T H E ROLE O F PHOSPHATES ON T H E TAKING UP OF DYES BY MORDANTS BY CHARLES E. WHITE AND NEIL E. GORDON
Many attempts have been made to advance a satisfactory theory to explain the effectof salts upon the dyeing process, but none of these have proved entirely satisfactory. One reason for this seems to be the lack of quantitative data based on investigations in which sufficient attention has been given to a control of the variables. The object of this investigation was to obtain quantitative data under such carefully controlled conditions that it might be possible to determine definitely the effect of phosphates on the taking up of dyes by mordants. Materials used The hydrous oxide gels of iron, aluminum and chromilun were used throughout the experiment. These were prepared by dissolving seventy-five grams of the respective chlorides in five liters of water a t a temperature of fifty degrees centigrade and then very dilute ammonium hydroxide was added until all of the metal had precipitated. The gel was washed free from chlorides by decantation. This usually required six or seven days. I n most of the experiments the gel was used mixed with such a quantity of water that it could be readily drained free from a fifty cubic centimeter pipette from which the tip had been cut off in order to allow passage of the large particles of gelatinous material. Where it was objectionable to have such a large quantity of water present the gel was filtered by suction until all the water possible had been removed. The gel in the resulting condition was weighed up on the balance in n stoppered weighing bottle. The silica gel was famished by' the Davison Chemical Company. Before using, it was ground small enough to pass a one hundred mesh sieve, washed by decantation and allowed to dry a t room temperature. The water content of the gel was determined by heating a sample for twenty-four hours a t 105' C. The iron, aluminum and chromum gels that were in a condition to be measured in a pipette contained about 98 per cent water. The dyes were donated by E. I. du Pont de Nemours and Company. The acid dye, Orange 11,is a sodium salt and the basic dye Crystal Violet is a chloride, potassium di-hydrogen phosphate, the salt, was J. T. Baker's C. P. material. Experimental Procedure In the adsorption of dyes by gels Gordon and Marker' have shown that equilibrium is reached if the dye and the gel are heated to 100' C for two hours with frequent shaking. Gordon and Starkeyl have shown that in order to 1 2
Ind. Eng. Chem., 16, 1186 (1925). Soil Science, 14, p. 449 ( 1 9 2 2 ) .
THE ROLE OF PHOSPHATES WITH MORDANTS
381
obtain maximum adsorption of potassium di-hydrogen phosphate by inorganic gels the mixture must be shaken almost continuously for seventy-two hours. Thus wyhen i t was desired to have comparable results with both the dye and phosphate, the solutions were put into 2 5 0 cc. ground-glass stoppered bottles, heated a t 100‘ for two hours, then shaken continuously for three days in a mechanical shaker. After this the solution was analyzed for materials unadsorbed. The solution was separated from the gel by an ultra filter. This was made by pouring the mixture of gel and solution over a filter paper until the paper had become thickly coated with gel. The whole solution was then filtered through this.
Determination of Adsorbed Materials The adsorption was determined by analyzing the solution before mixing with the gel and then again after adsorption had taken place. The difference between these determinations was called the amount of material absorbed. The dye was determned by titration with titanium tri-chloride according to the general directions in Knecht and Hibbert’s “Manual of Volumetric Dye -1nalysis.” Specific directions were furnished by E. I. du Pont de Nemours and Company. Their directions are as follows: “DuPont Orange 11: Dissolve .2 gms. dye in IOO C.C. water and 35 c.c of sodium tartrate ( ~ 5 7 ~ ) Boil . gently three minutes and titrate hot under COS with .o5N TiC13solution. The final color change is from red to gray or brown and is sharp. The titration should be carried out slowly as the end point is approached.” I n determining potassium, and the phosphate radical in Orange I1 solutions, it was necessary first to remove the dye by precipitation with HC1 or to decompose i t by boiling with aqua-regia. Both methods were used and the latter found to be the most satisfactory. The potassium was determined by precipitation with platinic chloride according to the Lindo-Gladden method. ’I he phosphate was determined by the ammonium molybdate method. I n the case of Crystal Violet the dye was removed by precipitation with sodium hydroxide. The hydrogen ion concentration of all clear solutions was determined by means of the Bailey Electrode. h normal calomel electrode was used with this and the readings taken on a Leeds and Northrup “Student Type” potentiometer. I n order to obtain the pH of the dye solutions and of solutions containing any colloidal material a modified form of the Hildebrand type electrode was used. The electrode was a single platinum wire coated with platinum black. Eefore using this it was saturated with hydrogen by electrolysis in a sodium hydroxide solution. It was found that this treatment insured much steadier readings in dye solutions. I n making a determination the solution was saturated with hydrogen before the electrode was immersed in it. For Crystal 1-iolet solutions and clear solutions the readings of this electrode could be checked with those of the Bailey electrode. But with Orange I1 solutions this is not true. K i t h Orange I1 all readings were determined in duplicate on the
CHARLES E. WHITE AND NEIL E. GORDON
382
electrode described above as soon as a steady reading could be obtained after the electrode was immersed in the solution. While it is not claimed that these values are absolute, they are the best it is possible ta obtain under these conditions and they allow excellent comparison for this work. The Adsorption of Crystal Violet by Silica, Iron and Aluminum Gels in the Presence of Phosphates Tables I-VI show a comparison of the Crystal Violet dye adsorbed with KH2P04present and when no salt was used. In these experiments the p H was varied by using sulphuric acid and sodium hydroxide. The solutions used were .osper cent dye and 0.5 N KH2P04.
