FLUOSILICIC ACID. I11 : METHOD OF TITRATING AND PROPERTIES BY C. A. JACOBSON
In previous communications1the author has reported upon the preparation of fluosilicic acid having a concentration in water solution of nearly 6 1 % ~which he considers the maximum obtainable a t room temperature. He has also shown by several different types of experiments that this acid does not exist as such in the vapor phase, but decomposes into SiF4and HzFzthe moment i t passes out of liquid water, and is similar to carbonic and sulfurous acids in this respect. It has also been shown that when the acid is thus decomposed the silicon tetrafluoride formed may exist in the presence of water vapor without combining with it to precipitate fluosilicic acid and silicic acid or silicon dioxide; and, whenever hydrofluoric acid reacts with silicon dioxide or glass in the presence of water, the SiF4 first formed immediately reacts with the water to form fluosilicic and silicic acids according to the equation: 3 SiF4+4Hz0= zHeSiF6+H,Si0, From the fact that so many different methods for the quantitative determination of this acid in solution 'have been proposed one may rightly infer that the determination is fraught with many difficulties. Other acids like hydrofluoric and hydrochloric may accompany it except when especially purified, so that when the fluosilicic acid is determined volumetrically the results will be too high. Various methods have been recommended to circumvent, the difficulty but none are entirely satisfactory. Katz2 suggests first to titrate the HzFz and one third of the H2SiFs in 50% alcohol, and then the remaining two thirds of the latter acid in water, using normal alkali, but found that the first stage titration was indefinite on account of the color of the indicator fading out more or less rapidly. Others have tried to precipitate the hydrofluo,ric acid as insoluble fluorides and then titrating the remaining fluosilicic acid, but these methods have also been discarded for the reason that insoluble fluosilicates are formed together with the fluorides. S. Honig3 has taken a long step forward in working out a satisfactory method by suggesting that the acid can be titrated in two stages, but did not follow up his preliminary work until a really satisfactory method was evolved. It is along this line that the author has expended most of his energies, and now believes that he has a method, though not yet perfect, may be considered an improvement over the older methods. The -principles of this method were first suggested to him by the late Dr. B. F. Lovelace of Johns Hopkins University, although the essential details have been worked out independently. J. Phys. Chem. 27, 577, 761 ( r g q ) , Chem. Z. 28, 356, (1904). Chem. Z. 31, 1207.
FLUOSILICIC ACID
507
The method consists in adding a suitable neutral salt which reacts with the acid and precipitates the insoluble fluosilicate, allowing of two independent titrations of the original portion of acid, thus letting one serve as a check upon the other. A volume of the water solution of the acid, not exceeding I O cc, and containing not over 0.1-o.2g.HzSiFfiis cooled in an ice-bath to approximately zero degrees. To this solution is added about a gram of KC1 or NaCl, and it is then titrated with one normal alkali, using methyl orange as indicator. A sharp end-point is obtained. The reactions that take place may be represented as follows: H2SiFfi+2KCl=K2SiFfi+zHC1 2HC1+2KOH = 2KC1+2Hz0 This is the first stage of the titration and gives uniform results provided no other acid is present, in which case the results would be too high. Water is then added to the titrat)ionmixture until the volume amounts to 50-75 cc, but not in excess of the latter volume, and heated to near boiling, (95'). About 5 drops of a phenolphthalein solution are added and the solution titrated to color while near the boiling point. The methyl orange first added does not interfere with the detection of the end-point in the second stage. The reaction taking place in this stage may be represented thus: K2SiF6+4KOH= 6KF+H4Si0, where it is seen that, twice as much alkali is required in the second titration as in the first. One titration therefore serves as a check upon the other, but a small correction must be applied to the second stage, which is negative and amounts to about 0.67~ for an acid titrating 30-soyo by the first stage. While this method for determining H 8 i F 6 seems to yield more consistent results than the others, it is by no means perfect. The two principal imperfections may be given as follows: It is very difficult to maintain the temperature so low and the solution so concentrated that the reaction products of the first stage will not begin to pass oves into the second before the first-stage reactions are completed. The temperature should therefore be kept as near zero as possible and the concentration of acid not less than one or two percent, while the actual amount of acid titrated should be small. The last-mentioned condition will prevent the dilution of the solution with a large volume of the base and cause an appreciable temperature rise due to the heat of reaction. When these conditions are not fulfilled there will be a more or less gradual fading of the indicator due to the products of the reaction passing into the second stage, which is more pronounced the higher the temperature and the more dilute the solution. Hudleston and Bassettl have carried out an elaborate investigation of these fading phenomena, but all their data seem to support the view that the fluosilicate goes over t o the fluoride and silicic acid in the presence of alkali, and that this transformation is accelerated by temperature and dilution. Their fading experiments, therefore, seem to have but, minor significance. J. Chem. SOC. 119.405 (1921)
508
C. A . JSCOBSON
The other imperfection in the method applies to the second-stage titration where slightly higher results are obtained than demanded by theory, and therefore a negative qorrection must be applied to the results in this stage. No satisfactory explanation of this is at hand, but it may be due, at least in part, to t,he adsorption of free acid by the precipitated fluosilicate, which is removed from the ephere of action in the first stage but liberated and titrated when the salt is decomposed. Another, and more plausible explanation may be found in the fact that strong alkalis attack glass and various silicates. The
TABLE J Exp.
