882
Vol. 62
NOTES
cipitated alone early in the run and then had to dissolve and recrystallize in solid solution with the silver bromide as the system attempted to follow the changing equilibrium curve of Fig. 1. The results of changing both the precipitation rate and the KBr excess are summarized in Fig. 3. Clearly, an increase in the time of precipitation increased the homogeneity of the crystals in the double-jet method, just the reverse of the single-jet method. The curves of Fig. 1 give the clues to the reasons for this behavior. Slow precipitation by the double-jet method encourages the attainment of the equilibrium condition which is the same at every instant during the experiment. But, with the single-jet method, the equilibrium changes continually during the run so that a uniform iodide distribution can occur only when the crystals are in a highly reactive state (ie., more finely divided as in the faster precipitations) near the end of the runs.
STUDIES ON SILICIC ACID GELS. XIX. THE EFFECT OF ELECTROLYTES UPON THE VISCOSITY O F THE SOL AND UPON THE TIME OF SET BY CHARLEB B. HURD,JOHN W. RHOADES, AND ARTHURC. SANTORA WILLIAMG. GORMLEY Department o j Chemistry, Union College, Schenectady, New York Received March .9q9 1968
The change in viscosity of hydrosols of silicic acid, during setting, is interesting because the curve of viscosity against age of sol resembles curves for other properties, such as the amount of light scattered. We have studied the effect of changing p H and of added salts upon these curves, also upon time of set. I n part 1, shown by Fig. 1, mixtures were made
24 *: 20
4
0 I
I
0
I
2
3
4
5
6
7
8
9
10
Groms per I00 ml of He0 Concentration of KBr in precipitation vessel,
Fig. 1.-Time
I 100
1 I 1 200 300 400 500 600 Time after mix. (min.).
700
800)
of flow of silicic acid gel mixtures as a function of age of sol and of pH.
by pouring solutions of E brand silicate (Philadelphia Quartz Company) into HC1 solutions. Concentrations were SiOz, 0.60, and NaCl from reaction, 0.36 mole per liter; excess HC1 for p H 7.0,. Silver bromoiodide crystals are usually bounded none, for pH 4.70, 0.0093 mole per liter. Curves by { 1111 faces. This most densely packed plane shown in Fig. 1, composed of many points, are of fulfills the Donnay-Harker4 criterion for minimum the same shape and can be made to superimpose b y surface energy. However, it was interesting to plotting time of flow against relative age. Relative note that, in the double-jet experiments, regular age is the ratio of actual time, since mixing, divided cubes were produced when there was no excess by time required for time of flow to reach an arbiKBr in the precipitation vesseL6 With no halide trary value, 10 seconds here. Time of flow was measured in an ordinary Ostions in solution, the reduction in surface energy aswald viscometer pipet. No attempt was made t o sociated with electrically neutral f 200) planes beactual viscosity, but the actual viscosity comes important. It is also pertinent that no tabu- calculate must be approximately proportional to time of lar crystals were observed among the cubes. This flow. No kinetic energy corrections were made. is reasonable in view of the explanation that tabu- All work was done at 25'. lar crystals of triangular and hexagonal outline are To study effect of electrolytes added, upon time caused by twinning on (111) planes parallel with of flow and time of set, we used mixtures of sodium the platelets.6 Since growth twins cannot occur on silicate and acetic acid solutions. If extra salt was (200) planes in the NaCl type of structure, no added, it was in the acetic acid solution. Each mixture contained Na+ 0.42 mole per tabular grains having [ 200 1 faces are t o be expected. liter, SiOz 0.70, CH&OOH 0.68 and extra electro(4) J. D. H. Donnay and D. Harker, Camp!. rend., 204, 274 (1937). lyte, if added, 0.30. Silicate always was poured (5) Also noted by V. N. Zharkov and E. P. Dobroserdova, Zhur. into the acid. Time of set was measured by the Nauch. i Prkklad. Fotograjiz i Kznematografii, 1, 250 (1956). tilted rod method. Figure 2 shows the results with (6) R. W. Berriman and R. H. Here, Nature, 180, 293 (1957). Fig. 3.-Phases present a t the end of double-jet precipitations of 12 mole yo bromoiodide a t 60'; effects of varying the precipitation time and the KBr concentration.
.