TABLE I The Effect of Phosphates on the Adsorption of Crystal Violet (a basic dye) by Silica Gel pH of salt solution
pH of bath after ads.
1.1
I .os
1.1
1.1
I .42
1.52
2.3
2.74
2.75 3.14 3.16 4.43 4.70
2.4 3.1 4.I 4.2 4.6 4.7
4.62 4.67 6.68 6.98
Mgm. of dye ads. per gm. dry gel no salt pres. with KH2P0, present
2 . 5
3.2
3.5 7.3 9.0
TABLE I1 The Effect of Phosphates on the Adsorption of Crystal Violet (a basic dye) by Aluminum Gel pH of 8alt solution
pH of bath after ads.
KH2P04present
1.52
2.91 5.36 5.98 6.12 6.30 6.54
49.2 173.4 216.5 439.3 449.1
2.81 4.63 6.12 6.49 6.85
Mgm. dye ads. per gm. dry gel without salt
-
3.0
4.0 7.0 13.0
16.0 18.0
TABLE 111 The Effect of Phosphates on the Adsorption of Crystal Violet (a basic dye) by Iron Gel pH of salt solution 2.59
4.45 6.12 6.49 6.85
pH of bath after ads.
5.27 6.04 6.14 6.42 6.81
Mgm. dye ads. per gm.dry gel KHIPO present without salt,
3i8 460.9 886.8 1041.4 1118.3
28.0 40.0 46.2 47.5 48.0
T H E ROLE O F PHOSPHATES WITH MORDANTS
383
It is seen from the above data that the effect of the phosphate salt added is to increase the adsorption of the basic dye. The pH could not be carried any higher due to the dye pfecipitating out of solution. Iron gel seems to be the best adsorbent for the basic dye and as will be noted in the following tables it is also the best adsorbent for phosphates. Both the potassium and the phosphate adsorbed in the above experiment were determined. The data are for all three gels. TABLE IV The Effect of Crystal Violet Dye on the Adsorption of Potassium Acid Phosphate by Silica Gel pH of bath after ads.
PO, ads. dye present
I .os 1.52
75 3.14 3.16 4.43 2
'
-0.8 -0.6
Mgm. adsorbed per gram of dry gel. PO, ads. K ads. dye no dye present
I< ads. no dye
2.7
-0.2
-2.7
0.5
-0.2
-2.4 -1.6 -1.4
-0.5
-0.3
-0.16
-0. I
-0. I
-0.13
0.I
-2.0
0.01
-1.5
0.2
-2.3
0.5
-0.2
TABLE V The Effect of Crystal Violet Dye on the Adsorption of Potassium Acid Phosphate by Aluminum Gel pH of bath after ads.
PO4 ads. dye present
2.91 5.36 5.98 6.12 6.30 6.54
Mgm. adsorbed per gram of dry gel. PO, ads. K ads. dye no dye present
127.8 124.9
318.5 249.0
4.4
11o.c)
233.0
101.2
227.5
104.3 98.8
223.3
11.4 9.4 9.5 9.9
218.0
7.1
K ads. no dye
4.7 9.4 10.6 10.8 11.1 11.5
TABLE VI The Effect of Crystal Violot Dye on the Adsorption of Potassium Acid Phosphate by Iron Gel pH of bath after ads.
PO, ads. dye present
5.27
272.5
6.04 6,14 6.42 6.81
254.2 258.6 247.3 255.6
Mgm. adsorbed per gram of dry gel PO, ads. K ads. dye no dye present
__ ~.
__ 339,o 276.0
'7.9 26.I 28.7 30.2
28.I
K ads. no dye
91 . o 90.0 82.0 76.0
CHARLES E. WHITE AND NEIL E. GORDON
384
In every case it is shown that even though the dye is very much more highly adsorbed with the phosphate present, the PO4 adsorption is not great enough to account for the high adsorption of the dye. I t is believed that the salt increases the size of the dye particle to colloidal dimensions and then it is merely filtered out of solution by the gelatinous particles of metallic oxides. It will be noted in subsequent data that the action of phosphates with acid dyes is directly opposite to that with basic dyes. Adsorption of Orange I1 (an acid dye) by Silica, Iron,Aluminum and Chromium Gels With silica gel there was no adsorption of the acid dye Orange I1 either with or without a salt present. Table VI1 shows that with aluminum gel the adsorption of Orange I1 was greatly decreased by the presence of the phosphate. In this experiment the pH was varied with sulphuric acid and sodium hydroxide. The solution used was 0.5 per cent dye and 0.05 N KHzPOa. TABLEVI1 The Effect of Potassium Acid Phosphate on the Adsorption of Orange I1 by Aluminum Gel pH of original salt solution
2.68 3.18 3.38 4.48 6.49 7.02
Final pH of bath
RIgm. dye ads. per gm. dry gel. Phosphate present no salt present
38 26
5.05 5.51 5.43 5.73 6.02 6.56
'5
279 I 86 185 I77
I4
170
I
166
27
This data are illustrated more vividly in Fig. I . It is remarkable that the small amount of salt added to the dye bath should make such a marked decrease in the adsorptive power of the gel. I t might be expected that the gel has preferentially adsorbed the phosphate, but the following data show that there is also a great decrease in the phosphate adsorption. TABLEVI11 The Adsorption of Potassium Acid Phosphate by Aluminum Gel in the Presence of Orange I1 Dye pH of bath after ads.
