547.292-133-179.1 6 THE PECULIAR BEHAVIOUR OF THE SODIUM-URANYL

TRIPLE ACETATES BY

N. SCHOORI,.

By a determination of the water of crystallization in the triple acetates of Na, Mg and UO, ( B l a n c h e t i e r e ) or Na , Z n and UO, ( B a r b e r and K o l t h o f f ) , it has been found that six molecules of water of cryst,dlization are taken up with an unmeasurably small vapour-pressure and that these 6 H 2 0 are present as “facultative water of crystallization”. In addition yet a 7th molecule of water is taken up as a “zeolithic increment”. Consequently the weight of the triple acetates in the airdried state can still fluctuate by about 0.25 %.

K a h a n e ‘ s triple acetate was found to contain not 8 H2O but about 5 H?O and in addition about 1.5 mol. ethylalcohol. It is stable in air up to 110” and shows a very small zeolithic increment.

The alcohol content of the liquid can vary in the precipitation between 30 arid 60 56, without the content of “volatile matter” (sum of water and alcohol) in the crystals varying more than 0.2 %.

Into the anhydrous sodium magnesium uranyl triple acetate 4 molecules of ethyl alcohol can be introduced artificially; they are taken up without any rioticeable vapour-pressure but are readily replaced by H,O.

The triple acetates of sodium, magnesium (or zinc) and uranyl have come into use as a suitable form in which to weigh sodium in the gravimetric determination of this element. The magnesium triple acetate NaMg (UO,) ,Ac, was recommended by B 1 a n c h e t i P r e (1923) 1 ) and later the zinc triple acetate NaZn(UO,),Ac, by B a r b e r and K o l t h o f f (1928)z) .

In the quantitative determination when the calculation is made with these triple acetates with 6 molecules of water of crystallization, the result does indeed agree very well, in other words the yield of triple acetate 1:s 65.11 times greater with the magnesium salt and 66.89

l ) A. I? 1 a n c h e t i 6 r e, Bull. SOC. chim. 141, 33, 807-818 (1923). 2 , H. H. B a r b e r and I. M. K o l t h o f f , J. Am. Chem. SOC. 50, 1625-1631

(1928); also Chem. Weekblad 26, 294-298 (1929).

LIX 12

306 N . Schootl.

times greater with the zinc salt than the amount of sodium in the weighed sodium salt.

Furthermore these 6 molecules of water of crystallization appear to be very tightly bound, since the triple acetates show no change in weight after drying at 110' C and cooling in air. The relative humidity of this tenaceously held water of crystallization is thus certainly less than 0.01.

It is however not so certain that this water of crystallization is bound in the stoechiometric ratio 1 : 6. Two analyses of the sodium magnesium triple acetate occur in the literature. C a 1 e y and F o u 1 k (1929) 3) have determined all the constituents of this salt (Na, Mg, UO, and acetic acid) accurately and in this way found a water content corresponding to 6% molecules by subtraction from the total weight. K a h a n e (1930) 4 ) determined the residue on ignition of B 1 a n c h e t i 6 r e ' s triple acetate, consisting of Na2U207 t + MgU,O, and from the results decided on 6 molecules H 2 0 . In doing so he made a small concession since the calculated ignition residue amounts to:

ignition residue for triple acetate with 6 H,O . . . . 62.10 "/o t . t . ,, 7 H 2 0 . . . . 61.36 %

whereas K a h a n e finds on the average an ignition residue of 61.90 %, which would correspond to 6.27 mol. H 2 0 .

The water of crystallization in the triple acetates cannot be determined directly from the reduction in weight which they mdergo on drying at still higher temperatures than 110' C in air, because somewhat above this, namely at about 125' C, decomposition with evolution of acetic acid sets in. This can however be done successfully by drying in vacuo in the presence of phosphorus pentoxide at 80° C, whereby a constant weight is reached in 24 hours. After complete cooling and after letting in air dried over phosphorus pentoxide, the dry triple acetate must be weighed in a hermetically tight weighing- bottle, since it increases very rapidly in air by the absorption of water.

In this way I found in various analyses of the air-dried triple acetates water contents corresponding to: 6.4, 6.5, 6.67 mol. H,O for the magnesium triple acetate,

6.6, 6.8 mol. H,O for the zinc triple acetate. The speed with which the triple acetates dried in this way, absorb

water again from the air and are regenerated to give the original weight of the salt with water of crystallization, is remarkable. This is complete in 24 hours in ordinary air or in a hygrostat of relative

3, R. C a l e y and C. W. F o u l k , J. Am. Chem. SOC. 51, 1664-1674 (1929). 4 , E. K a h a n e, Bull. SOC. chim. [4] + 47, 382-404 (1930).

