T H E BEHAVIOUR OF T H E TUNGSTIC ACIDS TOWARDS SODIUM HYDROXIDE BY ARTHUR M. MORLEY
I n a previous publication' the preparation of four types of tungstic acid has been described. Preparations of each type were subjected to various ageing processes, and from a combined analytical and X-ray study of the original and aged products, deductions were made as to the structure of the various tungstic acids. The present publication describes experiments which were carried out to study the action of solutions of sodium hydroxide upon tungstic acid products of the above mentioned types, with special reference to solubility, and the production of colloidal solutions. A search of the literature reveals the fact, that although there are many references of a general nature to the solubility of tungstic acids in sodium hydroxide, no systematic work has been carried out on this subject, and no quantitative data are available. I t is generally agreed, however, that one of the chief characteristics of tungstic acid is the ease with which it dissolves in solutions of the strong alkalies, giving perfectly clear solutions. Since in the experiments carried out by the author it was desired to study the peptisation of the tungstic acids, the sodium hydroxide solutions employed were, in most cases, of low concentration.
Experimental Preparations. All the preparations employed were made from solutions of ammonium tungstate and hydrochloric acid, as described in detail in a separate paper.' The analyses and general characteristics of the preparations are as shown in Table I. A pure primrose-yellow tungsten trioxide was also prepared, by heating a tungstic acid of type A in a platinum basin for three hours a t 8oo°C. in an electric furnace. All preparations were sieved through a 90 mesh I.M.M. sieve. I n the paper previously referred to, the structures of the tungstic acids employed were shown to be as follows: Type A-H2W04, H 2 0 ; Type Ba hydrated amorphous variety of either H2W04 or H1W04, H 2 0 ; Type Ca mixture, consisting chiefly of an imperfectly crystallised or a condensed acid, with smaller amounts of H2W04 and amorphous tungstic acid; Type D -H2W04. l
Morley: J. Chem. Soc., 1930, 1987T.
1656
ARTHUR M. MORLEY
Colour Appearance under microscope ( X 800)
3"
c1
Lemon yellow Flat leaflets exhibiting frequent twinning. Largest about IO^ in length, and 4p across the centre. May be regarded as crystalline
0.77 Trace 100.02
Colour Appearance under microscope ( x 800)
0.74
Nil -
99.92
Greenish yellow Amorphous. Very small, almost spherical particles, of average diameter I p
Greyish white Glassy amorphous particles of irregular shape. Size very variable. Maximum breadth between zp and sop
0.05
"3
c1
Trace 99.87
Deep yellow Identical with Type C products
* Matter insoluble in water after fusion with sodium carbonate. Due to absorption of calcium from tap water used for washing the precipitated tungstic acid. General Method The general method employed was to mix suitable quantities of sodium hydroxide solution and tungstic acid, in a flask coated internally with paraffin wax, which was then fitted with a waxed cork and suspended in a large water thermostat a t 2 5 ' C. * o.IOC. The contents of the flasks were shaken periodically, and were examined and analysed a t intervals, by determining the total amount of tungsten in the supernatant liquids, and in the liquids after ultrafiltration through collodion membranes. The difference between the values so obtained was regarded as a measure of the amount of tungstic acid present in the colloidal condition. One series of experiments was carried out with tungsten trioxide and tungstic acid D, but two series were performed with products A, B, and C, the separate preparations used being referred to as I and I1 in the analyses. The second short series served to confirm the first
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
I657
set of results, and in addition, certain other determinations were carried out vie. the determination of the pH values of the solutions, and the examination of the solid phase in a selected number of cases. Experimental Technique. (a) Alkali solutions. The specially distilled water employed for the experiments had a specific conductivity of ca. 2 X IO+ reciprocal ohms. The sodium hydroxide solution was prepared by subjecting pure metallic sodium to the action of water vapour in an enclosed space, free from carbon dioxide. The saturated solution produced was diluted with freshly prepared conductivity water in the usual type of enclosed apparatus, waxed internally,
FIG.I Tungsten Trioxide in NaOH Before Ultrafiltration
and with burette attached. This stock solution was diluted as required to prepare the solutions for the experiments. (b) Determination of hydrogen ion concentration. The standard potentiometric method was used. The h i t of accuracy of the apparatus employed may be regarded as + 0.02 of a pH unit. Both the hydrogen and quinhydrone electrodes were used, but since in certain cases a t low pH values poisoning of the hydrogen electrode occurred,' the values given in the tables are those obtained with the quinhydrone electrode, which behaved normally throughout the investigations. (c) Ultrafiltration. The method of ultrafiltration through collodion membranes was that previously described by Collins and Wood.2 Tests by Bechold's method3 showed that the filters held back particles with diameters greater than I pp. It was shown by direct adsorption tests with pieces of collodion membrane and solutions containing tungstate, metatungstate, and Cf. Britton: d. Chem. SOC.,1927, 147(T);1930, 124gT.