5.05
5.51 5.43 5.73 6.02 6.56
PO, ads. dve present
143.0 117.6 105.8 96.7 75.7 46.3
Mgm. adsorbed per gm. of dry gel PO, ads. K ads. dye no dye present
258.5
00
244.0 247 ' 5 238.0 231.5
00
217.5
00
4.67 6.12 __
K ads. no dYe
9.0 9.8 9.7 10.2
10.6
_-
385
THE ROLE O F PHOSPHATES WITH MORDASTS
A comparison of the phosphate adsorptions is given in Fig. 2 . The great decrease in the adsorption of the phosphate cannot be balanced by the very slight adsorption of the dye. The same phenomenal decrease in adsorption is noted in the following data on iron gel.
FIG. I
.idsorption of Orange I1 by Aluminum Gel
TABLE IX The Effect of Potassium Acid Phosphate on the Adsorption of Orange I1 by Iron Gel pH of original salt solution
pH of bath after ads.
Ngm. dye ads. per gm. dry gel phosphate present S o salt present
6.09 6.25 6.42 6.60 6 79
2.75
3.16 4.66 6.54 7.05
00
225
00
83
00
70
00
63 62
00
The phosphate and potassium adsorption was also decreased below normal even though no dye was adsorbed.
TABLE S The Adsorption of Potassium Acid Phosphate by Iron Gel in the Presence of Orange I1 Dye pH of bath after ads.
6.09 6.25 6.42 6.60 6.79
hlgm. ads. per gram of dry gel.
PO, ads. dye present
PO, ads. no dye -
Ii ads. dye
dye
91 .o 88.0 83.0 80.0 74.0
200. j
-
21.4 24.3
188.6 160.3 130.6
3 40
25.1
330 3'9
23.6 19. I
211. I
Ii ads. no
present
CHARLES E. WHITE AND NEIL E. GORDON
3 86
In order to determine whether this decrease in adsorption was due to the PO, ion alone or to other ions present, an experiment was run where phosphoric acid was added to the dye solution. Varyiug amounts of the acid were used in order to vary the pH. Of coume there might be expected a change in adsorption due to the varying concentration of the negative radical. I n the following experiment one gram of Orange I1 dye was dissolved in 2 0 0 cc. of phosphoric acid solutions and added t o I . 0 2 grams of gel.
FIQ.2 Adsorption of Phosphate by Aluminum Gel
TABLEX I The Effect of Phosphoric Acid on the Adsorption of Orange I1 by Iron Gel Final pH of bath
-
PO, originally present
Mgm. per gram of dry gel. PO4 ads.
Dye ads.
4.30
263 5
233.5
00
4.85 5.16 5.29 5.36 5.44
217.8
202.9 153.9 97.9 86.6 58.7
00
'
164.0 102.9 92.3 61.9
00
00 00 00
Table XI shows conclusively that the elimination of the adsorption of the dye is due to the phosphate and not to the presence of other ions. Having established the fact that the phosphate was the effective agent in decreasing the adsorption of the dye it became our problem to determine if possible just why the phosphate should do this. Gordon and Marker1 have shown that the taking up of Orange I1 by aluminum and iron gels is a chemical reaction and not merely a ease of surface adsorption. According to this then the observed facts should offer explanation either on the basis that the phosphate is acting on the gel and preventing its reaction with the dye, or it is acting upon the dye in solution in such a manner that it prevents the reaction taking place.
* Ind. Eng. Chem.,
16, 1186(1925).
THE ROLE OF PHOSPHATES WITH MORDANTS
387
Taking the latter idea first, it waa thought possible that the phosphate might have an effect of increasing the solubility of the dye in solution and thus making its taking up by the gel more difficult. So an experiment was run to determine this. It was found that a t the concentrations of phosphates used in the experiment there was no detectable iafluence upon the solubility. But if a large quantity of phosphate is added, the solubility of the dye decreases and the dye may be precipitated out of solution. This would have the effect of increasing the adsorption by gels rather than decreasing it. Another thought was, that owing to a change in the thermodynamic environment of the solution on the addition of phosphate, it might take a longer time for the dye and gel t o reach equilibrium. So an experiment was run in which the dye was allowed t o remain in contact with the gel for a period of six months. Samples were removed at intervals for analysis. Solutions of 0.j per cent Orange I1 dye and .os normal potassium dihydrogen phosphate were mixed with 1.08 grams of gel. This mixture was heated to boiling for four hours and placed in a mechanical shaker and agitated. This treatemnt was repeated at frequent intervals over a period of six months. The pH was varied with phosphoric acid and potassium hydroxide. TABLEXI1 The Result of allowing Orange I1 and Phosphate Solution to act on Iron Gel for a Deriod of six months pH of dye bath Jan. I
pH of dye bath July 3
5.02
5.46 5.69 5.87 5.95 6.07 6.07
5.27
5.66 5.86 5.95 5.71
Dye ads. Jan. I 00
00
Mgm ads. per am.dry gel. Dye ads. PO, ads. July 3 Jan. I 232.3 00 00 229.4
00
00
202.7
00
00
00
00
00
00
191.0 187.1 146.3
PO, ads. JdY 3 219.6 238.7 206.8 207.3
171.7 164.0
The data show that there was no adsorption of dye under these conditions even after being in the presence of the gel for a period of six months. The phosphate adsorption changed only very slightly and the pH shows a correspondingly slight change. I t was concluded then that the reason for phosphate cutting out the adsorption was not t o be found in its effect upon the dye in the solution. On the basis of the other idea mentioned above that the phosphate decremed the adsorption of acid dyes because of its effect upon the gel, it was thought possible that the phosphate might be adsorbed on the surface of the gel and thus prevent the gel reacting with the dye. In order to determine this it waa decided to run an experiment in which the gel would be allowed to adsorb the phosphate from the solution before adding the dye. If no adsorptidn took place it was thought it might be concluded that the phosphate coated the gel in such a way as to prevent its reacting with the dye.
