Table I. Determination of Azide Ion in Synthetic Primer Mixtures

Azide Ion Azide (Ns) Relative

Present, Found, Error, Mg. M g . %

0.50 0 . 4 6 - 8 . 0 1.00 1.04 + 4 . 0 1 .00 1 .01 i-1.0 2.00 2 .01 +o 5 3 . 5 0 3 . 4 0 - 2 . 9 3 . 6 0 3 .49 - 3 . 1 4 . 0 0 3 . 8 8 - 3 . 0

Mean - 1 . 6 Standard deviation 3 . 9

Table II. Determinations of N3 in Lead Azide, Purity 92.05%

Azide Azide (NI) (No) Purity Relative

Calcd., Found, Found, Error, M g . Mg. % % 2 . 6 4 2 . 6 6 92 .7 + 0 . 8 2 . 3 8 2 . 4 2 9 3 . 5 + 1 . 7 1 .81 1 .74 88 .7 - 3 . 9 2 . 8 4 2 .67 86 .6 - 6 . 0 3.16 3 . 2 0 93 .3 + 1 . 3 1.49 1 .50 92 .5 $ 0 . 7 1 .80 1.85 94.6 + 2 . 8 2 .77 2 . 6 4 87 .9 - 4 . 7 1.68 1 .79 9 8 . 0 + 6 . 5 3 . 6 8 3 . 5 5 88 .7 - 3 . 5

Mean 91 .6 - 0 . 4 Standard deviation 3 . 6 3 . 9

to shift the standard curve slightly and it is advisable to prepare a ne\T stand- ard curve when new ferric nitrate so- lution is prepared. The necessity for preparing a new curve with each batch of reagent is found in the reaction.

N j , - + N O z - + 2 H f - t H 2 0 + r \ ‘ > + N 2 0

The use of ferric sulfate in sulfuric acid to eliminate this difficulty was unsatis- factory, as color development with a sulfate reagent was poor. Azide ion gave, in equivalent concentration, a 57% T in a nitric acid reagent and 86% T in the sulfuric acid media.

Samples of synthetic primer mixtures were prepared, using 10 mg. of antimony trisulfide (stibnite), 10 mg. of potassium chlorate (both approximate weights), and accurately measured amounts of sodium azide solution of known con- centration. Table I indicates results for determinations on such mixtures.

Table I1 shows azide ion values found from analyses of military grade lead azide, the purity of which was 92.05y0 by the Navy titration method ( 3 ) .

While the relative errors shown are not considered excessive with small amounts of azide, they are too large to permit the use of the method for purity determinations. This is clearly illus- trated by the purities calculated from the results and also shotvn in Table 11.

LITERATURE CITED

( I ) Davis, T. L., “Chemistry of Powder and Explosives,” p. 430, Wiley, Xe-x York, 1943.

(2) Feigl, F., “Spot Tests, Inorganic iip- plications,” 4th ed., Vol. 1, p. 268, Elsevier, New York, 1954.

(3) Military Specification, MIL-L-3055, Sept. 30, 1949.

(4) Ricca, B., Gazz. chim. i la l . 75, 71 (1945).

( 5 ) U. S. Naval Ammunition Depot, Crane, Confidential Rept. QE/C 56-40.

RECEIVED for review August 27, 1956. Accepted December 13, 1956. The opin- ions and assertions contained in this article are the private ones of the authors and are not to be construed a8 reflecting the views of the Navy Department.

CORRESPONDENCE

‘undue exposure to air. Ricca (4) has shown that aeration causes appreciable fading of the color and suggests that this is due to the loss of hydrazoic acid. He has also presented evidence that the color is due not to undissociated ferric azide but to the complex ion Fe(NS) ++.

Interference from sulfites, thiosul- fates, and sulfides is avoided by pre- liminary oxidation of the alkaline sample with hydrogen peroxide ( 2 ) . Cyanates and thiocyanates, however, interfere and render the method ineffective. Nitric acid batch variations may tend

Determination of Potassium as the Metaperiodate

SIR: After publication of “Determi- nation of Potassium as the Metaperio- date” [ANAL. CHEM. 28, 2011-5 (1956)l our attention mas called to “Determination of Potassium in Soap and Mixed Caustic Lye” by W. T. Miller and J. T. R. Andrews [J . Am. Oil Chemists’ SOC. 26, 309-12 (1949)]. These authors report increased accuracy and sensitivity of the Willard and Boyle periodate procedure for potas-

sium through (1) hand stirring during precipitation of the potassium periodate, (2) totally reducing the periodate to iodine, and (3) titrating the iodine ivith sodium thiosulfate solution. Because our own paper deals in part with these points, we wish to call this paper to the attention of those interested in this procedure.

RALPH E. JENTOFT REX J. ROBINSON

154. Yttrium Trifluoride, YF,, Orthorhombic Form

155. 156.

Samarium Trifluoride, SmF,, Orthorhombic Form

Ytterbium Trifluoride, Y bF,, Orthorhombic Form

EUGENE STARITZKY and L. B. ASPREY, The University of California, Los Alamos Scientific Laboratory, Los Alamos, N. M.