* J. Chem. SOC., 121,
1 1 2 2 (1922). (190;).
2. physik. Chem., 60, 25;
1658
ARTHUR M. MORLEY
colloidal tungstic acid, that there is no adsorption of ions or colloidal particles containing tungsten, by the membranes. It was also shown by quantitative tests on the ultrafiltration of optically void solutions of sodium tungstate, and sodium metatungstate of known tungsten content, that at laboratory temperature, with ultrafiltrations carried out up to two hours, the experimental error due to evaporation of the solutions etc. was not greater than zyo. All ultrafiltrations were carried out under the above mentioned conditions. Dnvs fmom
... ............. ,
OamrrL C ~ M W W N
01
Nn(m
(*.fl*u&.
FIG.z Tungstic Acid (ty e A)-in NaOH Series I-Before 8ltrafiltration
(d) Estimation of Tungsten. All analyses were carried out by the standard gravimetric mercurous nitrate method. The method was carefully tested, and shown to be accurate with tungsten in the forms of sodium tungstate, sodium metatungstate, and tungstic acid in the colloidal condition. Since small quantities of solutions had to be used in many cases, all portions for analysis were accurately weighed out, and all results were expressed as grams of WOa per 100 grams of solution. All estimations were carried out in duplicate. In all Series I experiments, portions of the solutions were taken for analysis by allowing the solids present to settle for several days and carefully pipetting off the supernatant liquids. Filtration of the solid had been found to be unsatisfactory in many cases, owing to its finely divided nature. The settling method of necessity involved a slight concentration error caused by drops of condensed liquid on the upper portions of the flasks.
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
1659
This was avoided in the various Series I1 experiments, a centrifuge then being available. The standard method for obtaining a sample was to shake up the contents of a flask, centrifuge a portion for 30 minutes a t 2,000 revs. per minute, and pipette off and weigh part of the supernatant liquid. The centrifuging was carried out in an asbestos box at a temperature of 25’ C. + 0.5’ C., so that there was no appreciable change in temperature when the solution was transferred from the thermostat to the centrifuge. (e) The solid phase. Wo. Ostwaldl and von Buzagh? by their work on the “solid phase” rule have shown that generally, in a colloidal system, the amount of substance peptised is dependent on the amount of solid phase present. The author has shown by special tests, that this effect applies t o the peptisation of tungstic acid by sodium hydroxide. For this reason, the exact quantities of solution and solid used have been given in the following tables.
Analytical Results All columns of figures headed W03 refer to grams of W03 per I O O grams of solution. This also applies to the “Colloid content.” The pH values are those obtained with the quinhydrone electrode on solutions which had been centrifuged in the manner previously described. The ratio W03/Na20 gives an approximate idea of the type of tungstate (e.g. whether normal or metatungstate) present in solution; the wo3 values are those determined in IOO grams of the final ultrafiltrates, divided by its molecular weight, and the NazO values, that present in grams in IOO C.C. of the original sodium hydroxide solution divided by its formula weight. Since the densities of the solutions involved are near unity, it is not necessary to introduce the values for the purpose of these calculations. If sodium be adsorbed by the solid phase, it follows that the W03/Na20 ratio will not accurately represent the composition of the salts in solution. The colloid contents are given to the second place of decimals. Figures of greater accuracy would be meaningless, as, assuming an ultrafiltration error of zyo,even the figures in the second decimal place may be slightly incorrect. Graphs are Given showing the relation between the concentration of the original sodium hydroxide, and the WOS in solution before ultrafiltration, in the tungsten trioxide series, and the Series I experiments for tungstic acids A, B, and C. (Tables I, 11, IV, VI). Tungsten Trioxide I 2 No. of flask Conc. or original NaOH (Normalities) 0.001 0.020 Wt. of WOa (gms.) 3.0 3.0 In all cases joo C.C. of alkali were used.