CHARLES E. WHITE AND NEIL E. GORDOK
388
.5 gram of gel and IOO cc. of potassium dihydrogen phosphate was boiled for two hours, then shaken for three days in a mechanical shaker. 2 s cc. of the solution was then removed and tested for phosphates. Then 100 cc. of a one per cent dye solution were added and the mixture again boiled and shaken the same as before. The following results were obtained:
FIG. 3 The Relation between the Amount of PO, adsorbed and the Amount present in a Bath of Orange I1 Dye.
TABLE XI11 The Effect of Iron Gel adsorbing all the Phosphate from the Solutionfirst, then adding Orange I1 Final pH Orig. P0,mgm PO, in superDye Mgm PO, ads. of bath
per gm dry gel
5.53 5.73 5.75
63.7 127.4 190. I
5.86
254.8
5.27
63.9
natant liquid before dye added
none trace quantity quantity none
ads
rgm. gern fhal bath
00
59.4
00
105.4
00
126.8 154.7 59.7
00 00
I t will be noted that there was no dye adsorbed even when there was no phosphate in the supernatant liquid when the dye was added. The data show, though, that some of the adsorbed phosphate was released to the solution after the dye was added. This might have been expected, since in Table XI it was shown that in the presence of Orange I1 the amount of phoaphate adsorbed is proportional to the concentration of the solution. Hence when the solution was diluted with the dye we would expect some of the phosphate to be released. This point is illustrated in Fig. 3.
THE ROLE O F PHOSPHATES WITH MORDAS'TS
389
Since some phosphate was released to the solution this experiment does not prove conclusively its original purpose. I t does show, however, that the dye will not replace the adsorbed phosphate on the gel. I t then became important to know whether, if the dye was adsorbed first, the phosphate would displace it. In order to determine this the following experiment was performed. 150 cc. of a 0.75 per cent solution of Orange I1 was boiled with the gel for two hours and samples were removed for analysis. IOO cc. of 0.05 ? KHtPO, I were then added to each solution. This was shaken for three days to attain equilibrium. The pH of the original dye solutions was varied by adding sulphuric acid and sodium hydroxide. The results of this experiment are given in Tables XIV and XY.
TABLE XIV The Effect of allowing Aluminum Gel to adsorb Orange I1 first then adding Potassium Acid Phosphate to the Solution pH after dve ads.
5.2
5.3 5.56
pH of final bath
4.8 4.9 5 . 2
dye ads. Lfore salt added
439.4 311. I 67.0
Mgm. adsorbed per gm. dry gel dye ads. dye after released salt added 119.5 373.9
24.3 15.0
286.8 52 .o
PO, ads. 150.7
119.1 86.9
TABLE XV The Effect of allowing Iron Gel to adsorb Orange I1 first then adding Potassium Acid Phodphate to the Solution pH after dye ads.
pH of firial bath
dye ads. before salt added
5.05
4.56 5.42 5.69 5.65
407.6 ,245.7 9.7 9.7
5.11
5.48 5 ' 49
Mgm adsorbed per gm.dry gel. dye ads. dye PO, ads after released salt added
24.4 21.6 2 .o 2
.o
383.2 224.7 7.7 7.7
166.0 141 .o 114.4 120.0
The data show that a very high percentage of the dye is displaced by the phosphate. At first thought it would seem that if this were chemical replacement one equivalent of dye should be replaced by one equivalent of phosphate. These values were calculated and not found to agree in any case. This id as it should be for undoubtedly some of the dye was released on dilution of the solution. Or in other words some of the dye compound already formed is dissolved on the addition of the phosphate solution. The data presented thus far seem to show that the effectiveness of the phosphate in reducing the adsorption of the dye is due to its effect upon the gel. The thought was then advanced that the phmphate covered the exterior
CHARLES E. WHITE AND NEIL E. GORDON
3 90
surface with an insoluble phosphate and did not leave any ferric hydroxide exposed to react with the acid of the dye. It is well known that iron and aluminum gels are precipitated in very large particles, and just rn the reaction of sulphuric acid on lead is stopped by lead sulfate, here the reaction may be prevented by the formation of iron phosphate on the surface of the gel. If this concept is true we should be able to increase the adsorption of a salt by taking these larger particles of gel and breaking them apart so as to expose new surfaces. I n order to prove this point samples of iron and aluminum gels were allowed to reach a maximum adsorption with potassium acid phosphate. The gel was then removed and ground thoroughly in a mortar for ten minutes; it was then returned to the solution and placed in the mechanical shaker again. Samples were removed, before and after grinding the gel, for analysis. The pH of the solutions was varied by adding potassium hydroxide. 1.05 grams of gel (90 percent Fe203and 74. j percent A12O3 respectively) were used with 2 0 0 cc. of 0.05 N KHZPOI.