The peculiar behaviour of the sodium-uranyl triple acefates.

humidity 0.68, in even 1 hour in a hygrostat of relative humidity 0.94. This fact finds its explanation in two circumstances: Firstly the

vapour-pressure of the water of crystallization (at any rate up to an amount of 6 H,O) is extraordinarily small. The fresh triple acetate can not only be dried at 110' C in air without diminution of weight but it also retains 6 H,O when placed in a vacuum desiccator with conc. sulphuric acid or phosphorus pentoxide.

On microscopic investigation of the triple acetate freed from its water of crystallization and embedded in oil of aniseed it subsequently appears that the crystals have not crumbled to a fine powder as is the case in the ordinary weathering of a salt hydrate, but that the original crystalline form 5 ) is retained. The crystal lattice is thus in this case not broken up by the escape of the water of crystallization and the latter c,an also be reabsorbed in it again without change of the crystalline form. W e thus have a case here of "facultative" water of crystallization (see Chem. Weekblad 30, 361 (1933), case 11).

Besides the 6 molecules of water of crystallization with unmeasurably small vapour pressure, the triple acetates also take up a further quantity of water dependent on the water vapour-pressure in the surroundings, as is to be seen from the following experiments. Triple acetate of an accurately known water-content (determined in a separate portion by drying at 80' C in vacuo in the presence of P,O,) was placed successively in vacuo and in hygrostats of different relative humidity. Remarkably enough a constant weight had already been reached within one hour (probably much earlier still). The following changes of weight were found.

307 _ _ _ _ _ _ . _ ~ -

Sodium magnesium uranyl acetate ( B I a n c h e t i 5 re ) , NaMg(UO,),Ac, = 1388.5.

Air-dried salt, in which 0.049 g water was found in 0.632 g triple acetate by drying at 80' C in vacuo over P,O,; this corresponds to 8.41 '/o of water calculated on the dry substance.

An amount of 1.883 g of this triple acetate showed the following changes in weight in hygrostats of various relative humidities.

______-- ") The triple acetates have the appearance under the microscope of flattened

octahedra which are however weakly anisotropic and hence not cubic. The crystals are in fact hemihedral hexagonal as Dr. P. T e r p s t r a (lector a t Groningen) was kind enough to ascertain for me. He describes the crystals as showing two faces of the basal pinacoid and six faces of a flat rhombohedron making an angle of 40%", as well as six smaller faces of a steeper rhombohedron making an angle of 60%".

308 N . Schoorl.

Change in weight in

Rel. hum. mg o(pzo5) -6.6

0.68 -0.3 0.34 -2.6

0.94 + 3.1 0.98 + 6.0

Change in O/O water calc. on dry O/o water in

subst. dry subst. mol. H@ -0.41 8.00 6.17 -0.16 8.25 6.36 -0.02 8.39 6.47 + 0.20 8.61 6.64 + 0.38 8.79 6.78

Sodium zinc uranyl acetate ( B a r b e r and K o 1 t h o f f ) , NaZn(UO,),Ac, = 1429.6.

Air-dried salt in which 0.0646 g water was found in 0.8434 g triple acetate ( = 8.30 % water calc. on dry substance) by drying at 80’ C in vacuo over P,O,.

An amount of 1.716 g of this triple acetate showed the following changes in weight in hygrostats of various relative humidities.

Change in weight in calc. on dry O/o water in

Change in O/O water

Rel. hum. mg substance dry subst. mol. H20 -7.8 -0.53 7.77 6.17 -2.5 -0.18 8.12 6.45

O(p205) 0.34 0.68 -1.1 -0.08 8.22 6.53 0.94 + 0.9 + 0.05 8.35 6.64 0.98 + 3.7 + 0.25 8.55 6.80

The curves representing the changes in weight practically coincide. It appears that these triple acetates, besides 6 (or more than 6) molecules of facultative water of crystallization very tightly held, also contain in addition a “zeolithic increment” of water of crystallization (see Chem. Weekblad 30, 361 (1933), case IV), the magnitude of which depends on the water vapour-pressure of the surroundings and which can increase to practically one extra molecule of water of crystallization in the case of very high water vapour-pressure.

For the gravimetric determination of sodium the behaviour of the triple acetates has the significance that one can make an error of about 0.25 % in the weight, according to whether carries out the weighing of the air-dried triple acetates at a very low (rel. humidity 0.15) or a very high (rel. humidity 0.85) hygrometer reading.

A similar difference can also be observed when the triple acetates are dried at looo C and then cooled in a vacuum desiccator over sulphuric acid or phosphorus pentoxide ( a ) , or simply cooled in the air (b) .

