Download DETERMINATION OF RUBIDIUM IN SEAWATER

DETERMINATION
Raymond
C. Smith, K. C. Pillai,”
Scripps Institution
of Oceanography,
OF
RUBIDIUM
IN
SEAWATER’
Tsaihwa J. Chow, and Theodore
University
of California,
San Diego,
R. Folsom
La Jolla, California
ABSTRACT
Rubidium concentrations
in seawater from various locations and depths to 3,900 m have
been determined
using two independent
techniques--direct
flame photometry
and mass
spectrometry.
Results of these two techniques agree within estimated errors and indicate
that the rubidium content in the oceans is uniform within H$% with values varying about
120 pg/liter.
Sediment from the sea floor below a depth profile, analyzed for rubidium and
potassium, gave a rubidium concentration
factor of 150 and a potassimil to rubidium ratio of
1,270.
INTRODUCTION
Soon after his discovery of rubidium,
Bunsen evaporated large quantities of seawater in an attempt to demonstrate its
existence in the sea but obtained negative
results ( Bunsen 1861) . Similarly,
other
early investigators (Grandeau 1863; Sonstadt 1870; Schmidt 1878; and Thompson
and Robinson 1932) failed to detect rubidium in seawater. Results of more recent
investigations, usually surface coastal waters,
are summarized in Table 1. There is considerable variation in these values, from 35
to 450 ,g/liter,
and there are no accurate
representative values from various locations
and depths in the oceans. However, previous analytical techniques have been cumbersome and time-consuming and did not
lend themselves easily to routine analysis of
samples.
Direct flame photometric determinations
of alkali-earth elements in mg/liter quantities have been developed, and alkali metals
like lithium, rubidium, and cesium in the
,g/liter range have been assayed by mass
spectrometry, neutron activation, and flame
l The authors are indebted to their colleagues
who collected the samples, especially Daniel M.
Brown of R. V. Agussiz and Jan B. Lawson of R. V.
Argo of the Scripps Institution.
They also wish to
thank Lawrence E. Finnin for contributing
so much
to the construction
and improvements
in the flame
photometer.
This work received substantial
support from the U.S. Atomic Energy Commission,
the Office of Naval Research, and the National
Science Foundation.
3 On deputation from Atomic Energy Establishment Trombay,
Bombay, on an IAEA Fellowship
administered
by the U.S. National
Academy
of
Sciences-National
Research Council.
photometry after enrichment.
The work
described in this paper employs two independent techniques-direct
flame photometry and mass spectrometry-to
ascertain
values for rubidium concentration in seawater.
APPARATUS
AND
METHODS
A stock solution containing 125.6 mg rubidium/liter
was prepared by drying and
weighing spectrochemically
pure rubidium
chloride and dissolving it in a known volume
of double-distilled
water. The stock solution was stored in a Teflon bottle, and
rubidium
standards were prepared from
this by dilution.
The Pacific Ocean seawater samples were
collected in Nansen type bottles made of
Lexan plastic and brought to the laboratory
in polyethylene
bottles. Those from the
Indian and South Atlantic oceans were
taken with a large metal sampler and stored
in SO-liter polyethylene carboys. Salinities
of these samples were determined by the
conductivity bridge method. The rubidium
content of each sample was then independently determined by both of the following
techniques :
1) Flame Photometry: To make quantitative measurements of rubidium in seawater without recourse to chemical separation and concentration, a low-noise system
with high signal-to-noise ratio was constructed with the following components :
226
Beckman hydrogen
and housing
Bausch and Lomb
chromator
atomizer-burner
grating
mono-
DETERMINATION
TABLE
Date
this
1.
Rubidium
OF
Investigators
Source
Burkser, et al.
1933
1940
Goldschmidt, et al. *
Kovaleva and Burkser
1942
1944
Ishibashi and Harada
Borovick-Romanova
1955
1957
1959
1964
1965
Smales and Salmon
Smales and Webster
Ishibashi and Hara
Bolter, Turekian and Schutz
Average of this work
paper
(Goldschmidt
corrected
to 20
IN
et al.
mg/kg.
