tion to the effect of chloride on the electrode process. For instance, the inhibition of T I upon addition of 12 is greater in 0.0531 TLIXC than in 0.1M LiC104. It is possible t h a t the specific adsorption of TRIAC results in a blocking of the electrode surface to I-. This would explain the general inhibition of T~ in T X i C and the shift in the of step (1) from +0.2 in 0.1M LiC104 t o +0.35 in 0.05X T X A C . Cathodic Chronopotentiometry of N-Chloro Compounds. The results
of cathodic chronopotentiometry of chlorine. four chloramide compounds, and hevachloromelamine are shown in Table 111. T h e purpose of this work was to examine the reactivity of t h e S-C1 bond in a series of similar compounds in ternis of different potentials of cathodic reduction. In this EtudyI precautions were taken to exclude all halide ions and maintain a low level of water ( 5 0.03y0). Thus, hydrolysis or disproportionation reactions of the S-chloro compound to give chlorine or. in the absence of chloride, hypochlorous acid n ere prevented or made insignificant. The reduction steps observed for most of the X’-chloro ronipounds in Table 111 n ere poorly defined probably because of a complicated, s l o ~c,!ectrode process occurring a t a low leiel of available protons which nould be involved in a two-electron reduction of a n X-Cl function. T n o possible steps in the general electrode process for S-C1 reduction are --T--Cl
+ e-
-f
=N,
+ C1-
(4)
Although the chronopotentiometric constants for the compounds described in Table I11 indicate electron change values in the 1 to 3 range, in some cases, further reduction of the test compound was observed at more cathodic potentials. However, these further reduction steps were so poorly defined in most cases t h a t no meaningful measurements could be obtained. Also, because of the poor definition of the first cathodic waves, the tangent potential (Et’)values are the most characteristic potentials for comparison purposes. Trichloroisocyanuric acid shon ed the best n-are shape and the most reproducible potential and io+?C values. On the basis of the chronopotmtiometric constants shown in Table I, the value of 0.58 for trichloroisocyanuric acid represents a two-electron reduction. Both chlorine and 1,3-dichlorourea, a n extremely unstable compound, showed reduction steps a t - 0.1 to -0.2 volt which may be accounted for b y chlorination of the solvent. However, cathodic tests on model compounds such as I-chloro-1-nitroethane and 1,ldichloro-1-nitroethane shon ed no evidence of reduction. Both tetrachloroglycouril compounds h a r e apparent reduction potentials ( E t ) very much loner than trichloroisocyanuric acid or hexachloronielamine. Both compounds show a fused double n a v e with a rather low chronopotentiometric constant as the major reduction step. The diphenyl substituted compound also shon-s a small reduction step which may be caused by a small amount of chlorine or other impurity in the test compound. These low reduction potentials of the chlorinated glycourils may be an indication of greater stability
of the K-Cl bond in these compounds with respect to trichloroisocyanuric acid and hexachloromelamine. CITED
. CHEM.33.
1123
Halogenation,” Chap. S and 9, Butterworths, London, 19,59. (6) International Critical Tables, McGrar-Hill, Xew Turk, 1926. a. T’ol. 5, p. 65. b. Vol. 6, p. S3 c. Yo1 7, p. 213. ( 7 ) .Inamoto, R. T., Private Coinmunication and Paper S o . 41 Division of Bnalytical Chemistry, 140th Meeting, ACS, Chicago. September 1961. (8) Klingsberg, E., J . d n i . Cheni. SOC. 83, 2934 (1961). (Second paper in press.) (9) Lorenz, IT., Muhlberg, H., Z. Elektrochem. 59, 736 (1953); %. PhUszk. Chena. (S.F.) 17,129 (1958) (10) Selson, I. IT., In amoto, R. T., A s . 4 ~CHEW . 33, 1795 (1961). (11) Newson, J. D., Riddiford, A . C., J . Electrochem. SOC.108,699 (1961). (12) Kicholson, bl. AI., J . -4111 Chem. SOC. 76,2539 (1954). (13) Popov, A. I., Geske, D. H., Ibzd., 80, 1340 (1958). (14) Rapoport, L., Smolin, E. >I., “sTriazines and Derivatives,” pp. 330333, Interscience, Yew York, 1959. (15) Reilley, C. S , Everett, G. IT., Johns, R. H., A \ A I . CHEV. 27, 483 (1955). (16) Streuli, C. A , , American Cyanamid Co., Stamford, Conn., private communication, 1961. (17) Wawzonek, S., Berkey, R., Blaha, E. IT., Runner, hl. E , J . Eltactrochem. SOC.103, 456 (1956). RECEIIEDfor review Januarv 16, 1962. iiccepted May 10, 1962. Metropolitsn Regional Meeting, ACS, KeTv York, January 1962.
