with cold water, dissolution in ammonia and determination of silver by gravimetric analysis of this solution after acidification with sulphuric acid. Duplicate analyses would agree as closely as 0.02% on both Na₂O and V₂O₅ determinations. If the sample for analysis were more alkaline than the orthovanadate, standard sulphuric acid was added in known amount to bring the solution between the ortho and meta salt in composition. Similarly if the sample were more acid than the meta salt sufficient standard alkali was added to produce a colourless solution.

Equilibria with the colourless vanadates.

It will be convenient to describe first the equilibrium at 25°. between the aqueous phase and solid phases of the colourless ortho, pyro and metavanadates. These salts were prepared by fusion of vanadium pentoxide and sodium hydroxide in different proportions and the finely ground salt was then stirred with water for a period of one to three weeks, employing a mercury seal stirrer to exclude air. The thermostat was maintained at 25°±.01. In some cases solid sodium hydroxide was also added to produce a more alkaline mixture. After setting, samples of the liquid phase were withdrawn by filtration through glass wool into a pipette and the solid was sampled by rapid filtration through a sintered glass filter and the “wet residue” analysed. Analyses of 30 pairs of liquid and “wet solid” phases are given as weight percentages in Table I and plotted on the usual triangular diagram in Fig. I.

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By drawing tie-lines on the triangular diagram it can be seen that at least five solid phases are stable at 25° in very alkaline solution NaOH,H₂O is the solid phase, but the addition of a very small amount of vanadium pentoxide suffices to bring about a transition to Na₃VO₄,7H₂O, a compound which is stable only in presence of an excess of alkali and which would recrystallize from its own solution as the dodecahydrate. The tie-lines are sufficient to characterize it as Na₃VO₄,7H₂O, the formula given by Hall (1887), and not Na₃VO₄, 8H₂O given by Baker (1885). Further addition of vanadium pentoxide leads to Na₃VO₄,12H₂O, the pyrovanadate Na₄V₂O₇,18H₂O and the metavanadate NaVO₃,2H₂O; the tie-lines for the last salt, not drawn in the diagram, show, in agreement with McAdam and Pierle (1912), that the dihydrate and not the anhydrous salt is stable at 25°. The diagram affords no evidence for the stable existence at 25° of the compounds 4Na₂O,V₂O₅,26(or 30)H₂O obtained by Ditte (1887), Na₃VO₄,16H₂O by Roscoe (1871), Na₃VO₄,10H₂O by Baker (1885) or Na₄V₂O₇,8H₂O by Ditte (1887).

After the appearance of the paper by Kiehl and Manfredo (1937) we made a special search for the compound Na₃VO₄,2½H₂O, which they found to be stable at 30°. We were able, after stirring for a month, to obtain three sets of points whose tie-lines passed approximately through the point corresponding to this compound.

The following were the analytical data obtained:—

Experimental work in this region is very difficult; moreover, this hydrate is stable over only a small range of solution concentration; this makes it impossible for the tie-lines to converge on the solidus point through a large angle and hence it is not possible to locate the composition of the compound accurately. The data are, however, sufficient to show that the compound is neither the dihydrate nor the trihydrate and that the formula Na₃VO₄,2½H₂O given by Kiehl and Manfredo is likely to be correct.

Preparation of Polyvanadates.

A polyvanadate to which we give the formula 4Na₂O,7V₂O₅, 33H₂O and which we shall refer to as the 4:7 salt, was prepared in a number of ways:—

The best preparation is secured by adding two mols of acetic acid to one of sodium metavanadate, throwing down the polyvanadate by addition of alcohol and then recrystallizing from water by evaporation over sulphuric acid in vacuo.

Analysis of Polyvanadate.