1933)
227
SEAWATER
content of seawater reported
1932
* In the original
was mistakingly
RUBIDIUM
by various investigators
No. of
samples
of seawater
Azov Sea
Black Sea
North Sea
Black Sea
Mediterranean
Sea
English Channel
Japan Coast
Japan Sea
Barents Sea
Greek Archipelago
Okhotsk Sea
Black Sea
North Atlantic Ocean
North Atlantic Ocean
Japan Coast
Atlantic Ocean
Pacific, Atlantic and Indian
a value
Grating: 675 grooves/mm blazed for
0.7-1.6 ,J,
Filter:
Ednalite Duraklad R3 29F
dark red
Farnsworth (ITT) FW-118 photomultiplier with an S-l photocathode.
Throughout the determination the flame
photometer settings were as follows:
Wavelength:
scanned from 787 rnp to
803 rnp in 33 seconds
Photomultiplier
voltage: 1811 v
Rubidium spectral line: 795 rnp
Slit width: 0.1 mm
Oxygen: 15 psi (1.06 kg/cm”)
Hydrogen : 6.5 psi ( 0.46 kg/cm” )
The photometer was adapted from one
designed for cesium analyses at Oak Ridge
National Laboratory (Feldman and Rains
1964), but the electronic circuitry
was
simplified.
Since the FW-118 photomultiplier has unique signal-to-noise properties
( Eberhardt 1963n, b, and c), it is an excellent low-noise amplifier. Use was made of
this characteristic to eliminate intermediate
amplifiers and the output of the photomultiplier was fed directly into a self-balancing
potentiometric
chart recorder (10 mv full
scale) across a 6,700 ohm load resistor. A
of 200
Kg/kg
was
given.
Oceans
In
Goldschmidt
Rubidium
,ug/liter
1
1
1
1
1
1
1
1
2
2
1
1
6
1
1
11
16
( 1958,
200
320
200
450
340
240
35
210
200
190
180
100
120
121
190
125
120
p.
169 ),
simple potentiometer circuit was used for
background current and sensitivity adjustment. While input impedance of this circuit is higher than that recommended for
the recorder, this did not lower the full
scale response or increase the response time
significantly.
Since special difficulties attend elimination of flame background when it is impractical to prepare a blank solution, the
standard additions technique (Chow and
Thompson 1955) was used to eliminate
interferences caused by changes in physical
and chemical conditions of the seawater
sample.
Fifteen-ml aliquots of a seawater sample
were pipetted into four different Pyrex
bottles, and known quantities of rubidium
standard were added in the ratio of 0, 1, 2,
and 3. The volume of each solution was
adjusted to 2,O ml with double-distilled
water. The quantity of additions was approximately one-half, equal to, and one and
one-half times the original rubidium present
in the solution. This represented a compromise that minimized
the number of
standard additions to be made per sample,
kept the standard deviation of the extrapolated intercept
a minimum
for given
deviations in single intensity measurements,
228
2”
60
5
ii
lx
40
RAYMOND
C. SMITH,
K. C. PILLAI,
TSAIHWA
20
795
WAVELENGTH
[wl
FIG. 1.
Schematic diagram of rubidium 795 rnp
spectral line overlapping
large potassium 766/769
rnp background
line, -- net rubidium
after
background is subtracted.
and retained the linearity of the emission vs.
concentration curve.
The net emission intensity of each of the
four portions was measured as follows:
After sufficient instrument warm-up, measurements of four aliquots of each of the
samples were carried out rapidly in a uniform rhythm to avoid drift of sensitivity
during measurements. The aliquots of the
samples were analyzed in order of increasing concentration
and repetitions
were
made by re-running the four aliquots. The
atomizer was rinsed with warm distilled
water between each measurement.