Unusual Matrix Effects in Fluorescent X-Ray Spectrometry L. S. BIRKS U. S. Naval Research laborafory, Washingfon 25, D. C.
D. L. HARRIS American Chrome Co., Nye, Monf.
b Certain systems o f elements may b e postulated in which the x - r a y intensity from some element, A, will decrease rather than increase as the concentration o f A increases. The requirement for such a situation i s that as the concentration o f element A increases, the concentration o f another element, B, which has a high x-ray absorption coefficient for radiation from element A, increases a t an even faster rate. Iron and chromium in the mineral
system chromite-olivine form such a pair o f elements and the predicted phenomenon i s observed; i.e., FeKa intensity decreases steadily as the iron content increases from 8 to 17%.
I
fluorescent x-ray spectrometry, x-ray intensity is not linear with concentration ( 1 ) . Matrix absorption and enhancement are the causes for nonlinearity and are well understood. N
It has usually been assumed that a n increase in the Concentration of a given element would result in an increase rather than a decrease in ite x-ray intensity; that is, i t has been assumed that calibration curvee n-ould ah-ays have a positive slope. Recently, a practical mineral system was found in n hich this is not true. As the iron content of the system increases, the F e K a intensity decreases. The conditions for such a negative slope are rather restrictive as VOL. 34, NO.
a,
JULY 1962
943
L' 90 92
IO
14
I2
W%
16
I8
Iron
Figure 1. Relative x-ray intensity of FeKar vs. weight per cent in chromiteolivine mineral system Intemity from unity
5% chromite specimen is taken as
might he expected. Also, particle size and specimen homogeneity are of increased importance in such a delicately balanced situation. I n this paper the 1-
d )r Le ie 3-
tt 'y In 1-
is b
1-
"..*
"pb".LLLb"
U""S."J,
lluI"l"llyl
density of element A; p" contains the sum of the mass absorption coefficients p and p' for the incident and emergent wavelengths times the cosecants of their respective incidence and emergence angles. The terms Q and p" vary with incident wavelength. The relative in-
ent in Chromite-Olivine Mixtures Ca.
Si
Al
Me
0
is merely the weight fraction , 'E of element A . By making this suhstitution and canceling the &IO terms, Equation 2 transforms simply to
The requirement for a negative slope is t h a t as W increases, p" must increase by a greater relative amount. T o illustrate, suppose that as W increases from 0.3 for W 1to 0.4 for W,, p" increases from 100 for p2" to 150 for pl". The intensity ratio 1 1 / 1 2 = 2.7 X lo-$/3X = 0.89. Thus, as the concentration has gone up from 30 to 40%. the intensity has actually decreased. The conditions outlined in the preceding paragraph will be fulfilled when an increase in concentration of the element being measured is accompanied by an even greater increase in some matrix element t h a t has a very high absorption coefficient for the radiation being measured. MEASUREMENTS IN A PRACTICAL SYSTEM
LOSS Absorption
:a 10 20 56 07
coerricienrs
AI
Si 120
97
82 57 38
44 30
64
Mg 77 52 36 25
0 22 15 10 7
The practical system that first illustrated the phenomenon of negative slope in calibration curves was chromiteolivine mixtures in mill concentrates. Chromite contains oxides of iron, chromium, magnesium, aluminum, and
silicon. Olivine mill tailings contain the same oxides in different proportions plus calcium oxide. Table I s h o w the composition for mixtures containing 5 to 1007, chromite. When the FeKa intensity for the 5% chromite specimen was taken as unity, the relative intensities for the 50 and 1007, chromite specimens, for insbance, were measured as 0.983 and 0.979, respectively. T o calculate the relative F e K a x-ray intensity for comparison with the measured values, one should take account of all the polychromatic incident radiation shorter than 1.i -4. Three wavelengths were actually used in the and 1.3 A. calculation, namely 1.7, I t n 3 l be shown below that these nere sufficient. Table I1 lists tlie mass absorption coefficients used in the calculations and Table III lists tlie total ahsorpt’ion cocfficients p ” for the various concentrations :ind lI-,’p”values for each incident \mvelmgtli. Table IT gives the calculatetl rclative intensities. Figure 1 is a plot of rvlative F e K a intensity 21s. iron concentration for calculated and measured vulue.. The measured values fall nicely in i,ptmen the cdculated values from 1 .T- :3nd 1.5-A. incident radiation. The con\.ergcnce of the 1.5- and 1.3-A. curves indicates that even shorter incidmit wavelengths will not change the relative intcwit>-.