HE trifluorides of yttrium, sama- The precipitates were oven-dried a t 110” The yttrium oxide used as starting material for this preparation is believed to be about 99% pure. The samarium was purified by E. I. Onstott of this

T r i u m , and ytterbium were precipi- tated with hydrofluoric acid from aque- ous solutions of corresponding chlorides.

C., dried under vacuum a t 1000” C.. and then heated under argon to about 100” C. above their melting point.

VOL. 29, NO. 5, M A Y 1957 855

laboratory by an electrolytic process (1). It contained 0.07% neodymium and 0.003y0 europium (determined spectro- photometrically). No other rare earth elements were detectable spectro- graphically in the ytterbium oxide used, which was pursed by ion exchange methods.

The structure of these orthorhombic trifluorides, belonging to the space group Pnma - D:! with a unit cell containing four formula units, has been determined by Zalkin and Templeton (3). They re- ported the following unit cell dimen- sions:

YFs SmFa YbFs ao, A. 6.353 6.669 6.216 bo, A. 6.850 7.059 6.786 cn. A. 4.393 4.405 4.434 Volume per for-

Density (x-ray), mulaunit,A.a 47.79 51.84 46.76

gramspercc. 5.069 6.643 8.168

The powder x-ray diffraction pattern of yttrium fluoride is given on ASTM cards 50546, 5-0547, of samarium fluoride on card 5-0517, and of ytterbium fluoride on cards 5-0551, 5-0552.

The density of the trifluorides crystal- lized from melts was determined with a Berman microbalance as 6.61 grams per cc. for the samarium salt; 8.17 grams per cc. for the ytterbium compound.

CRYSTAL MORPAOLOQY. The tri- fluorides crystallized from their melts as aggregates of coarse anhedrons cbarac- teriaed by prominent (0101 cleavage and twinned polysynthetically on (101) (Figure 1).

Figure 1. Cleavage fragments of samarium trifluoride showing repea

Crossed polarized light

OPTICAL PROPERTIES. respect to the trace of the composition plane (1011. The angle between the Z-

YF, SmFa YbF3 directions in adjacent twin hmellae ie Refractive in- 67" for samarium tduoride, 71" for

A.) Color. Samarium trifluoride is pink; n, 1, 536 1 , 577 569 yttrium and ytterbium fluorides Yare

dices (5893 ytterbium fluoride.

w 1.553 1.597 1.580 colorless. nz 1.569 1.608 1.599

mean 1.55% 1.594 1.58% Geometric LITERATURE CITED

Molecuiar re-

Optic m i d

(1) Onstott, E. I., J . Am. Chem. Sac. 77,

(2) Zalkin, A,, Templeton, D. H., Ibid., fraction,cc. 9.21 10.60 9.41 2129-32 (1955).

sngle,2V 85' 72' 78' 75, 2453-8 (1953).

WORK done under auspices of Atomic optic Orientation. x = c; y = b; E~~~~ commission. Crystallographic

data for pubhation in this section should Extinction Angles. Extinctions on be sent to W. C. McCrone, 500 East 33rd

St., Chicago 16, Ill.

Z = a.

(0101 cleavage plates are symmetric with

157. Lanthanum Trifluoride, LaF,

158. Neodymium Trifluoride, NdF,

EUGENE STARITZKY and L. 9. ASPREY, The University of California, Las Alamos Scientific Laboratory, Lor Alarnos, N. M.

LN'PHAN UM anu nsouymiurn IIUIITKXS L- were precipitated with hydrofluoric acid from aqueous solutions of corre- spnnding chlorides. The precipitates were heated to 400' C. in an atmosphere of gaseous hydrogen fluoride, heated under vacuum to about 1000" C., and then melted under argon a t about 1400" C. The fluorides crystallized from their melts as coarse-grained anhedral aggre- gates.

The lauthanum source material used was purified by ion exchange methods. Spectrographic analysis indicated the presence of O.O2y0 calcium aud 0.005y0 magnesium; no rare earth elements were

856 ANALYTICAL CHEMISTRY

aecemed. Spectrographic analysis of the neodymium salt indicated the pres- ence of O.lyo magnesium, 0.01% calcium, 0.03rr/o iron, and 0.2y0 cerium. No other rare earth elements were detected.

Oftedal (9) proposed a structure for hexagonal rare earth fluorides belonging to the space group PG8/mcm - D ~ B with a cell containing six formula units. Cell dimensions reported by Oftedal (1 ) for lanthanum fluoride are, after converting from ICX to Angstrom units, aa = 7.117 zt 0.007 A,, co = 7.344 * 0.007 A.; for neodymium fluoride corresponding fig- ures are a. = 7.035 i. 0.007 A., co = 7.210 5 0.007 A.

X-RAY DIFFRACTION DATA.

LaFr NdFt Cell dimensions

7.186 zt 7.030 f 0.001 0.001

eo, A. 7.352 zt 7.200 i 0.001 0.001

ao, A.

1.023 1.024 Volume per formula

unit, A.8 54.80 51.36 Formula weight 195.92 201.27 Density, grams per

CC. 5.936 6.506

da,

The above cell dimensions were determined hy linear extrapolation against the function (eos%/sin8 + Cos28/8) to the zero value of that func-