TABLEXVI The Effect of Further Subdivision of the Particle of Aluminum Gel pH of original Salt solutions
4.683 5.799 6.287 7.084 10.718
pH of bath before grinding
7.168 7.328 7,532 8.081 9.417
pH after arjndina -
7.405 7.422 7.641 8.182 9.552
Mgm. PO, ads. per gm dry gel Before After grinding grinding Increase
138.4 135.6 127.6 94.4 41. I
I47 ' 7 143.3 132.5 97 ' 1 46.4
9.3 7.6 4.9 2.7 5.4
TABLE XVII The Effect of Further Subdivision of the Particle of Iron Gel pH of original salt solutions
4.683 5.799 6.289 7.084 10.718
pH of bath before grinding
7.151 7.303 7,557 8.098 8.706
pH after grinding
7.354 7.439 7.624 8.149 9.028
Mgm. PO4 ads. per gm dry gel Before After grinding grinding Increase
166.2 163.4 154.2 121.2
58.7
171.5 166. I 160,7 126.0 68.8
5.3 2.7
5.5 4.8 10.1
The difference in pH between the first and second column does not give the pH change corresponding to the phosphate adsorbed because quite a bit of water was added with the gel. I n a total of 48.3 grams of gel plus water there was 47.3 grams of water. With the iron gel in the above experiment even after filtering there was considerable colloidal ferric phosphate in the solution. For the phosphate determination this was removed by saturating the solution with ammonium nitrate which precipitated the colloid. On filtering, the solution was perfectly clear.
THE ROLE OF PHOSPHATES WITH WORDANTS
391
The data given in the two preceding tables are presented in Fig. 4. From these curves it is seen readilythat there wm quite a definite increase in the adsorption of phosphates by both iron and aluminum gels when new surfaces were exposed by crushing the gel particle. This seems to show that the surface of the gel, when maximum adsorption is reached, is coated with a definite film of phosphate which is impervious to more phosphate or dye under the existing conditions. But when one of these phosphate-coated particles is broken and a new surface exposed, more of the phosphate can be taken up
FIG.4 The Effect of Further Subdivision of the Particles of Gel on .Idsorption of Phosphates
from the solution. This might appear to be then the explanation of the fact that phosphates cut out the adsorption of an acid dye by iron gel. If the layer of phosphate formed cannot be penetrated by additional phosphate from phosphoric acid, then it is reasonable to suppose that the dye cannot penetrate the film. But in experiments following this where less phosphate was used and was still effective in preventing the adsorption of the dye, it is hardly conceivable that there is a sufficient amount of the phosphate radical present to cover the whole surface of the gel. These data, however, certainly seem to explain the always troubling assertion that there is apparently never a definite chemical equivalent relationship between the amount of material adsorbed and the amount of gel present. All of the preceding data were recalculated on the basis that the dry gel was 74. j pe . cent .kl2O3and y o per cent Fe203respectively and no definite chemical combining ratios were found between the dye or salt adsorbzd and the iron or aluminum present as gels. None should be expected according to the data presented in Tables 95’and S V I because at all times there might be quite a large score of aluminum hydroxide or iron hydroxide within the gel particle that never comes in contact with the solution and hence cannot be acted upon. I n all of the data presented, the adsorption of the dye in phosphoric acid has been compared to the adsorption in sulphuric acid. I n order to have a comparison of the effects of the ions C1-, SO4- - and PO4- - - upon the ad-
CHARLES E. WHITE A N D S E I L E. GORDON
392
sorption of Orange 11, an experiment was run using hydrochloric, sulphuric and phosphoric acids of the same pH values to dissolve the dye, and iron, aluminum and chromium gels as adsorbing mediums. The same amounts of gel and the same precentage dye soltuion (0.5 per cent) were used in each case. The results are given in grams of dye adsorbed per gram of dry gel. The gel in this state in the case of iron is 90 per cent Fe203,that of aluminum 74.5 per cent A1203 and chromium 70.4 per c2nt Cr203.
500
300
100
rnqm. DYE AD50R0€D E FIG.
700
R GRAM DRY
G€L
5
Sdsorption of Orange 11 by Iron (:e1 in the presence of HCl, H,S04 and H3POI.
TABLE: XVIII The Adsorption of Orange I1 by Iron Gel in the Presence of three different acids under the same pH pH of acid sol. before dye added
HCI
Xlgm. of dye adsorbed per gm of dry gel H2SOI
Hap01
2.95 2.66
8
00
28
00
3.33
I43
00
2.01
440
30
1.94
540
I02
It is shown here that in their adsorption effects with iron gel, HCl and H2SOa are very much alike and H 3 P 0 4has cut out the adsorption entirely except at a very low pH. Fig. 5 shows these data plotted. The results with chromium gel are very similar; these are shown in the following table and in Fig. 6. It is apparent that hydrochloric and sulphuric acids have almost the same effect again except at a very low pH, and phosphoric acid has very greatly reduced the adsorption. The adsorption in the presence of phosphoric acid passes through a minimum. This result is explained in the theory of this reaction which is developed in the latter part of this paper. With aluminum gel the results are similar.