Table Ill.
Values of Total Absorption Coefficient p” and of W / p ” a t 1.7, 1.5, and 1.3 A. Incident Wavelengths
II
70
Chro-
mite 5 10 15 20 30 40 50
60
70 80
90
1 5
1 7
.
Total p ” 160.5 171 . 3 182.0 191.4 211.9 233.0 252.9 274.1 294.4 315.2 335.1
1T/p“ 0.0504 0.0502
0.0500 0.0502 0.0500 0.0498 0.0498 0 IO496 0.04!)6
0.0495 0.0495
Total p” 136.6 146.1 155.2 163.3 180.9 199.3 216.6 235.4 252.4 270,3 2 8 i .6
stituents in .I and B and the elemental composition in three different proportions of -4 and B. Just as in the chromite-olivine mixtures, the iron and chromium concentrations in A and B satisfy the theoretical requirements for negative slope as shown by the calculated FeKa intensity in the last column of Table Y. Figure 2 shons micrographs of the three mixtures after extensive dry ball milling. The darker particles are FesOl in a greenish matriv of C r 2 0 3and hIgO. Although the overall distribution of FesOl particles is uniform, the main contribution to F e K a intensity comes from near the surface and the iron occurs in an iron ovide ma-
\V/F
1 3 Total
0.0598 0.0589 0.0586
125.9 133 . 8 110,8
0,0586
15.5.9 171.8 186.7 202.5 217.6 233.0 247,9
0.0582
0,0582 0.0878 0.0578 0.057i 0.0577
5 10
0 0 0 0 0 0 0 0 0 0 0
117.8
0 . 0588
Table IV.
1\77
p”
0688 068’3
0680 0682 0680 06i5 0675
0672 0671 0670 0670
Calculated Relative X-Ray Intensity
100 0
15
20
30 40 50 60
io
80 90 100
100 0 99.3 98 8 99.1 98.8 98.1 98.1 97.4 97.4 97.3 97.3 97.0
99 6 99 2 99.6 99.2 98.8 98.8 98 4 98.4 98.2 98.2 08.0
100 0 99 2 98 8 99.1 98.8 98 1 98 1 97.6 97.5 97.3 97.3 9i.l
EFFECT OF NONHOMOGENEITY
I n the chromite-olivine system, the iron and chromiuin n-ere homogeneously distributed in the chromite and oliyirie individually. To shon- the effect of nonhomogeneous distribution of iron and chromium, two synthetic minerals d and B Tvere prepared by mechanicalljmixing oxides of iron. chromium, and in:igti(~sium. ‘Tahlt. T- chon-s the con-
0
C
I
2
’
4
E
8
W %
IO
I2
1
14
1
16
1
IS
Table V.
Composition o f M i x e d Synthetic Minerals A and
B
-4,10% FepOd; 2% Crz03; 88% bIgO B , 20y0 Fea04; 40% CrlOJ: 4 0 5 N g O Calculat ecl
hlixtures A 100 60 0
B 0
40 100
Fe 7 2% 10 1 14 5
FfhIiL2
14%
11 8 27 4
0 38 3‘; 36 6 34 0
53 lS0 41 5 24 1
intensit! 1 00 0 87 0 80
tris rather than in a chromiuni-magnesium matrix. ‘l’herefore the chromium is not effective in absorbing the F e K a radiation and the slope of the measured calibration curve is normal as shown in Figure 3. Even finer grinding as in a n-ater slurry 11-ould not’ change the chemical combination of the iron although it should lead to smaller particle size, Thus the mwsured values for the actual nonhomogeneous specimens are in disagreement with the calculated values for homogentous specimens as might be expected. Calculation of relative intensities in nonhomogeneous matrix is too difficult a t the present time.
tion curl-es of negative slope by postulating specimens in which the matrix absorption increases a t a faster rate than concent’ration. Second, practical examples of the negative slope phenomenon may be found in mineral systems and perhaps elsewhere. Third, the calibration curve changes quickly from negative to positive slope if the individual components of the system are not, homogeneously distributed.
CONCLUSIONS
RECEIVEDfor revieil March 1, 1962. Accepted April 20, 1962. 10th International Spectroscopy Colloquium, College Park, l f d . , June 1962.
LITERATURE CITED
(1) Birks, L.
S.,“5-Ray Spectrochemical Analysis,” Interscience, Xew York, 1959.
1
20
Iron
Figure 3. X-ray intensity vs. composition for synthetic mixtures of Figure 2
Three results have been shown in this paper. First, one may predict calibra-
vot. 34,
NO.
a,
JULY 1962
945