Vol. Temp. titrated
I
I O cc. 60 cc.
95O
IO cc. 60 cc.
95O
IO cc. 60 cc.
9s0
IO cc. 60 cc.
05O
2
3
4
5
6
7 8
9
IO
I1
I2
IO0
IO0
IO0
IO0
10 cc.
IO0
60'
95'
IO cc. 60 cc.
95O
IO cc. 60 cc.
95
I O cc. 60 cc.
95
IO cc. 60 cc.
95
IO cc. 60 cc.
95
60 cc. 60 cc. 60 cc. 60 cc.
IO0
IO
IO
IO
IO
IO
~ g Salt .
NaCl
NaCl
NaCl
KC1
KC1
KCI
KC1
KC1
KC1 KCl KCl
95 IO
95
KC1
N NaOH
%H,SiF,
Me-Or Ph-th
9.91 29.95
35.76 36.02
Me-Or Ph-th
7.21
52.03
21.98
52.87
-9.84
Me-Or Ph-th
8 .OI 24.34
57.80 58.34
-0.74
Me-Or Ph-t,h
3.47 10.58
37.56 38.I7
-0.61
Me-Or Ph- th
2.43 7.43
26.29 26.81
-0.52
Me-Or Ph-th
7.50 1 5 . I9
54.15 54.79
-0.64
Me-Or Ph-th
7.56
54.55
15.35
55.38
-0.83
Me-Or Ph-th
7.75 15.68
55.92 56.58
-0.66
Me-Or Ph-th
7.66 15.39
53.29 55.51
--c.22
Ph-th Ph-th
7.55 15.40
54.48 55.56
-I
Ph-th Ph-th
7.70 15.31
55 * 56 55.24
+ .32
Ph-th Ph-th
20.74
33.77 33.26
+.SI
Indicator
cc.
7.02
Correction
-0.26
.08
FLUOSILICIC ACID
509
silicic acid, which is formed in the second stage titration, being slightly soluble in hot water. would undoubtedly react with the alkali to form the soluble ortho or meta silicate. Consequently, the larger the volume of water used in the second-stage titration the greater would be this influence. This assumption has been verified by R long series of experiments. The volume of liquid in the second stage titration was gradually increased from 2 5 cc to 1000 cc, and the results found to increase in the same ratio as the volume, so that when the original IO cc liquid in the first stage was diluted to one liter and titrated in the second stage the results were about 4.5y0higher than by the first stage titration, that is, for a 55% HzSiF6solution. Further consideration will be given to this problem. Table I contains a few actual readings, illustrative of the method. Column I of the table gives the number of the experiment carried out in two stages. Column 2 gives the volume of the acid titrated, 3 the temperatures, 4 the neutral salt added, 5 the indicator, as explained in the note, 6 the number of cc of normal NaOH used for titration, 7 thc percentage of acid found and 8 the correction to be applied to the second-stage titration. Methyl orange is abbreviated to Me-Or and phenolphthalein to Ph-th in column 5 . From the first eight experiments, performed under similar conditions, it is seen that the average correction to be applied to the second stage titration is - . 64YGwhen acids analyzing between 26% and 58% are titrated. The ninth brings out the influence of dilution on the first-stage titration where it is seen that the first-stage reaction has been slightly exceeded, and in the tenth the influence of the indicator. Experiments I I and I 2 show that when the same volume of acid is used in the first and second stage together with phenolphthalein as indicator in both stages, the first-stage titration isslightlyhigher than the second. Experiment number eight was titrated with a 0.6628N KOH instead of a normal solution as the others. It is seen that the more dilute alkali increases the titration value of the acid about 2y0,due in all probability to the solubility and titration value of the resulting silicic acid. Properties of the acid. Fluosilicic acid is a non-volatile acid, and as far as we know, existing only in water solution. It can be concentrated at room temperature to approximately 61%. At this concentration the solution has R distinct odor, clue to the decomposition products of the acid. It has a sharp stinging taste which in dilute solution is suggestive of alum. The specific gravity of a 60.79% solution at 25' was found to he 1.4634, and the index of refraction a t the same concentration and temperature 1.3465. In dilute water solution the acid is quite stable, for it may be kept in glass vessels for a long period of time without etching, and the etching that eventually takes place is found right above the surface of the liquid. Whereas the acid was formerly supplied a t a concentration not much above 20Y0,it can now be purchased a t concentrations well above 50%. Some work has been done to determine the value of other indicators and neutral salts upon the two-stage titration of this acid; but the investigations are not far enough advanced to warrmt a report at this time. Morgantown, W . Vu. September 8, 1923.