This
periodic rinsing helped to prevent clogging
(more precisely the deposition of salt crystals on the atomizer tip) of the burner by
sea salts. The stability of the instrument
an d uniformity of burner atomization were
checked at the beginning and end of measurements by running a standard rubidium
solution.
The net emission intensity plotted against
rubidium concentration gives an intercept
from which the rubidium content of the
original sample can be determined. In this
paper, a best-fit to a straight line and the
extrapolated intercept were calculated numerically.
J. CHOW,
AND
THEODORE
R. FOLSOM
To measure the net emission intensity of
rubidium in the presence of large quantities
of other elements requires care. In particular, since the ratio of potassium to rubidium
is greater than three thousand in seawater,
the “tail” of the potassium doublet at 766/
769 rnp obscures the 780 rnp line of the
rubidium doublet and gives a high background emission under the 795 rnp line. The
2.5 rnp half-intensity
band-width
of the
grating monochromator
could have been
narrowed to lower this background intensity, but only with a sacrifice in photocurrent and hence in a lower signal-to-noise
ratio. To measure the net emission intensity
of rubidium in the presence of this high
background response, which varies more or
less linearly with wavelength, a chart recorder was used to plot total emission
intensity from 787 to 803 rnp. Fig. 1 illustrates the features of such a record. The
overlapping
interference
is corrected by
measuring the peak height, h, from the top
of the rubidium peak to a point directly
below on a line representing the background
emission of potassium. The height, h, is
taken as the net emission intensity of rubidium.
Shellenberger et al. (1960) showed that
the emission intensity of rubidium is enhanced by the presence of potassium. Their
data also show that after the addition of
large quantities of potassium, further increases have relatively little effect on the
rubidium emission. From these results and
because there is a large excess of potassium
in seawater, it is assumed that the amounts
of rubidium added, in the standard additions technique, have negligible effect on
the relative emission intensity of the rubidium line. Also, our measurements have
shown that the added rubidium is not sufficient to change the potassium, and hence
the background, emission intensity noticeably (less than 1%).
Interferences
that affect equally the
emission intensity of radiation from the
test element either originally
present or
added as the standard increment are referred to as multiplicative
because, in principle, the interference could be corrected by
multiplying
the emission intensities by a
DETERMINATION
OF
RUBIDIUM
constant factor. Interfering substances that
are corrected by the addition or subtraction
of a constant factor are additive interferences. The standard additions technique
permits determination of the slope of the
emission intensity vs. concentration curve
(working curve) in the presence of multiplicative interferences. But, if additive interferences are present and unaccounted for,
the working curve will be translated parallel
to itself up or down the emission intensity
axis and the extrapolated value giving the
original concentration will be in error. Our
procedure, taking the net emission of
rubidium as the height from the peak to a
point directly below on a line representing
the background emission, is a method of
correcting for additive interferences, since
the background, regardless of origin, is
automatically subtracted. This is true except when, by some mischance, a background line peak lies within the instrument
band-width of the analytical spectral line.
Search of flame spectra tables (Hermann
and Alkemade 1963) shows no known lines
arising from elements sufficiently
concentrated in seawater, that could affect the
rubidium measurements.
2) Mass Spectrometry:
Isotope dilution
technique was performed using a 60” sector
single focusing mass spectrometer with a
12-inch (30 cm) radius of curvature. An
enriched “TRb tracer, obtained from the
Oak Ridge National Laboratory, was dissolved in 1 N HCI to make up the spike
solution containing 68 pg rubidium per ml.
The isotope composition of the spike and of
the common rubidium were determined as
follows :
“Rb atom % “5Rb atom %
Tracer
99.19
0.81
Common
27.85
72.15
Seawater ( approx 100 ml) was introduced into a 250-ml Teflon beaker, and 100
~1 of rubidium
spike were added. The
combined solutions were reduced to a small
volume by evaporation. About 5 ,~l of this
brine were placed and dried on the surface
of a tantalum filament that had been carefully baked in vacuum at 2.5 amp until any
rubidium which might have been initially
IN
229
SEAM’ATER
present on the filament was removed. No
chemical separation or enrichment was required. Since no other chemical than the
rubidium spike was used, the contamination
could be controlled.
