Karol J. Mysels University of Southern California LOS Angeles 7
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Textbook Errors, 22
Miscellanea NO. 3
Thermal Decomposition of Alkali N i h l e s
Professor Harry Bloom of the University of Auckland, New Zealand, points out that textbooks' are generally in error when they discuss the effect of heat upon sodium and potassium nitrates. The statement is made that the decomposition temperature is low and that the corresponding nitrite can be thus prepared. The behavior of sodium and potassium nitrates a t atmospheric pressure has been clarified recently by Freeman (1). The liquids formed by melting a t 307' aud 33A0C, respectively, undergo little change with increasing temperature until about 600°C when evolution of gas becomes appreciable and increases rapidly with temperature. Between about 600 and 700°C -the principal reaction seems to be the formation of oxygen and nitrite. The reaction is reversible and under these conditions the nitrite reacts also with oxygen to form the nitrate. The reaction is slow below fi00"C but reaches completion within about 2 hours a t 700". The final composition of the equilibrium liquid depends on the partial pressure of oxygen. At 1 atm pressure of oxygen an equirnolar mixture of nitrate and nitrite is obtained at about 670" for the potassium salt and at about 730" for the sodium salt. At lower temperatures the proportion of nitrite decreases. When the temperature appreciably exceeds 750800'C in the presence of oxygen and a t lower temperat,ures in its absence, other reactions become important, leading to format,ion of nitrogen and some nitrogen oxides. These are mainly due to decomposition of the nitrite with nitrogen and alkali peroxides as principal products. The latter give an orange color to the melt. These studies were conducted in stainless steel containers which seem to have little effect upon the course of the reaction. In Pyrex (2) or silica (3) vessels the situation is much more complicated because of reaction hetween the molten nitrate and the silica which leads to appreciable decomposition by the time about 550°C: is reached. Clay vessels cause still earlier reactions (4). The end product of heating sodium nitrate or nitrite in air is obtained a t about 900°C and corresponds stoichiometrically to the complete loss of N206 leaving NapO which is the only oxide stable a t such high temperature (5). This result was obtained thermogravimetrically Suggestions of material auihhle for this column and guest columns suitable for publication directly are eagerly solicited. They ~houldhe sent with as many details as possible, and particularly h-ith references to modern textbooks, to Karol J. Mysels, DB partment of Chemistry, University of Southern California, Los Ingeles 7, California. ' Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the swrce oi errors discussed will not he cited. The error must occur in at least two independent standard hooks to he presented.
in a porcelain crucible but Dr. Freeman feels that the glaze was sufficiently inert to prevent any important side reactions in the short times involved (6). Thus it appears that the sodium and potassium nitrates are quite stable thermally up to about 60O0C, then decompose gradually to give an equilibrium mixture with the nitrite, and that further decomposition leading to peroxides and ultimately the oxide begins long before conversion to nitrite is substantially complete. The Conductivity of Complexes
Professor Carlos R. Piriz Mac-Coll of Montevideo, Uruguay, points out that several texts1 erroneously call the measured quantity used occasionally in determining the structure of complexes the "equivalent conductivity." Since the equivalent weight is unknown until the structure is determined, this may seem to be a meaningless operation. I n fact, however, the quantity involved is the molar conductivity, whose calculation requires only a knowledge of the empirical formula, so that the procedure is logically consistent. Thus Werner and Miolati (7) in their classical investigation of these conductivities studied two compounds of cobalt, each containing four ammonias and three chlorines. The empirical formulas were Co(NaH6)C1a and C O ( K H ~ ) ~ C ~ ~respectively. H~O, The clearly different colors led to their common names of "purpureo" and "roseo" pentamine chlorides. The molar conductivities (i.e., the specific conductivities of their solution divided by the formal concentration) were found to be about 261 and 393 mho, respectively, a t 25°C and !A concentration. By comparison with molar conductivities of other salts this can be interpreted as indicating that the former gives only three ions in solution while the latter gives four ions, so that their formulas are [CO(NH~)~C~]CI? and [Co(NH&H2O]Cl3, respectively. (Historically these formulas had been established earlier on chemical grounds and the measurements were used to establish the relation between structure and conductivit,y.) Once these formulas have been established, the equivalent conductivities can he calculated by dividing by 2 and 3 to give 130.5 and 131, respectively. It would seem reasonable to suggest that the distinction between molecular and equivalent conductivity be clearly distinguished by using a differeut symbol. I/ and A have been frequently used in this sense. The International Union of Pure and Applied Chemistry has however deemed it advisable (8) to recommend A for "equivalent or molar conductance of electrolyte or ion." Thus the use of A for molecular conductivity has support but its meaning should be made very clear by the context. Volume 36; Number 6, June 1959
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Gram-Molecular Weight
Dr. J . It. Lakshmana 1t:io of t,lie ITniversity of Mysore l,rings up the point t,hnt m;niy textl)ooksl are obsourr i l l their definit,ion of gram qu:u~t,it,iessuch :LS gr:zni-molecular weight,. This is sometimes given as the molecular weight expressrd in grams immediately after a diacussio~~ pointing out. that the molecular weight, is a dime~~sionless r:~t,io which clearly can he expressed only by a pure number. 111 fact,, of course, t,he gram quantities are definit,e amounts of matter whose weight in grams is numerically eqwd t,o the corresponding dimensionless qumit,ity.
t,he whole field of science, chemistry is well represented among his examples which include such common statements as "atoms strive to attain the stable arrangement of' electrons t,hat characterizes the inert gases." Lifemture Cited (1)
FREEMAN, E. S., J . Phy8. Chem.,
60, 1487 (1956); J . A m .
('hem. Soe., 79, 838 (1957).
(2) BULKOR, K., AN,) CHASSOVENNY, V., Acta Phyaieoehim U.R.S.S., 5, 137 (1936); C.A., 31, 5639 (1937). H., private oommunication. (3) B~.ao%s, (4) I.ESCHEWSKI, K., Bw., 72B, 1763 (1939). 15) CENTNERSZWER. M.. AND B ~ M E N T H A M.. L .BdI. acad. d o n .
Discussed Elsewhere Y'eleoloyy. I