Kolloid-Z., 41, 163;43, 227, 249 (1927).
* Kolloid-Z., 41, 169 (1927);46, 178 (1928).
3
4
5
0.078
0.157
0.261
5.0
10.0
10.0
1660
ARTHUR M. MQRLEY
TABLEI No. of
h k
Days from start
7 4
I
161
7 43 161
2
3
4
5
Before ultrafiltration
After ultrafiltration
(V. Fig. I )
PH
WOs
wo3
Colloid Content
wo
Appearance J
NalO
0.038 0.068 0.063
5.42 5.91 6.28
0.017
0.02
0.046 0.048
0.02
0.015
4.I
0.215
9.83 6.97 6.85
0.216 0.284 0.326
Nil Nil Nil
1.4
0.286 0.320
7 43 161
0.733 0.987
7 43 161
1.482 1.905
7 43 161
12.0
7.51
7.33
1.020
-
-
-
-
1.934
7.63
-
2.146 2.920 2.972
12.77
-
11.87
-
Clear 1.1
-I
Clear
1.1
-
-
8.04
Faintly
Clear
-
7.82
12.23
Of.
Solution
No change was observed in the appearance of the solid phase.
Tungstic Acid-Type
A
Series I
No. of flask Conc. of NaOH (N) Volume of solution
9 0.001
zoo
IO
11
12
13
14
0.005 0.0100.020
0.035
0.050
200
150
IOO
IOO
1.0
1.0
IOO
25.
Series I1 26
0.0100.020
500
500
27
0.028 500
(C.C.)
Wt. of tungstic acid(gms.)~.o
1.5
3.0
4.0
5.5
9.5
10.5
1661
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
TABLE I1 Series I No. of flask
9
Before ultrafiltration Days from (v. Fig. 2) start Wos
7 85 I 60
0.006
0.03
-
0.030
0.01
7 85
0.160 0.187
0.134
0.oj
0.221
0.208
0.01
0.212
0.01
686
0.226 0.228
0.218
0.01
7 85 I 60
0.282 0.411 0.518
205
0.527
250
686
0.517 0.518
0.388 0.523 0.519 0.519
7
0.646
160 205
I2
I
85 60
20.5
250
686
0.821
0.919 0.938 0.952 0,979
7 85
0.891 1.303
160
1.565 1 ' 565 I . 600 1 ' 594
205
250
686 I4
Colloid Content
686 IO
13
wo s
0,035 0.039 0.041 0.049 0.044
205
I1
After ultrafiltration
7 85 160 205
686
0.038
0.01
0,033
0.01
-
0.519
0.782 0.920 0,930 0.939 0.987
-
0.02
Nil 0.01
Nil Nil 0.04
Nil 0.01
0.01
Nil
-
-
I.
267 I .560
0.04
1.535
0.03
I . 604
Nil Nil
1
'
594
I .171
-
2.015
2.031
2.238 2.262 2.324
2.219 2.334
Nil
Nil 0.04 Nil
Appearance of Solution
Solutions 9-13 very slightly opalescent at commencement of experiments. later becoming clear Solution 14 clear throughout the experiments
1662
ARTHUR M. MORLEY D m
rn.n
..............