The spectrometric procedure has been
described by Chow and Goldberg ( 1962).
The filament current was slowly raised to
0.8 amp, corresponding to a temperature of
about 750C. At this temperature only the
rubidium spectrum was observed. The interference from 87Sr was carefully checked
by monitoring
the @Sr and Y5r peaks.
Throughout
the mass spectrometric measurements, no strontium ion beam was detected.
The electrometer
shunts and atomic
weight corrections were incorporated in the
calculation. Since measurements were made
with a Faraday cup collector, the square
root of mass ratio correction was not necessary. The spike solution and seawater were
mixed so that the ratio of sTRb to s5Rb was
about one, which minimizes the error and
facilitates measurement. The amount of
rubidium in the sample was calculated from
the following equation:
M=
m( iV x s5Rb, - s7Rb,)
=Rb, - N x ““Rb,
’
where
LM= rubidium in the sample (pg);
m = amount of 8TRb tracer added
(PdG
N = observed t(‘iRb/s5Rb ratio
with atomic weight correction;
“SRb,s,S5Rb, = atom % of 87Rb and 85Rb in
the rubidium tracer, respectively;
87Rblo,s5Rb, = atom % of 87Rb and X5Rb in
the common rubiduim, respectively.
RESULTS
AND
DISCUSSION
In flame photometric analysis, random
errors arise from a number of causes (Hermann and Alkemade 1963). The dominant
source in this determination is the variation
in emission intensity caused by the formation and dislodgment of salt deposits on
the atomizer tip. This error might be re-
230
RAYMOND
C.
SMITH,
K.
C.
PILLAI,
TABLE
2.
TSAIHWA
Rubidium
J. CHOW,
content
AND
THEODORE
Depth
(m)
FOLSOM
of seawater
Rubidium,
Location
R.
pg/liter
Normalized
Temp.
(Cl
Flame
photometer
Mass
spectrometer
s ‘%g,
Pacific Ocean
33”25’ N lat
122”36’ W long
( Agassix 80.80 )
8 Ott 1963
0
49
98
291
485
972
1,943
2,906
3,920
19.44
16.30
12.01
7.30
5.69
3.90
2.14
1.64
1.50
33.09
33.02
33.04
34.03
34.17
34.56
34.61
34.66
34.69
120
113
112
115
117
109
110
112
122
129
121
118
114
113
111
112
122
119
132
124
122
118
118
112
112
118
122
34”04’ N lat
122”02’ W long
( Agassiz 82.47 )
9 Ott 1963
0
474
18.46
6.81
33.59
34.25
127
128
N.D.
N.D.
133
131
33.76
119
115
121
32”52’
117”15’
( S.I.O.
27 Aug
N lat
W long
Pier )
1963
0
South Atlantic Ocean
19”44’ S lat
12”54’ W long
( Lusiad 51)
23 June 1963
0
3,000+
-
36.50
34.90
144
121
135
118
134
120
S. Indian Ocean
18”Ol’ S lat
40”51’ E long
( Lusiad Hydro 2 )
22 May 1963
0
2,500+
-
34.64
34.93
132
129
125
114
130
123
121
119
123
Average
duced and the precision of measurement
increased by chemical separation and concentration of rubidium, but our aim was a
procedure suitable for routine analysis on
unconcentrated seawater, and the precision
of the present method is greater than all
previous work except the single measurement of Smales and Webster ( 1957). By
comparison, the much smaller concentrations of cesium occurring in seawater (about
0.4 pg/liter ) were determined with a precision of about +4% (Folsom, Feldman,
and Rains 1964) using a similar photometer
and the aid of concentrating steps.
The precision of the extrapolated intercept value depends on the deviation in the
individual intensity values, the number of
individual intensity measurements, and the
slope of the best-fit line. Choice of these
variables has been discussed above. In the
standard additions method, the prime consideration is directed to minimizing changes
in burner characteristics during a determination while such changes between repetitions are of no consequence to the intercept
value, since they are multiplicative
in effect.
