4 SOLVEJTT
10 ml. of water
10 ml. of 1 volume of concentrated HC1 and 2 volume« of alcohol
0.4 ml. of water and 10 ml. of alcohol, both
■aturated with HC1
The separation of cesium from potassium is easier than separation of rubidium from potassium. T h e solubility indi
cated that about 2.7 mg. o f rubidium chloride might be ex
tracted from potassium chloride without carrying more than 0.6 mg. of potassium chloride, but tests show that the extrac
tion of rubidium proceeds slowly. The quantities extracted (Table II) were all actually determined as chloroplatinates,
November 15, 1934 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 441 to 25° C., but similar results were obtained after 30 minutes.
T a b l e II. E x t r a c t i o n o f a L i t t l e R u b i d i u m a n d C e s iu m
It m ay be safely concluded that if the first extract under the conditions specified is n ot greater than 0.6 mg., significant proportions o f rubidium and cesium are absent, and accord
ingly no further time need be spent in looking for them, unless spectrographic tests are of interest.
P r o c e d u r e . Evaporate the “ potassium chloride” to dryness in a 30-ml. Erlcnmeycr flask, preferably one with a glass stopper, at about 106° C. in an oven through which the neck of the flask emerges, or on the steam bath. Cool, add 0.4 ml. of water, warm, cool again, and saturate with hydrochloric acid gas from a small generating flask, by dropping concentrated hydrochloric acid into concentrated sulfuric acid. Then add 10 ml. of abso
lute alcohol, also previously saturated with hydrochloric acid gas.
Filter with suction through asbestos in a small funnel or through a sintered-glass filtering crucible. Wash with a displacement wash of 2 ml. of a mixture of absolute alcohol and ether. Evaporate the filtrate to dryness, ignite very slightly, but not to redness, and weigh. If the weight is not more than 0.6 mg., rubidium and first extract, but if this is appreciable a second extract should be made to remove any further cesium. Considerable ru
bidium may still remain with the potassium. The next step is to remove rubidium (and potassium) as sulfate from the extracts containing the cesium. The method here described was worked out by the junior author (R . E . S.).
Potassium and rubidium are precipitated by ammonium sulfate in alcoholic solution. T h e quantity of ammonium sulfate, determined by many trials, is such that 5 to 10 mg.
of rubidium sulfate are precipitated essentially free from cesium. A larger quantity of ammonium sulfate causes the precipitation of some cesium. Ammonium chloride is in
cluded in the alcoholic wash solution, together with am mo
nium sulfate to prevent precipitation of cesium sulfate during washing.
P r o c e d u r e . T o the dry alkali chlorides in a small dish or beaker add 0.1 ml. of 5 per cent ammonium sulfate and stir the solution until all alkali chlorides dissolve. T o the solution add 5 ml. of the alcoholic ammonium sulfate from the buret, dropwise with stirring. After a few drops of the solution have been added the sulfates of potassium and rubidium begin to form as a bulky precipitate. It is well to add the first milliliter very slowly, about one drop a second, as a coarser precipitate is thus formed.
Now allow the solution to stand a half hour with occasional stir
ring and then filter with mild suction through the asbestos pad.
Rinse the precipitate, beaker, and filter thoroughly with three portions of the wash solution, about 0.5 ml. each. A wash bottle giving a fine jet is useful for the purpose. Catch the filtrate and washings, containing the cesium, in a small tared platinum cru
cible or dish, and add a small quantity of powdered ammonium sulfate to convert cesium chloride to sulfate during the ignition to follow. Evaporate the filtrate on the steam bath until the salts
Cover the crucible with a watch glass and place in the radiator for the final drying of the residue and the removal of ammonium salts. A low flame is used; any salt which may spatter is re
flected back into the crucible by the watch glass. When ammo
nium chloride is seen on the watch glass it may be removed and the heat increased. After the ammonium chloride is all removed the mass begins to melt and ammonium sulfate to volatilize.
Care should be taken to avoid loss of cesium through spattering.
After most of the ammonium sulfate is removed, apply the full flame of the burner to the radiator for a short time, then move the crucible about in the direct flame of a burner, keeping the entire crucible at moderate redness. Place in a desiccator, covered with a watch glass, cool, and weigh. Repeat until a constant weight
servation. A quantity of cesium even less than 0.1 mg. can thus be detected.