883
NOTES
July, 1958
cerns the possibility that metal fluorides like NiFz or CuFz might be sufficiently volatile a t the high temperatures t o complicate the interpretation of the data.5 The vapor pressure of NiFz has been measured independently by two different methods a t the Jet Propulsion Laboratory and a t the University of Wisconsin to see whether this objection is valid for experiments in nickel containers. The only previous observation on NiF2 was that of Poulence who stated that NiFz sublimes without melting under an H F atmosphere at about 1000". Samples of NiFz may be prepared conveniently by direct fluorination of NiC12 at 350-400°.7 They 0 then must be protected carefully from water vapor 0 20 40 60 80 100 which could cause hydrolysis to NiO and HF on Time after mix. (min.). heating. The NiFz is contained without reaction in Fig. 2.-Time of flow of silicic acid gel mixtures as a platinum vessels up to a t least l l O O o . function of age of sol as influenced by alkali chlorides: The Jet Propulsion Laboratory measurements A, no salt; B, LiC1; C, NaC1; D, "&I; E, KC1. were Knudsen effusion studies over the range 1026alkali chlorides. Table I summarizes the results of 1104°K. The University of Wisconsin work was by the flow method with Nz as the flow gas, and many series of tests. yielded vapor pressures at 1233 and 1349°K. The TABLE I The solid TIMEOF SET A N D THE pH OF SILICIC ACID GEL MIXTURES results are shown graphically in Fig. 1.
,
CONTAINING ADDEDELECTROLYTES AT 25' Time of set in min. Li Na K NHa Li NaPH K
C1 Br I NOa CNS
74 58 66 77 70 55 60 66 51 55 72 58 60 63 52 56 With no electrolyte added, 100 85
C1 Br I NO3 CNS
I
"4
4.8 4.8 4 . 7 4.8 4 . 9 4.9 4.9 4 . 8 4.7 4.8 4.6 4.7 4.7 4 . 7 4 . 8 4.9 4 . 9 With no electrolyte added, 4.8
Efficiency of ions used, for decreasing time of set or decreasing time required to produce a given increase in viscosity is shown as Kf > NH: > Naf > Li+ and I- > CNS- > Br- > NOs- > C1- for acid gels of pH 4.8 containing excess CHICOOH and CH3COONa produced in the reaction. This order of cation effectiveness agrees with the results of Pappadal on effectiveness of coagulation of SiOzhydrosols. We believe the same process occurs in all of these gel mixtures because of the shape of the curves. Change of p H or presence of added salt merely accelerates the process. Further theoretical discussion is omitted because of space limitation. (1) N. Pappada, Qazz. chim. itat., 85, I, 78 (1905).
THE VAPOR PRESSURE OF KICKEL FLUORIDE1 BY TILTO ON FARBER, RICHARD T.MBYERA N D JOHN L. MARGRAVE
Jel Propulsion Laboratory, California Institute of l'echnolovy, Pasadena Calbfornia Department of Chemistry, Univemity pf Wisconsin, Madison, Wisconsin Received March 66, 1968
One of the objections raised to values for the dissociation energy of fluorine as determined by pressure or effusion studies in metal systems,2-4 con(1) Taken in part from the B.S. theeis of Richard T. Meyer, submitted to t h e University of Wisconsin, June, 1956. (2) J. L. Margrave, Ph.D. Thesis. University of Kansas, 1950: P. W. Gilles and J. L. Margrave, J . Chem. Phys., 21, 381 (1963). (3) R. N. Doescher. ibid., 19, 1070 (1951);20, 330 (1952). (4) H.Wise, ibid., 80, 927 (1952).
i
*
d
-4-
3
3
-5
*'
"\ \
i., Fig. 1.-Vapor
1
1
7.00
8.00
I
9.00
10.00
1 0 4 / ~ ,OK. pressure of nickel fluoride: A, JPL Rnudsen data; 0, UW flow data.
line drawn through the points indicates the vapor pressure equation to be log PNiF2(&tln)=
- 13,100 + 6.8
I___
(1026
< T < 1340°K.)
with the assumption that NiFZ(g) is the major vapor species. Simple thermodynamic arguments in combination with the observed weight losses show that decomposition to the elements or to NiF(g) and F(g) or Fz(g) are not important in vaporization a t these temperatures. From these data, for the reaction NiFz(s) .= NiF2(g), one calculates the heat of sublimation (5) R. T. Sanderson, J. Chem. Phys., 82, 345 (1954). (6) C. Poulenc, Ann. chim. phus., 2 , 41 (1894).
(7) "Inorganic Synthesas," Val. 111, McGraw-Hill Book Co., New Y o r k , N . Y., 1950, p. 173.