In analyzing the data, the extrapolated
intercept value and its standard deviation
were calculated for each determination.
At
least three sets of measurements were made
on each sample. The replicated intercept
values were averaged. The standard deviations ranged from 2 to 10% with the average
of the final rubidium values being about
6%.
A careful study indicated that the possible systematic errors (e.g., non-linearity
of the working curve, possible contamina-
DETERMINATION
OF
RUBIDIUM
tion, neglect
of interferences)
are small
compared to the random errors. However,
the effect of most of the possible systematic
errors is to increase the measured amount
of rubidium.
In mass spectrometric
analyses, random
errors arise from chemical
contamination,
fluctuation
of the ion beam intensity,
and
background
noise.
Because
there is no
chemical
operation
involved
and the isotopes are measured with a ratio of about
one, random errors are greatly reduced.
A
possible
systematic
error is isotopic
fractionation
during ion beam emission.
As a
test, the samples were “burned”
for considerable
lengths
of time.
No detectable
change in isotope ratio was observed.
The validity
of the isotope dilution calculation was tested by mixing known amounts
of standard solutions of common rubidium
and the tracer in which the predicted
X’Rb
to “sRb ratios ranged from 0.75 to 3.5. By
this test, the resulting
ratios were actually
determined.
The uncertainty
of the calculation was about 5%.
It is a significant
feature of this work that
each sample was measured by two entirely
independent
analytical
techniques.
The results of these measurements
are given in
Table 2. The flame photometric
analyses
and the mass spectrometric
analyses agree
within the estimated precision
(measure of
that
random errors ) , This is an indication
the accuracy ( measure of systematic errors)
is of the order of or smaller than the precision of the measurements.
An analysis of the data gives an average
for the rubidium
content in the oceans as
120 -C 9 pg/liter
and an average value for
the rubidium
to chlorinity
ratio as 6.34 +
0.41 x lo-“. This agrees with the work of
Smales and Webster ( 1957) and Bolter et al.
(1964 ) and indicates that the rubidium
content in the oceans is uniform within 28%.
The flame photometer
and mass spectrometer rubidium
values have been averaged
and then normalized
to water of salinity 35;;‘(,
in Table 2. The Pacific Ocean depth profile
shows a slight trend toward maximum
rubidium
at the surface and bottom with a
minimum
at intermediate
waters.
Seawater
Sediment
Sediment
Seawater
IN
231
SEAWATER
0.109
16.3
150
390
20,700
3,580
1,270
53
The flame photometric
procedure
was
also applied to the analysis of alkali metals
in one sample of marine sediment.
Sediment taken from a sample bottle which
struck bottom
at 3,929 m at the profile
station in the Pacific Ocean was analyzed
for rubidium
and potassium content (Table
3). The average- concentration
factor of
rubidium
in the sediment
to that of seawater was found to be 150. This sediment
had a potassium to rubidium
ratio of 1,270
as compared to 3,580 for seawater.
REFERENCES
E., K. K. TUREKIAN,
AND D. F. SCHUTZ.
1964. The distribution
of rubidium,
cesium
and barimn in the oceans.
Geochim.
Cosmochim. Acta, 28: 1459-1466.
BOROVICK-ROMANOVA,
T. F. 1944. On the ruDokl. Akad.
bidium content of sea water.
Nauk SSSR, 42: 216-218.
BUNSEN,
R. 1861. Chemical
analysis by spectrum observations.
Phil. Mag., 22: 329-349.
BUIIKSER, E. S., N. V. KOMA~I, S. G. RUBLEY, AND
1932. Researches exR. I. USTIL~VAKAYA.
perimentales sur la determination
du rubidium
dans l’eau de la mer noire, de la mer d’azov
et des limans cl’odessa.
Tr. Biogeokhim. Lab.