The weight of the triple acetates in the experiments described above was:

The peculiar behaviour of the sodium-uranyl triple acetates. 309

Mg triple acetate Z n triple acetate originally 1.385 g 1.218 g after drying (a) 1.381 ,, 1.2135 ,, in the air (b) 1.385 ,, 1.218 ,, diE. b-a 0.29 Olo 0.37 010

When the triple acetates are first of all completely freed from water at 80’ C in vacuo over P,O, and then regenerated in a hygrostat

Fig. 1. The full curves give the relative humidity of the zeolithic increment of the original triple acetates (a ,Mg, b Zn) ; the dotted curves

that (of the triple acetates after dehydration and regeneration (c Mg, d Zn) .

of relative humidity 0.94, they show a still larger zeolithic increment when weighed in air of various humidities.

Magnesium triple acetate original weight 0.632 g

n w t O,’o aq. mol. HzO vat. 80° (z . PzO5 -0.051 - - rel. hum. 0.94 -I- 0.0025 9.20 7.10 .. ., 0.68 -0.0005 8.69 6.70 ,, ,. 0.32 -0.0025 8.35 6.45 ,. ,, O(P2O5) -0.006 7.74 5.97

Zinc triple acetate original weight 0.8434 g

n w t O/o aq. mol. HzO -0.067 - - -0.001 8.50 6.75 -0.0035 8.17 6.50 -0.0055 7.91 6.29 -0.009 7.47 5.93

It can be seen from the graph that the zeolithic increment after this preliminary treatment is almost double that of the original triple acetates. The crystals thus appear to have become more porous.

310 N . &hood.

B 1 a n ch e t i e r e ’ s method for the quantitative determination of sodium has been altered by K a h a n e (1930) 0 ) . He uses a reagent mixed with alcohol (45 vol % ) and according to him the precipitate, which is produced thereby, corresponds to the composition NaMg(UO,),Ac, . 8 H,O deduced from the residue on ignition (see earlier). 1 did indeed find 10 % of “volatile matter’’ calculated on the precipitate, thus 11.1 % on the dry substance) in K a h a n e ’ s precipitate by drying at 80’ C in vacuo over P,O, and this does require the formula with 8 H,O.

Nevertheless these crystals do not only contain water, but also alcohol of crystallization, which can be detected simply by mixing the aqueous solution with concentrated sulphuric acid and a few drops of bichromate, whereby a green coloration (of Cr,O,) is produced as well as the smell of acetaldehyde.

I determined the ethyl alcohol content quantitatively by dissolving a weighed amount (200 to 300 mg) of K a h a n e ’ s precipitate in 10 cm3 water, mixing with 10 cm3 of a cooled mixture of equal volumes of concentrated sulphuric acid and water and 25 cm3 0.1 n per- manganate and allowing it to stand in a closed vessel during an hour, after which the excess of permanganate was determined iodometrically. Since the ethyl alcohol is oxidised to acetic acid’) 1 cm3 0.1 n = = 1.15 mg C,H,OH.

10 $% of “volatile matter”. of which 4.3 % was alcohol and thus 5.7 % water, was found in this way in K a h a n e ’ s precipitate, prepared by mixing 1 cm3 NaCl solution containing 10 mg NaCl with 10 cm3 of K a h a n e ’ s reagent (containing 45 vol ”/o ethyl alcohol). Calculated on the dry substance this becomes 4.8 % alcohol and 6.3 ”/o water, which corresponds to about 1.5 mol C,H,OH and about 5 mol H,O per molecule of triple acetate.

Remarkably enough K a h a n e ’ s triple acetate can be dried at 105’ C in air. without the weight changing. Neither is there any change in weight on standing 3 days in the air or 3 days in a desiccator over sulphuric acid and in all these treatments the alcohol content also appears to remain unchanged. Only by putting the precipitate in a hygrostat of relative humidity 0.94, thus in a water-rich atmosphere, had the alcohol content diminished to 4.0 % after 3 days and after having remained for about M year in this atmosphere the alcohol was found to have disappeared completely and the content of “volatile

6 ) E. K a h a n e , Bull. SOC. chim. [4] , 47, 382-404 (1930). ’) According to C h a p m a n and S m i t h , J. Chem. SOC. 20, 301 (1867):

employed by J. G. I m h o f , Dissertation 1932, p. 77.

The peculiar behaviour of the sodium-uranyl triple acefates. 31 1

matter” (water) to be 8.7 “/o. The water molecules are thus able to displace the alcohol molecules in the crystal.