-.-.H I -
.PI
O a r e r ~ aCo+ioc.nrno* ~
em NIOU (w-)
FIG.3 Tungstic Acid (Ty e B)-in NaOH Series I-Before dtrafiltration
TABLE111 Series I1 No. of Days from flask start 25
26
27
91 161 218
13 63 91 161 218 91 161
Before ultrafiltration
woa
p~
After ultrafiltration Colloid WOJ Content
0.455 0.469 0.468
5.64 4.85 4.46
0.424
0.03
0,457 0.477
0.01
0.601
-
0.561 0,766 0.880 0.939 0.950
0.04 0.09
0.922 0.937 0,954
5.41 4.63 4.29
1.156 1.299
5.89 4.96
1.072
0.08
1.239
0.06
0,854
Nil
0.04
Nil Nil
WOS Appearance of Solution -N
-
4.0 -
All solutions - > resembled - 9-13in 4.1 Series1
-
-
1663
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
TABLEIV Series I No. of flask 15
Days from start
7
0 .I O 0
64
0.105
0.072
0.03
165
0. I 1 2
0.107
0.005
0.104 0.125
0.091 0.114
0.01
0.296
-
687
wos
0.06 0.015
20
0.880 1.250 I . 506 I . 690 I . 900 1.478 I . 674 2.129 2.164
-
0.01
0.IO
0,930 7.379 0.3;1
0.03
0.08
-
0.05 0.09
1.569 1.894
0. I 2
-
Same as 1 5
0.01
0.03
2.277
,644 1.930 1.93' 2.250
1.631
-
-
1,904 2.173 2.971 2.114
0.04 0.14
1.942 2.312 3.018 2.148
Same as 1 5
0.01
I . 198 1.418
I
Same as 15
0.01
0.612
-
Slight opalescence, decreasing with time
-
0.337 0.334 0.342
0.456 0.714 1.007
Appearance of Solution
0.01
0.269
1.348 0.382
I9
-
0,352
0.351
18
3
0,331 0.346
I7
Colloid Content
wo -
205
16
Before ultraAfter ultrafiltration filtration (v. Fig. 3)
0.20
Clear
0.23 0.03
0.05
0.03
Clear
1664
ABTIIUR Y. MORLEY
&,,cirrrL
Cmcmrnmmu
01 NrOH
(uornam:l*
FIG.4 Tungstic Acid (Type C)-in, NaOH Series I-Before Ultrafiltration
TABLEV Series I1 No. of flask
28
29
Days from start
87
170
1.147 1.236
226
1.244
WOa
-
1,239 1,354 1.565
0.04 0.01
-
1.829
0.23
-
1.873
0.21
4.6
1.282
0.12
-
1.671 1.761
0.22
-
0.04
2.9
226
2.081
87 170
1.397 1.888 1.805
6.41 5.75 5.92
0.02
A pearance
KOo r Solution
0.04 0.09 0.14
170
226
Colloid Content
1..107 1.145 1.105
5.42 2.87 2.79
1.282
After ultrafiltration WOS
3.65 2.73 2.79
1.377 1,578 2.057
15 65 87
30
Before ultrafiltration woa p~
-
Clear
4.8
Clear
Clear
The following changes in the solid phase were observed: in Series I, solid 15 was definitely yellow after 5 days, the remaining solids being unchanged. As time progressed, solids 16-19slowly changed to a dull yellow colour, but no change in structure was visible under the microscope. After about go days, the solid in flask 20 consisted of small white crystals (hexagonal plates) of Na2W04, 2H10, and unchanged tungstic acid. These observations were confirmed generally in Series 11, e.g., white hexagonal crystals appeared in flask 30.
1665
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
Tungstic Acid-Type C Series I This series actually consisted of three separate short series, the results being combined for convenience The flasks, therefore, are not numbered consecutively.