T H E ROLE O F PHOSPHATES WITH MORDANTS
3 93
TABLE XIX The Adsorption of Orange I1 by Chromium Gel in the Presence of three different acids under the same DH HC1
Mgm. of dye ads. per gm. of dry gel. HtYOI
24 41
24 35
I55
150
00
2 .OI
475
1.94
500
3 46 3 90
'5 88
pH of acid sol. before dye added
2.95 2.66 2.33
mpm.DY€ADSOR,Y€D P E R p . DRY
H,POI 5
4
GEL
FIG.6
.Idsorption of Orange I1 by Chromium Gel in the presence of HCI, H?SO,, and H,PO,.
TABLE SS The Adaorption of Orange I1 by Aluminum Gel in the Presence of three different acids under the same aH pH of acida sol. before dye added
2.95 2.66 2.33
HC1
1Igm of dye ads. per gin. of dry gel. H?SO,
90 95
95 96
222
215
HzPOI I5 I
25
543 I 60 620 363 These results are brought out more clearly in Fig. 7 . Here again hydrochloric 2.01
1.94
5 60 650
acid seems to offer the best adsorption medium with sulphuric acid very nearly equal to it and with phosphoric acid there is very little adsorption at all. At a very low p H the amount of dye adsorbed with phosphoric acid present increases very rapidly. Since the control of the pH has been shown to be a very important factor it was carefully followed in all the stages of the above experiment. The results are given separately for each acid.
CHARLES E. WHITE AND NEIL E. GORDON
394
When the gel was added to the dye solutions it was mixed with a considerable quantityof water, (47 g m s . water plus r gram gel). The dilution and the pH of this water had an effect upon the final pH of the bath, but since this was constant in every case it does not enter into the comparison of the effects of the acids. The water with iron gel had a pH of 7 .j, that with aluminum 8.36 and with chromium 8.3 j.
ZOO
400 FIG.7
600
800
Adsorption of Orange I1 by Aluminum Gel in the
TABLE XXI The pH Changes during the Adsorption of Orange I1 in the Presence of HCl pH of original H of solution acid solution ager adding I gm before dye added of dye to 200 cc sol
2.95 2.66 *,33 1.96 1.65
6.4 6.0 X Y
pH of bath after equilibrium was reached A I gel Cr gel Fe gel
6.63 5.85 5.48 5.24 3.82
6.91 6.93 6.27
6.74 6.34 6.12
j.8j
j.IO
x 2.47 __ In the solutions marked x the pH could not be taken due to the dye precipitating out on the electrode. T.4BLE X s I The pH Changes during the Adsorption of Orange I1 in the Presence of Sulphuric Acid. pH of original pH of solution acid solution after adding I gm before dye added of dye to 200 cc sol. 2.98 6.4
2.69 2.38 2.01 1.79
6.3
AI gel 7.12
6.71
X
6.08
X
-
X
3.82
pH of bath after equilibrium was reached Cr gel 7.14
7,I3 6.40 5.58 4.72
Fe gel
6.30 5.85 5 ' 48 4,94
-
THE ROLE O F PHOSPHATES WITH MORDANTS
395
TABLE XXII The pH Changes during the Adsorption of Orange I1 in the Presence of Phosphoric Acid pH of original acid solution before dye added
pH after adding I gm dye to 200 cc solution
3 ' 29
6.66 6.59 4.9
3.01 2.74 2.42 2.18 1.94
X X X
pH of adsorption bath after equilibrium w a reached AI gel Cr gel Fe gel
7.40
7.20
7.13
7.10
7.05 7.46
7.00
5.10
6.10 5.88
3.20
2.57
6.47 6.32 6.28 6.20 4.81 2.08
The increase in pH of the solution on mixing the dye with the acid is due to the formation of the acid dye, It was the formation of this acid dye that made it impossible to determine the pH of the solutions marked x, for when this material is present in large quantities it precipitates on the electrode. It will be noticed in the above dats, that when the final pH of the bath was in general below 6.5 with hydrochloric and sulphiiric acids quite a large amount of dye was adsorbed. This point varies, of course, with the different gels. But with phosphoric acid before any quantity of adsorption could be obtained the final pH of the bath was lowered to 3.2 for aluminum gel, 2.57 for chromium gel and 2.0; for iron gel. X few drops of chromium, aluminum, and ferric chlorides were added to the respective phosphoric acid dye baths which had given little or no adsorption. d precipitate of the metal dye compound immediately resulted. This seemed to indicate that one of the conditions necessary to get the dye removed or adsorbed was to have the metal present in an ionizable compound. Had the ions been present before, adsorption would have resulted. A t almost any pH value when sulphuric or hydrochloric acid is in the presence of aluminum, iron, or chromium gel, metallic ions will be liberated because of the format'ion of a soluble sulfate or chloride. With phosphoric acid reacting with the gel, aluminum, iron or chromiumphosphatewould form, all of which are relatively insoluble except in more concentrated acid solutions. If this theory is true, that the liberation of the metallic ion is necessary in order for it to react with the dye and form the metal-dye compound, then we should find, a t the pH values where adsorption was obtained with phosphoric acid, that iron phosphate is soluble and will yield ferric ions in solution. In order to determine this the following experiment was performed. Ferric phosphate was prepared by treating potassium dihydrogen phosphate with ferric chloride. h white flocculent precipitate was obtained which became curdy on boiling. To equal amounts of this were added varying amounts of .2K phosphoric acid. This was boiled and let stand for a few hours. In some of the tubes it could be seen that some of the ferric phosphate had dissolved, but some white precipitate still remained in all of them. The pH values of the solutions were determined and they were tested for ferric
3 96
CHARLES E. WHITE AND NEIL E. GORDON