Dissolve the precipitate formed in the treatment with alcoholic ammonium sulfate in water, take to dryness in a second tared
Gram Gram Gram Gram Gram Gram Gram Gram
7 0 .0 1 0 4 0 .0 1 1 3 0 .0 1 0 5 + 0 .0 0 0 1
442 A N A L Y T I C A L E D I T I O N Vol. 6, No. 6 crucible, ignite, and weigh to give the weight of rubidium and
potassium sulfates in the first extracts.
Se p a r a t i o n o f Ce s i u m a n d Ru b i d i u m
Experiments were made on a range of mixtures, containing quantities of cesium chloride up to 30 mg. and of rubidium chloride up to 10 mg., to illustrate the use of the method and the errors involved. Complete precipitation of the small quantity of potassium, left after treatment with hydrochloric acid and alcohol, was shown by an experiment with 2 mg. of potassium chloride. The crucible used to weigh the alkalies in the cesium fraction gave no change in weight, showing that potassium is quantitatively precipitated as sulfate. It was thought that sodium might also be removed completely, as its solubility as sulfate in water is less than that of potassium, but experiments showed that about 0.7 mg. of sodium sulfate remained in the filtrate, enough to affect the results and make spectroscopic identification of cesium difficult. T o a still larger extent lithium remains in solution; an experiment with 0.0025 gram of lithium chloride yielded 0.0030 gram of lithium sulfate in the cesium fraction. Care must therefore be taken in the chloroplatinate procedure in order to avoid the presence of lithium and sodium with the cesium.
The method works best for mixtures containing up to 10 mg. of rubidium chloride and not more than 20 mg. of cesium chloride. With more rubidium its removal becomes some
what incomplete, and with more cesium chloride it also begins to precipitate as sulfate. If larger percentages of these ele
ments are present, an aliquot portion of the potassium group should be taken for analysis or preliminary fractional separa
tions made. The results given in Table I I I were obtained at a room temperature of about 22° C., but during hot weather slightly different results were obtained and it was found necessary to cool the solutions.
On account of the difficulty of manipulating very small weights of material, temperature fluctuations, etc., the errors shown in Table I II have a relative rather than an absolute significance. Nevertheless they offer the possibility of mak
ing some corrections. When the quantities of the two chlorides involved are small, as in experiments 23 to 25, no corrections are indicated. The removal of larger quantities of rubidium is less complete and a correction of about 0.3 mg. is shown. With 30 mg. of cesium chloride the results are dependent on the quantity of rubidium chloride that accompanies it; if little or no rubidium is present the results are low b y about 1.3 mg., but when 10 mg. of rubidium chlo
ride are also present the results are high b y about 0.4 mg.
This behavior is due to the removal of the sulfate ion as ru
bidium sulfate.
A trace of cesium was sometimes observed by means of the spectroscope in the rubidium separated in this way, but that the separations were n ot due to a compensation of errors is clearly shown by the results with the single salts. Even less than 0.1 mg. of cesium sulfate can be shown b y the spectro
scope and a spectroscopic quantity would have little signifi
cance in the separations.
Ru b i d i u m
If the cesium calculated to cesium chloride, plus 0.6 mg.
of potassium chloride for each extraction with alcohol and hydrochloric acid, accounts for all the salts extracted, ru
bidium is absent; otherwise some rubidium is present, but that separated from the cesium m ay be only part of the total rubidium. If rubidium is found here the extraction of the potassium chloride with alcohol and hydrochloric acid is re
peated as long as any more rubidium chloride is extracted, allowing 0.6 mg. of potassium chloride for each extract. The small quantities of rubidium ordinarily found in succeeding extracts m ay be freed from the little potassium present by
separation as chloroplatinate in 5 ml. of 15 per cent alcohol, as shown below. T h e calculated weight of rubidium chloride can then be checked by the weight of rubidium chloroplatinate.
Se p a r a t i o n o f a Li t t l e Po t a s s i u m f r o m Ru b i d i u m a n d Ce s i u m
As both rubidium and cesium chloroplatinates are consider
ably less soluble than potassium chloroplatinate at ordinary temperature (7), rubidium and cesium m ay be separated from 1 or 2 mg. of potassium chloride, as obtained in the extractions with alcohol and hydrochloric acid. The chlorides are evapo
rated nearly to dryness with a slight excess of chloroplatinic acid, the salts well stirred with 5 ml. of 15 per cent alcohol, and the less soluble chloroplatinates filtered off, washed with 95 per cent alcohol, and weighed. Tests of this procedure are
rated nearly to dryness with a slight excess of chloroplatinic acid, the salts well stirred with 5 ml. of 15 per cent alcohol, and the less soluble chloroplatinates filtered off, washed with 95 per cent alcohol, and weighed. Tests of this procedure are