Akad. Nauk SSSR, 2: 65-84.
CHOW, T. J., AND E. D. GOLDBERG.
1962. Mass
spectrometric
determination
of lithium in sea
J. Marine Res., 20: 163-167.
water.
AND T. G. THOMPSON.
1955. Flame
phbtometric
determination
of strontium in sea
water.
Anal. Chem., 27: 18-21.
EBERHARDT,
E. N. 1963a. Noise in multiplier
phototubes.
International
Telephone
and
Telegraph
Industrial
Laboratories
Research
Memo No. 309. Fort Wayne, Indiana.
-.
1963b.
Single photoelectron
counting
properties of multiplier
phototubes.
International Telephone
and Telegraph
Industrial
Laboratories
Research Memo No. 387. Fort
Wayne, Indiana.
BOLTER,
232
RAYMOND
C. SMITH,
K. C. PILLAI,
TSAIHWA
1963c. Test results in FW-118.
International Telephone
and Telegraph
Industrial
Laboratories
Research Memo.
Fort Wayne,
Indiana.
FELDMAN,
C., AND T. C. RAINS.
1964. The collection and flame photometric
determination
of cesium.
Anal. Chem., 36: 405-409.
FOLSOM,
T. R., C. FELDMAN,
AND T. C. RAINS.
1964. Variation
of cesium in the ocean.
Science, 144 : 538-539.
GOLDSCHMIDT,
V. M. 1958. Geochemistry.
Oxford Univ. Press, London.
730 p.
-,
H. BERMAN,
H. HAUPTMANN,
AND CL.
PETEHS.
1933. Zur Geochemie
der Alkalimetalle.
Nachr.
Akad.
Wiss.
Gottingen,
Math.-Physik.
Kl., III Nr. 34, p. 235-244.
GHANDEAU,
L. 1863. Sur la presence du rubidium dans certaines matieres alcalines de la
nature et de l’industrie.
Ann. Chim. (Paris),
67 : 155-236.
HERMANN,
R., AND C. T. J. ALKEMADE.
1963.
Chemical analysis by flame photometry.
Interscience, New York. 644 p.
ISHIBASHI,
M., AND T. HARA.
1959. On concentrating rubidium
and cesium from a large
volume of aqueous solution.
Bull. Inst. Chem.
Res., Kyoto Univ., 37: 172-178.
-,
AND Y. HARADA.
1942. On rubidium
-.
J. CHOW,
AND
THEODORE
R. POLSOM
content in sea water and bittern.
J. Chem.
Sot. Japan, Pure Chem. Sect., 63: 211-216.
KOVALEVA,
K. N., AND E. S. BURKSER.
1940. Determination
of rubidium in sea water.
Dokl.
Akad. Nauk SSSR, 5: 31-36.
SCHMIDT,
C. 1878. Hydrologische
Untersuchungen. Bull. Akad. St. Petersburg,
24: 177258, 41943Ei.
SHELLENBERGEJX,
T. E., R. E. PYKE, D. B. PARRISH,
1960. Some factors
AND W. G. SCHRENLS.
affecting
flame photometric
emission of rubidium in an oxygen-acetylene
flame.
Anal.
Chem., 32: 210-213.
SMALES,
A. A., AND L. SALMON.
1955. Determination by radioactivation
of small amounts
of rubidium and cesium in sea water and related
materials
of
geochemical
interest.
Analyst, 80: 37-50.
-,
AND R. K. WEBSTEH.
1957. The cletermination
of rubidium
in sea water by
the stable isotope method.
Geochim.
Cosmochim. Acta, 11: 139.
SONSTADT, E.
1870. Preliminary
note on a newly
discovered property belonging to rubidium and
cesium.
Chem. News, 22: 2543.
1932.
THOMPSON,
T. G., AND R. J. ROBINSON.
Chemistry of the sea, p. 95-203.
In Physics
of the Earth, v. 5, Oceanography.
Natl. Res.
Council, Bull., No. 85. Washington,
D.C.