ions with potassium sulfo-cyanate. At a pH of 6.88 no ferric ions were present. A t 2.40 there was a faint teat for ferric ions, lower than this a t pH 2.077 and 2 . 0 2 9 the number of ions in solution increased greatly. This can be tested qualitatively by adding KCNS to ferric chloride and then adding Na2HP04. Under these conditions the color will disappear. If phosphoric acid is added, the characteristic red color of ferric .ions will reappear due to the ferric phosphate which was first formed disdolving again. In order to confirm this and have conditions exactly the same as in the dye adsorption experiment, iron gel was mixed with phosphoric acid solutions of various concentrations and subjected t o the same conditions under which the adsorption of the dye was carried out. Results were obtained confirming those given above. With solutions of pH above 4.00 no ferric ions were found in solution. Below this the lower the pH the more ions there were present. Aluminum phosphate was prepared by treating a solution of potassium monohydrogen phosphate with aluminum chloride. This was treated in the same manner as the iron. A total of seven solutions were used. In these the number of aluminum ions present seemed t o pass through a minimum a t pH of 5.6 above this number of ions present increased and below this the lower the pH the greater was the number of ions present. It is believed that this is due to the amphoteric nature of the aluminum. Referring back to Table X X and Table X X I I I we find that the amount of Orange I1 adsorbed passes through a minimum when the final pH of the bath corresponds very closely to this. Chromium phosphate was prepared in exactly the same manner as iron phosphate and a similar experiment tried. At a pH of 6.0 there was a trace of chromium ions in solution but they did not appear in an appreciable amount until a pH of 2.8 was reached, lower than this the amount present increased rapidly. Referring back to Tables X I X and XXIII, it will be noticed that with chromium gel a small amount of dye was adsorbed in the presence of phosphoric acid at a pH of 5.6 but the first adsorption of a large amount of dye was at a pH of 2.6. In every case the adsorption of the dye checks very closely with the amount of metal passing into solution in the form of ions. Here we believe is the explanation of the effect of the phosphate radical on the adsorption of an acid dye. The amount of dye removed depends upon the number of metal-ions there are in solution to react and form the metal-dye compound. Hydrochloric and sulphuric acids react with the gels, form soluble salts and furnish the metal ions to the solution. The more acid there is present the more metal ions there will be in solution, hence we find the greater adsorption of the dye a t the lower pH. With phosphoric acid, however, reacting with the gel relatively insoluble salts are formed and no ions are furnished to the solution until the acidity is of such concentration that it dissolves these salts. As was shown the solubility of these phosphates vary, iron being the most insoluble. The series of reactions through which this process of dyeing the mordant, goes are thought to be as follows: For simplicity let x represent the dye radical.
THE ROLE O F PHOSPHATES WITH MORDASTS
397
Gordon and Marker have shown that the compound formed by the dye with the metals is AlX,, FeX3 and CrX3. Aluminum gel and hydrochloric acid are used as an illustration. NaX
+ HCl*NaCl+
HX
(1)
That this reaction takes place and forms the weaker acid HX is shown by the great increase in pH when Orange I1 is added to an acid solution. Data illustrating this are given in Tables XXI-XXIII. 3HC1
+ .41(OH)3a-hlClr + HzO
(2)
This reaction was proven by taking various mixtures of the acid and gel and testing for the ions formed. hlC13
+ s N a X I - A l X 3+ 3NaCl
(3 1
The salt A1X3 is relatively insoluble and is thus removed from the field of action. This reaction has been illustrated by adding crystals of aluminum, iron and chromium chlorides to a water solution of Orange 11. In every case an immediate precipitate results. It is believed that most of the metal-dye compound is formed by this reaction. A small fraction may be formed by the reaction of the dye acid and the gel but this is highly improbable since the base is extremely weak and the acid is relatively weak. Al(OH) 3+ 3HX*AlX3+
3Hz0
(4)
If this is the essential reaction then phosphoric acid should assist the dyeing just aa effectively as sulphuric or hydrochloric acids, because with phosphoric acid the dye acid, HX, is formed just as in the other two cases as the data in Table XXIII show. If no acid is present some small amount of the metal dye salt may be formed due to the slight ionization of aluminum hydroxide. Al(0H) 3+3NaX$-hlX3+NrtOH
(5)
There is also the possibility that some of the dye acid formed reacts with the .i1C13 formed by reaction ( 2 ) . AlC1~+3HX~AlX~+3HCl However the evidence seems to indicate that the essential reaction for the formation of the metal-dye compound is reaction (3). On the basis of this we can explain the very sudden increases in adsorption of Orange I1 at a low pH which were noted by Gordon and Marker. They found that the adsorption of Orange I1 increased steadily until a certain relatively low pH was reached; at this point further addition of acid caused very rapid increase in the adsorption. We explain this point by claiming that it occurs a t the hydrogen ion concentration where enough acid has been added to shift reaction ( I ) as far as possible to the right. Any more acid added would react directly with the gel and proceed through reactions ( 2 ) and (3).