No. of flask Conc. of NaOH (N) Volume of Solution (C.C.) Wt. of tungstic acid (gms.) No. of flask Conc. of NaOH (N) Volume of Solution (C.C.) Wt. of tungstic acid (gmS.1
44 0.001
46
45 0.004
47
39
0.007
48
0.010
0.017
0.025
200
200
200
I50
IO0
IO0
1.0
1.0
1.0
1.0
1.5
1.5
40
41 0.049
0.059
0.079
0.098
IO0
IO0
50
50
50
2.0
2.0
2.0
2.0
2.Q
0.035
21
23
22
24 0.118
50 2.0
TABLE VI
No. of flask
44
45
46
Days from start
Before ultrafiltration (v. Fig. 4) WO 3
After ultrafiltratio? W08
Colloid Content
7
0.047
-
71 I18
0.058
0.006
0.05
0.065
0.012
0.05
201
0.06~
0.026
0.04
7
0.113
-
71
0 .‘45
0.095
0.05
I18
0.I73
0.I 2 0
0.oj
201
0.186
0.119
0.0;
71
0.164 0.189
0 .I 5 5
0.03
I 18
0.229
0.164
0.065
201
0.254
0.I 7 0
0.08
7
-
Appearance of Solution
~
Colloidal in appearance ; bluish white by reflected light, reddish-orange by transmitted light
1666
ARTHUR M. MORLEY
TABLE VI (Cont,inued) No. of
.flask
Days from start
7
39
71
I I8 201
7
47
71 I18 201
Before ultrafiltration (v. Fig. 4) WO,
After ultrafiltration WOa
Co1Ioid Content
0,325 -
0.187 0.238 -
0.06 0.09 -
0.414
-
0.481
0.398 0.421 0,435
0.209 0.244
0.529 0.545
71
0.523
0.555 0.575
0.563
0.632 0.728
-
-
0.707
0.02
0.834
-
0.843 -
Nil -
0.849
201
0.801 0.860 0,900 0.890
7
I.18j
-
71
1,352
1.321
I8
1.451 I . 456
1.312
40
7 71 I I8
0.472
201
7
&I
71 I I8
21 I
201
7 71 I I8 201
23
7 71
I18 201
24
Colloidal in appearance; 0.I 1 bluish-white 0.11 by reflected light, reddish0,035 orange by Nil transmitted 0.01 light
I18
I
201
22
-
0.08
0.488
48
0.572
-
0.894 0.866
1.447
I , 542
1.665 1.677
1,525 I. 636
I .696
-
767 1.910 1.933
1.747 1 ' 799 I.882
'
0.01
O'OI 0.02
i
Clear
J
0.14
Clear
I . 420
I . 562
I
A pearance oPhlution
0.14 0.04 J
0.I 1
Clear
Clear
0.05
7
I .88I
-
71
2.000
1.936
I I8
2.118
2.007
0.I 1
201
4.124
2.040
0.08
Clear
1667
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
Series I1 No. of flask Conc. of NaOH(N) Volume of solution (c.c.) Wt. of tungstic acid (gms.)
31 0.010 500
3.3
32 0.040
33 0.060
400 8.0
400 8.0
34 0.080
400
10.8
TABLE VI1 No. of
h k
31
Days from start
93 170
226 32
93 170
226 33
I4 64 93 170
226 34
93 170
226
After ultrafiltration Colloid WOa Content 6.32 0.161 0.03
Before ultrafiltration WOs pH
wo3
~
0.193 0.293 0.324
5.95 5.99
0.232
0.709 0.853 0.890
6.69 6.67 6.54
0.707 0.831 0.881
0.876 0.957 0.961 1.139 1.164
7.10 6.95 6.96
0.843 0.933 0.938 1.119 1.161
0.02
Nil
1.7
1.288 1.436 1.439
0.03 0.06 0.03
-7
1.318 1.499 1.471
7.10
7.08 7.11
0.261
0.06 0.06
Nil
A pearance
s of'solution l ~
- - } Colloidal 2.21
0.02
Very 1 faintly
0.01
I .9
opalescent
0.04 0.02 0.02
1.6
The solid in flask 44 remained pale greenish yellow; the remaining solids of Series I, and all those of Series I1 changed rapidly to bluish white products, In both series, approximately similar quantitative results were obtained up to a concentration of initial alkali of 0.05 N. Above this concentration, the values for the total WO1 in solution were higher for Series I than for Series 11.
Tungstic Acid-Type D Owing to shortage of time, only one series of experiments was carried out with tungstic acid D.
No. of flask Conc. of NaOH(N) Volume of solution (c.c.) Wt. of tungstic acid (gms.)