K a h a n e ’ s triple acetate shows only slight changes in weight in the presence of various degrees of humidity. In an investigation in the same way as B l a n c h e t i e r e ’ s and B a r b e r and K o l t h o f f ’ s precipitates (see above) the change of weight of 2 g of K a h a n e ’ s sir-dried precipitate was found to be:

O/o ..volatile matter”

in a hygrostat of rel. hum. 0.94 . . . . . . 3-0.003 10.15 I. *. ., ,, ,, 0.60 . . . . . . + 0.001 10 05 . I ., ., ,, ,, 0.32 . . . . . . 0 10.0 I t I, ,, ., ,, O( PZOS) . . . . . -0.00 1 9.95 then after 2 hours drying at 100’C , . . . . -0.003 9.85 afterwards in the air . . . . . . -0.001 9.95

Altogether the fluctuation in the weight thus amounts to 0.2 ‘/o SO

that in weighing in very dry air (rel. hum. 0.15) or in very moist air (rel. hum. 0.85) one can reckon on no greater error in weighing in the analysis than 0.1 %; and similarly after drying at 100’ C and cooling in the air.

The content of alcohol (4.3 % ) and water (5.7 ”/.) found in K a h a n e ’ s precipitate does not represent a stoechiometric ratio, but is dependent on the medium in which it is precipitated (here 41 vol. % alcohol). I have observed this by producing the triple acetate at various alcohol concentrations in the liquid. In every case I started from 1 cm3 NaCl solution containing 20 mg NaCI, mixed the latter if necessary with absolute alcohol (see the following table) and then with 1 0 cm3 of B l a n c h e t i e r e ’ s reagent (which is more than twice as strong in uranyl acetate as K a h a n e ’ s ) , previously mixed with various amounts of absolute alcohol. The precipitates are collected after stmding over night, washed three times with 3 cm3 strong spirit, once with 3 cm3 acetone and then dried for a short time at 100” C. The plzrcentage of “volatile matter” in these precipitates was determined by drying at 80° C in vacuo over P,O, and in addition the alcohol content by the method described above. 1 em3 NaCI

+ cm3 a h . 10 em3 Blanch. vol. O/o alc. in matter in in solution 0,’o volatile O/o alc.

alcohol + cm3 abs. alc. the liquid precipit. precipit. 0 1 8 9.2 3.44 0 2 15 9.4 3.97 0 5 31 9.6 4.19 5 5 47.5 9.8 4.67

10 10 64.5 10.0 4.97 10 20 73 10.2 5.34 10 40 82 10.4 5.85

312 N . Schoorl.

Between 30 and 60 vol. % alcohol in the liquid, there is thus but little change in the ”/o alcohol and the % water in the precipitate and, by partial compensation, also little difference in % “volatile matter” 8 ) .

Altogether the % alcohol fluctuates so that it lies between 1 mol and 2 mol alcohol of crystallization.

o 20 40 60 ao vol.%A

Fig. 2. The percentages by weight of alcohol and water (along the vertical axis) are represented by two curves for various alcohol

contents of the liquid (in vol. % along the horizontal axis).

Various attempts were made to introduce further molecules of alcohol of crystallization into the triple acetate, of which only the following succeeded. One must be very careful to exclude the smallest traces of water, since the latter readily displace the alcohol of crystallization.

K a h a n e ’ s triple acetate was completely freed from “volatile matter” at 80’ C in vacuo over P,O,, cooled in a vacuum desiccator over P,O, and transferred in the weighing bottle (which was quickly cjpened at the last moment) to a tube through which a stream of dry air saturated with the vapour of absolute alcohol was passed. The latter was previously distilled over calcium and then stored with some pieces of bright calcium.

After dry alcohol vapour had been passed for 5 hours the weight had increased to 13.25 % (calculated on dry triple acetate). After- wards by placing it in the sulphuric acid desiccator, pumping out the latter and again filling with dry air, the increase in weight fell to

*) There is a small difference in this case in % “volatile matter“, compared with the precipitate made with K a h a n e ’ s reagent. This is possibly attributable to small differences in composition compared with B 1 a 11 c h e t i e r e ’ s reagent which contains proportionally much more magnesium acetate and acetic acid.

The peculiar behaviour of the sodium-uranyl triple acetates. 313

12.4 %. The alcohol content in this state was determined in the manner indicated above and the value 12.37 % was found (calculated on dry triple acetate). There is therefore now only alcohol of crystallization present and no water of crystallization.

Since 1 mol alcohol of crystallization (C,H,OH = 46) to 1 mol triple acetate ( = 1388.5) represents an amount of 3.32 "/o (on dry substance), there were thus 3.75 mol alcohol present in the preparation investigated containing 12.4 % alcohol and 4 mol alcohol, as a maximum, had been introduced into the crystals of the triple acetate, namely before pumping off over sulphuric acid.

That this alcohol of crystallization is readily replaced by water of crystallization in moist air, has already been mentioned.

U t r e c h t, Pharmaceutical Laboratory of the University, January 1940.

(Received February 7 th 1940).