RESEARCH· ·1970·

B182
ANALYTICAL METHODS
in the aqueous layer that quench the fluorescence of. the
carbonate-fluoride phosphor. Pipet a 2-ml aliquot of the
ethyl acetate into a 7-ml platinum dish of the dimensions
given by Grimaldi, May, Fletcher, and Titcomb
(1954, p. 103). Place the dish in a shallow pan that contains
about one-eighth of an inch of water to keep the
bottom of the dish cool. Ignite the ethyl acetate with a
lighted taper and aJllow the ethyl acetate to burn completely.
Dry the residue remaining in the platinum dish
on a steam bath, then heat the dish briefly over an open
flame, below a red heat; to. remove any free nitric ·acid
and organic matter in the residue before adding flux.
The flux is -a carbonate-fluoride mixture that contains
by weight 45.5 parts of sodium carbonate, 45.5 parts of
potassium carbonate, and 9 parts of sodium fluoride.
Two g of this flux are added to the residue in the platinum
dish. Heat the dish over a burner at a low temperature
until the flux melts. Then heat for an additional
minute to keep the flux a little above the melting point,
whi.le swirling the flux to dissolve all the uranium and
to obtain a uniform melt. Set the dish on a level Alundum
plate to cool. Measure the fluorescence of the phosphor
with a transmission fluorimeter· such as that described
by Kinser ( 1954). Determine the uranium in
parts per million by reference to a standard curve.
Standardization
Standards containing 0.0 0.025, 0.050, 0.2, and 0.5 p.g
of uranium are included with each set of samples, after
the step of the addition of nitric rucid, and carried
through all subsequent steps, to prepare working curves
and to correct for small time-distributed changes that
may occur. Two linear working curves covering two
scales of the fluorimeter are used. One curve ranges from
0.0 to 0.05 p.g uranium and the dther from 0.0 to 0.5 p.g.
STUDY OF PRECISION
Many determinations of uranium in plants collected
during the season 1954-55 were made on two separate
portions of ash from the same ashed sample. As a
routine checking procedure, samples so duplicated included
all those for which the first value obtained
exceeded 1.0 ppm uranium and a considerable number,
randomly selected, below this value. An equation (standard
deviation =.15+0.0063 U, where U ~s ·the observed
uranium concentration in parts per million) based on
326 duplicate determinations allows the precision of
other routine determinations to be predicted with
considerable assurance, if analyses are made by the
described method.
The possibility that the precision of the uranium
determination might be different for different plant
species was first studied. The four species of plants
studied were Artemisia sp? (sagebrush), Pinus edulis
(pinon pine), Juniperus sp? (juniper), and Pinus ponderosa
(ponderosa pine). The duplicate uranium determinations
were collected for each species and subdivided
into statistical classes, the ranges of which are
given in table 1; then the arithmetic means, variances,
and standard deviations were calculated. The equation
used in calculating the variances (V) was
'};d2
V=- 2n
where d is the difference between the duplicate determinations
for each sample and n is the number of paired
determinations.
Table 1 shows that the standard deviations are
virtually independent of the species but are dependent
on the uranium content. Therefore, table 2 was prepared
to show the determinations combined without
regard to species. When these standard deviations are
plotted against the corresponding arithmetic means, a
close approximation to a straight line results (fig. 1).
The regression line shown in the figure was obtained
by the method of least squares; its equation is standard
deviation = 0.15 + 0.063 U, where U is the uranium
concentration in the ash, in parts per million. This
equation allows a standard deviation to be estimated
from a given uranium concentration within a range of
0.4 to 35 ppm uranium.
Following are examples of the use of this equation
for estimating expected standard deviations ( s.d.) :
1. Observed concentration of uranium in ash = 0.7
TABLE 1.-The arithmetic mean, variance, and standard deviation
for duplicate uranium determinations grouped by plant species
and ranges within species
Plant
species
Sagebrush __
Pinon pine_
Juniper ___ _
Ponderosa
pine.
Range
(ppm)
0. 8- 1. 6
1. 6- 3. 2
3. 2- 6. 4
6. 4-12. 8
12. 8-25. 6
25. 6-51. 2
. 0- . 8
. 8- 1. 6
1. 6- 3. 2
3. 2- 6.4
. 0- . 8
. 8- 1. 6
1. 6- 3. 2
3. 2- 6. 4
6.4-12.8
. 0- . 8
. 8- 1. 6
1. 6- 3. 2
3. 2- 6.4
Number Arithmetic
or pairs or mean Variance
determi- (ppm) (ppma)
nations
7 1. 26 0. 02429
29 2.43 . 09328
18 4.40 . 1839
10 9. 64 . 4005
20 18. 22 2. ·202
11 34. 77 5. 104
15 . 65 . 01700
46 1. 13 . 02717
22 2. 31 . 1182
14 4. 42 . 1321
22 . 43 . 01636
30 1. 19 . 05667
17 2. 26 . 07559
11 4. 28 . 1218
7 9. 20 . 7329
6 . 68 . 03583
23 1. 10 . 05957
10 2. 04 . 08700
8 4. 85 . 1788
Standard
deviation
(ppm)
0. 16
. 31
. 43
. 63
1. 48
2. 26
. 13
. 16
. 34
. 36
. 13
. 24
. 27
. 35
. 86
. 19
. 24
. 29
. 42
1 .•