69
70
71
72
0.001
0.005
0.010
0.060
500
500
500
500
5 .o
5.0
5.0
12.0
1668
ARTHUR M. MORLEY
TABLE VI11 No. of flask
69
Days from start
6 27
70
71
After ultrafiltration Colloid WOa Content
0.038 0.045
4.29 4.09
0.006 0.014
0.03 0.03
WOs Appearance N X of Solution
-
0.253
4.19
0.147
0.11
0.270
4.10
0.183
0.09
7
0 .5 0 2
4.24 4.14
0.370 0.395
0.13
0,519 1,915 2.004
6.03 6.17
1.836 1.980
0.08
-
0.02
2
7
7 27
0.12
II
Colloidal in appearance; bright yellow - by reflected 3 . 2 light, reddishorange by - transmitted 3 . 4 ) light I. 2
27
27
72
Before ultra- filtration WOa pH
Veryslightly
. 8 J opalescent
The solids in flasks 69-7I remained unchanged throughout the experiments. The tungstic acid in flask 72, however, slowly changed to a white solid. The X-ray examination of this product has been described in the previously mentioned publication. In certain cases, after the final analyses, the solids were taken out of solution, pressed between filter papers to remove excess liquid, and air-dried to constant weight on porous tiles. The following results were obtained:
TABLE IX Type
A B C
D
No. of flask
Logs on ignition (%)
25
14.41
27
11.I 7
28
29
11.46 12.30
31
IO.2 5
33
9.78
72
9.24
Loss on ignition (%) of original tungstic acid
A. 1 4 . 4 3
B. 14.69
c.
9.92
D. 8.43
Discussion A study of the pH values of the various solutions, obtained a few minutes after commencing each series, showed that neutralisation of the alkali had taken place, probably producing Na2WOr. Thus, the experiments described really record the solubility of the tungstic acids in sodium tungstate solution. With anhydrous tungsten trioxide, the sodium hydroxide was only neutralised slowly, except in the most dilute solution, when immediate “neutralisation”
TUNGSTIC ACIDS AND SODIUM HYDROXIDE
1669
occurred, this suggesting that adsorption was the original cause of the removal of alkali from the solutions. The soluble sodium tungstate produced would then slowly attack the tungstic acid present, forming salts with increasingly large tungsten content. The peptisation of tungstic acid by alkali may be conveniently explained by the theory of Zsigmondy, which was worked out in detail in connection with the stannic acids. If sodium hydroxide is added to tungstic acid, sodium tungstate will be produced, and if it is assumed that the tungstate ion is strongly adsorbed by the tungstic acid particles, the latter will become negatively charged. If sufficient charging is effected by this process, dispersion of the particles will result, and a sol will be produced. Alternatively, one may assume that tungstate is formed on the surface of the tungstic acid particles. Dissociation of the product so formed may occur, the sodium ions diffusing into the liquid, leaving the particles negatively charged. The final result of such processes is to produce charged micelles containing water, some form of tungstic acid or oxide, and probably sodium, together, of course, with free sodium ions. A study of the colloid contents of the various series shows that with a given tungstic acid, and increasing sodium hydroxide concentration, the colloid content frequently increases from a small value to a maximum, and then decreases. This is particularly well shown in the colloid contents for Types B and D. The effect would be anticipiated, on the following grounds: very small quantities of alkali will produce small quantities of tungstate, and the adsorption of the tungstate ion will charge the particles, but not sufficiently to cause peptisation. Medium quantities of alkali will produce sufficient salt to finally bring about dispersion of the tungstic acid, whilst higher alkali concentrations will finally lead to a coagulation effect on the tungstic acid. It must also be noted that at and above a certain concentration, disintegration of the solid phase occurs, producing molecular tungstate and metatungstate solutions. The maximum concentration of sodium hydroxide which produced immediate peptisation with the various products was as follows: WOa, 0.001N ; tungstic acid A, 0,035 N ; B, 0.020 N ; C, 0.03 j N ; with D, colloidal matter was present in all solutions tested, i.e. up to 0.06 N. On general grounds, it would be expected that little peptisation would occur with ignited tungsten trioxide, owing to the complex structure and dense nature of the particles. Also, the crystals of tungstic acid A, and the comparatively large glassy particles of tungstic acid B are not suitable products for extensive sol formation. Tungstic acids C and D being of a more finely divided nature and possessing greater adsorptive power than the previously mentioned products (shown e.g. by their ammonia contents), are far more likely to peptise when treated with alkali. These general deductions are in harmony with the observations given in the various Tables under “Appearance of Solution.” During prolonged experiments of the type described in this paper, there are many variable factors, so that the prediction of the possible behaviour in any particular case becomes very difficult. The following possible sources of variation may be considered :-
1670
ARTHUR M. MORLEY