398
CHARLES E. WHITE AND NEIL E. GORDON
With sulphuric acid the reactions given above would be of the same nature as with hydrochloric acid. With phosphoric acid, reaction ( I ) is the same as with hydrochloric. But in reaction ( 2 ) there would be formed either A1(H2POJ3, A12(HPOS3 or AlPOd. Since there is a large amount of gel present and only a small amount of acid it is believed that the major compound would be AIPOa. This compound was shown to be relatively insoluble and corresponding salts of chromium and iron still more insoluble. The formation of this insoluble product in reaction ( 2 ) would prevent reaction (3) from taking place and hence would prevent adsorption, by this reaction taking place. This is borne out by the experimental data given above. Aluminum phosphate was found to be slightly soluble under all the conditions used and some adsorption of the dye was found in all cases. The pH values at which chromium gel gave adsorption of dye in the presence of phosphate were found to correspond with those a t which chromium phosphate yielded chromium ions to the solution. And finally, it was found that iron gel in the presence of phosphates adsorbed dye a t only a very low pH value. This corresponded to the point a t which iron phosphate yielded ferric ions to the solution. The experimental facts seem to indicate that in order to have an acid dye removed from solution by a gel, in any large quantity, the metal ion must be free. This is accomplished by adding sulphuric or hydrochloric acids which react with the gels to form soluble salts. Phosphates decrease the adsorption because of the formation of difficultly soluble salts with the gels.
Summary of Experimental Results I. The presence of .05 normal potassium dihydrogen phosphate greatly decreased the adsorption of a n acid dye with aluminum gel and iron gel. 11. Using H3P04solution in the dye bath the adsorption of Orange I1 was very much decreased with aluminum gel and cut out entirely with iron gel except at a very low pH. 111. Iron gel was allowed to adsorb all of the phosphate from . o I N K H ~ POc and .o12N H3P04,then Orange I1 dye was added. KO dye was adsorbed. IV. Iron and aluminum gels were allowed to adsorb dye from a .5 per cent sohtion of Orange I1 at p H values varied with KOH and HzSOd. Potassium acid phosphate was then added. Most of the dye was released and the phosphate adsorbed. V. Iron gel was allowed to remain with dye solutions in .ojN potassium acid phosphate for a period of six months. Even at the end of this period no dye waa adaorbed. VI. Iron and aluminum gels were allowed to reach adsorption equilibrium with .o5N KH2P04. The gels were removed, and ground in a mortar then put back into the solutions. There was an increase in adsorption in a11 solutions used. VII. The adsorption of Orange I1 by iron, aluminum, and chromium gels was tried with the following results.
399
THE ROLE O F PHOSPHATES WITH MORDANTS
Xluminum gel: High adsorption of dye with HCl and H2S04. Low adsorption of dye with H3P04. Chromium gel: High adsorption of dye with HC1 and H2S04. Very low adsorption of dye with H3PO4. Iron gel:
High adsorption of dye with HC1 and HzS04. except at a very low pH S o adsorption of dye with value. The order of increasing adso-ptive power for Orange I1 is: iron, chromium, aluminum. VIII. I n cases where phosphate is adsorbed from solutions having varying phosphate concentrations the amount adsorbed in the presence of Oragne I1 dye is a straight line function of the amount present. IX. Chromium, iron, and aluminum as chlorides when mixed with sollutions of the sodium salt of Orange I1 give an immediate precipitate of the respective metal-dye compound. X. Chromium, iron, and aluminum chlorides dropped into the H 3 P 0 4 solution from which no dye was adsorbed gave immediate precipitates of the metal-dye compound. XI. The solubility of aluminum phosphate in phosphate solutions of various hydrogen ion concentrations was found to coincide with the adsorption curve of Orange I1 by aluminum gel in phosphate solutions. Iron phosphate was found to be soluble at the same pH at which the iron gel adsorbed Orange 11. Chromium phosphate was also found to be soluble in phosphate solutions at the same pH where adsorption of Orange I1 was found.
Conclusions The decrease in the adsorption of an acid dye, Orange 11, by iron, aluminum and chromium gels (mordants) in the presence of phosphates is not due to a n y effect of the phosphate adical upon the dye or the solution. It is due to the phosphate forming a relatively insoluble compound with the gels, hence not furnishing any metal ions to react with the dye and form the metaldye compound which is the cause of adsorption. At high hydrogen ion concentration adsorption takes place because the metal phosphat,e becomes soluble. The reactions supposed to take place during the adsorption of an acid dye Orange I1 by an inorganic gel are best expressed in the following equations: Let x represent the dye radical and using iron gel with a hydrochloric acid solution of the dye for an example:NaX 3HCl FeC13 3HX 3NaX
+
+ + + +
HCl Fe(OH)3 3NaX Fe(OH)3 Fe(OHj3
e e -I-
NaCl FeCl3 FeX3 FeX, FeX3
+ + + + +
HX 3H20 3NaCi 3H10 3SaOH
(1) (2)
(3) (4) (5)
400
CHARLES E. WHITE A N D NEIL E. GORDON
It is postulated that most of the adsorption is due to reaction (3). Some little may be due to reactions (4) and ( 5 ) . Sulphuric acid acts very similar to hydrochloric in these reactions. Phosphoric acid acts in reaction ( I ) similar to hydrochloric. I n reaction ( 2 ) phosphoric acid forms an insoluble iron phosphate thus preventing reaction (3) from taking place and no adsorption results unless the solution is made sufficiently acid to dissolve the iron phosphate. Aluminum and chromium gels act similar to iron.