Twenty-four analyses of this polyvanadate have been made. The percentages of Na₂O and V₂O₂ so found we do not regard as significant by themselves, because it was always difficult to determine the point at which air drying ceased to remove adhering moisture and began to cause efflorescence. The significance of the analyses can be better appreciated if they are expressed as molar ratios of Na₂O: V₂O₅. The following table summarizes the analyses in this form:—

The last two methods of preparation give the most reliable analyses, and it is evident that the simplest compound with this analysis would be 4Na₂O,7V₂O₅. Taking into consideration the appearance of the crystals before analysis, i.e., the appearance of moisture or efflorescence, we believe that the most reliable formula is 4Na₂O,7V₂O5,33H₂O although our most reliable analyses would not be markedly inconsistent with the formulation 4Na₂O,7V₂O₅,34H₂O.

An analysis of this salt by the method of Kiehl and Manfredo corresponded to 4Na₂O,6.89V₂O₅,34H₂O but we do not consider this method of analysis so reliable.

Crystallography of Polyvanadate.

We are greatly indebted to Dr. F. J. Turner for an examination of crystals of this polyvanadate. His report is as follows:—

Habit variable as a result of a tendency for crystals to be flattened parallel to any one of the three forms (l, g and h) of the principal zone. Two other zones (containing l x and y, or l m and n respectively) may be developed, but usually either m and n or x and y are omitted. The following angles, measured from ten different crystals, are probably correct to ±5′.

• l∧g = 42° 30′

• g∧h = 49° 5′

• h∧l′ = 88° 25

• l∧m′ = 56° 52′

• m∧n = 59° 17′

• l∧n = 63° 51′

• x∧g = 46° 51′

• m′∧g = 48° 36′

• x∧l = 56° 52′

• x∧y = 61° 25′

Symmetry appears to be triclinic, closely approaching monoclinic (the axis of the principal zone is almost an axis of binary symmetry). Microscopically examined, the crystals are biaxial, negative, and distinctly pleochroic (α < δ).

Preparation of a Polyvanadate unstable at 25°.

Occasionally in the course of the above preparations the salt formed could be seen to be heterogeneous and a second polyvanadate could be separated by hand from the mass of crystals. The formation of these brownish-orange lath-like crystals was favoured by low temperature, e.g., if the evaporation in vacuo were conducted in the ice chest. Otherwise their formation seemed to be fortuitous and not controllable, but as they could not be recrystallized from water without decomposition into pentoxide and the 4:7 salt, and no sign could be found in the subsequent phase rule investigation of their stable existence at 25°, this polyvanadate does not concern the 25° isotherm. The mean of five analyses gave to this salt the formula Na₂O, 1.99V₂O₅,10H₂O. The crystallographic features, measured by Dr. F. J. Turner, are as follows:—

Habit lathy, flattened parallel to the optic axial plane. Extinction angle(δ to direction of elongation) = 23°.Distinct pleochroism (δ = orange, α = lemon-yellow, δ>α). Good cleavage parallel to elongation, crossed at 85° by a less perfect transverse cleavage (δ lies in the acute angle of intersection of the cleavages).

The 3:5 Polyvanadate.

We obtained a number of analyses corresponding approximately to the salt 3Na₂O,5V₂O₅,22H₂O but these were always obtained from solutions with a higher ratio of Na₂O:V₂O₅ under experimental conditions which excluded washing the crystals free from mother liquid. A crystallographic examination by Dr. Turner showed that they were identical with authentic crystals of the 4:7 salts and we concluded that sufficient mother liquid had been left entangled with the crystals to give a misleading analysis. We do not, therefore, claim the existence of this salt.

Equilibria with the coloured Polyvanadate.

By using different mixtures of sodium metavanadate, the 4:7 and 1:2 salts and the solution obtained by saturating sodium carbonate solution with vanadium pentoxide, it was possible to follow the equilibria between solid and saturated solutions of composition between the metavanadate and what was practically an aqueous solution of vanadium pentoxide. The last three lines of Table I refer to solutions prepared from the 4:7 salt with sodium metavanadate as solid phase. The remainder of the analyses are given in Table II.