(a) Change in the dispersion medium may occur owing to the early formed sodium tungstate passing through various intermediate stages to metatungstate, and possibly still higher polytungstates (v. WO3/NaZ0ratios. Table V, 2 8 and 29.). This will probably cause varying adsorption of the ion containing tungsten, by the solid phase. This tungstate-metatungstate change is shown by the decrease in the pH values of the solution, and the condition of the final ultrafiltrates is approximately indicated by the WOS/NazO ratios. Solutions of tungstic acid C were exceptional, in that the pH values remained relatively high. This is accounted for by the fact that the solubility of C was rather low, and therefore only incomplete formation of metatungstate would take place. A study of the solubility “curves” shows that speaking generally, a condition of equilibrium was approached with tungstic acids A and C, and with tungsten trioxide. Abnormalities were observed with product B whilst D had not sufficient time to attain equilibrium. Since A and D are essentially definite compounds, it would be anticipated that the products would easily dissolve in sodium hydroxide, and would finally attain an equilibrium. The much lower solubility of product C is one of the facts which most strongly suggests that it might contain a condensed acid, as previously suggested.1 Such a product would not readily be attacked by reagents, and one would expect its solubility in sodium hydroxide to be less than the solubilities of H2WO4, HzOor HzWO4. (b) Change in the solid phase. Possible ageing effects are ( I ) a general coarsening of amorphous particles to larger amorphous particles or ( 2 ) crystallisation, either partial or complete. The evidence (observations and Table IX) shows that such changes were slight with tungstic acids A and C, and not of large extent with D, but were much more pronounced with B. This greyish white product frequently changed to a yellow substance. The author has shown by X-ray analysis2 that such changes are due to crystallisation into HzW04 or HZW04, HzO or into a mixture of these products. Also, it was shown by centrifuging, that a tungstic acid B from a solubility experiment contained grey, yellow, and white particles. Since in a solubility series with products of this type, each solid will have assumed a different composition after a short time, and therefore any equilibrium which may have been set up between solid and solution will alter, a smooth solubility curve, and accurately reproducible results would not be expected. Such considerations will account for the irregular results shown in Fig. 3. The large decrease in the tungsten content of solution 17, after 687 days (v. Table IV) may be due to the decomposition of unstable higher tungstates, brought about by the above mentioned causes. Solutions 2 0 and 30 were exceptional in that white hexagonal crystalline plates were deposited. This crystallisation accounts for the fact that in the IQC.cit. 2
Loc. cit.
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final ultrafiltrates for 2 0 and 30, the tungsten contents are less than in 19and 29, respectively, (Tables IV and V) whilst the pH value of 30 is much higher than for 2 8 and 29. (Table V). (c) Change in the particles of the disperse phase. The remarks made under (b) apply in general to the particles of sols; ageing usually produces larger particles with less stability, and which therefore frequently coagulate. (d) Hydrolysis in the solutions may produce colloidal tungstic acid. If the particles produced were small and heavily hydrated, clear sols may be produced. This was the case in the following experiments: Type B, (Table IV, 18, 19,20;) Type C, (Table VI, 21, 2 2 , 23, 24). In all these cases, the colloid content of the solution first increased, and then decreased with time; in some cases the value decreased to zero. This effect is probably due to an increase in the size of the particles, accompanied by a change in the degree of hydration, followed by coagulation of the larger particles. In some cases, presumably the whole of the colloidal tungstic acid was removed in this way. In general, the changes in the amount of tungstic acid present in the colloidal state will be the resultant, of the following effects: (a) dispersion of particles of the solid phase, (b) coagulation of aged particles in the sol. summary
Tungsten trioxide and four standard tungstic acids whose structures had been previously determined, have been subjected to the prolonged action of solutions of sodium hydroxide of varying concentration. At stated intervals, the amount of tungsten in the solutions was determined before and after ultrafiltration through collodion membranes. The pH values of certain solutions are recorded. Changes in the solid phase have also been studied. The results are discussed. I n conclusion, the author wishes to express his sincere thanks to Dr. J. K. Wood, F.I.C., for the keen interest displayed in the progress of the investigation, and for the helpful suggestions given during the consultations held in connection with the research. Municipal College of Technology, University of Manchester, June 15, 1981.
