1080
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
VOl. 15,
K O . 10
ness in Fig. 3. The errors for determinations above 100 parts per million have been divided by a factor corresponding to the quantity of water used in the soap test, as was done with the sulfate and calcium results. It is evident that the soap method for hardness is more reliable than the rough turbidimetric methods for sulfate and for calcium. The average percentage or difference between the calculated and determined hardness for the one hundred seventy-four samples is 8 per cent of the calculated hardness.
detail of the soap method as given in Standard Methods of Water Analysis of the American Public Health Association. If the hardness of a sample was over 100 parts per million, less than 50 cc. were taken for the test and the volume made up to 50 cc. with distilled water. The hardness of each water was calculated from the calcium and magnesium determined in the regular analysis. The difference between the determined and calculated hardness for each sample is plotted against the calculated hard-a
T h e Titration of Hydrofluoric and Hydrofluosilicic Acids in Mixtures Containing Small Amounts of Hydrofluosi li ci c Acid’ By Paul H. M.-P.Brinton, Landon A. Sarver, and Arthur E. Stoppel UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MI”.
HE determination of hydrofluoric and hydrofluosilicic acids in mixtures of the two has been investigated by a number of workers,2 and the problem is of considerable importance because it is really encountered in every analysis of the hydrofluoric acid of commerce. A study of the method given by Scottaforms the subject of this paper. In this method the sample of the mixed acids, to which has been added some potassium nitrate, is first titrated ice cold with standard sodium hydroxide, using phenolphthalein as indicator; and then the titrated solution is heated to about 80’ C., and again titrated to a pink end point. The reactions for these two titrations are as follows: For the cold end point: HzFz + 2NaOH -+ 2NaF -I- 2H20 HzSiFef 2KNOs + 2NaOH + KzSiFGf 2NaNOa f 2H20 For the hot end point: KzSiFa f 4NaOH +4NaF f 2 K F f Si(0H)d From the equations the results of the titrations for the two acids are readily calculated. Results of analyses of the same sample made in three different laboratories by this method showed wide divergence. Not only were the h a 1 results a t variance, but there was considerable difference in the ea88 with which the end points, particularly in the cold titrations, could be detected. Investigation of the causes of these differences has led to the uncovering of sources of error which the writers believe are responsible for the dissatisfaction with the method which has been expressed in some quarters. WEIGHINGOF SAMPLE It is customary in many laboratories to transfer the sample from the bottle to a platinum weighing vessel by means of a lead thief or lead pipet. The writers noticed in some of their early experiments that the tip of the lead tube became corroded, and found that if the tip of the pipet were allowed to touch the platinum as the acid was flowing from the tip, a brisk evolution of gas occurred and the lead was rapidly at-
T
1 Presented before the Division of Inorganic and Physical Chemistry a t the 65th Meeting of the American Chemical Society, New Haven, Conn , April 2 to 7, 1923. 2 Guyot, Compt rend., 71, 224 (1870); Katz, Chem. Ztg., 98, 356, 387 (1904); Greef, Ber., 46, 251 (1913); Dinwiddie, Am. J . Sci., 192,421 (1916); Huddleston and Bassett, J . Chem. SOG.(London), 119,403T (1921). 8 “Standard Methods of Chemical Analysis,” 1992, p. 1019, D. Van Nostrand Co.
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tacked as a result of the galvanic couple. If the lead is not allowed to touch the platinum there is no action. With small platinum weighing vessels, even if opened under alkali, there is apt to be loss of hydrogen fluoride vapors when the vessel is first opened. This source of error has been corrected by weighing the sample in a rubber bottle4 which has been nearly filled with cracked ice and then weighed. To overcome the change of weight due to condensation of moisture on the cold bottle during weighing, the bottle is weighed in a stoppered weighing bottle, well lined with asbestos paper, and counterpoised by a weighing bottle of similar external form on the opposite balance pan. After weighing, the bottle is shaken once or twice to absorb any vapors of hydrogen fluoride, and the sample, then in the form of an ice-cold, dilute solution, can be poured and rinsed into the alkali without loss. ENDPOINTAND INDICATOR Methyl orange cannot be used because of the weak nature of the second hydrogen of H2F2. This leads to the formation of acid salts as intermediate reaction products, which have the strength of weak organic acids. Phenolphthalein, however, is satisfactory. ScottSsuggests as the end point for the cold titration a pink that will persist for 15 seconds. It is self-evident that an alkali solution free from carbonate must be used, for even a t this temperature the end point with phenolphthalein when using a solution contaminated with carbonate is not only misplaced, but is also faint and fleeting. If the solution is not icecold for the fist end point, the color will rapidly fade due to hydrolysis of potassium fluosilicate. K a t d has suggested that this hydrolysis proceeds much more rapidly in the presence of calcium salts, owing to disturbance of equilibrium due to formation of insoluble calcium fluoride, and it was thought that perhaps calcium oxide would be inadmissible as a reagent for removing carbonate from the standard alkali; but the amount of calcium oxide which can exist in a sodium hydroxide solution is found to have a negligible effect. EFFECT OF SILICA I N THE STANDARD ALKALI bpparently, the most serious source of error in the use of this method is the silica content of ordinary alkali solutions. In tracing out the cause of the divergence in the analytical results obtained in three laboratories on the same sample, 4 Hard rubber bottles of 0 5-ounce capacity have been found very satisfactory for this purpose. 6 Chem. Ztg., 98, 356 (1904).
INDUSTRIAL A N D ENGIiVEERIiVG CHEMISTRY
October, 1923
it was found that one alkali solution was freshly made, one was several months old, and one was two years old. The carbonate had been recently removed from all the solutions by calcium oxide. The percentages of hydrofluosilicic acid reported by the three laboratories were 1.54, 2.24, and 5.52, in the order of the ages of the sodium hydroxide solutions. A determination of the silica in these standard alkali solutions showed that they contained 0.02,0.07, and 0.32 gram of silica per liter, in the order of their ages. Tests with standard alkali solutions intentionally contaminated with silica, by the addition of varying amounts of water glass, showed that the presence of this silica exerted a tremendous effect in lowering the apparent percentages of hydrofluoric acid and increasing that of hydrofluosilicic acid. Lots of sodium hydroxide made from sodium metal by the action of water vapor, the sodium hydroxide being caught in platinum and stored in bottles lined with ceresin, were used by the analysts in the different laboratories, and the former discrepancies disappeared. The rapidly fading “cold end points” of the earlier analyses were no longer in evidence, and sharp end points in which the pink persisted, not for 15 seconds, but for from 1 to 4 minutes, were easily obtained. Electrometric titration curves obtained in a hydrogen electrode apparatus, all the glass parts of which were heavily paraffined, showed that in the absence of silica in the standard alkali solution the hydrolysis of the potassium fluosilicate in ice-cold solution was sufficiently slow to allow a ready reading of the cold end point, whereas in the presence of appreciable amounts of silica in the alkali, the change in the hydrogen-ion concentration on standing was much more rapid, and the end-point color correspondingly fleeting. In order to determine quantitatively the exact effect of silica on the analyses, a series of standard alkali solutions of varying silica content were made from sodium metal, with subsequent additions of pure sodium silicate which had been obtained by acidifying sodium silicate solution with hydrochloric acid, boiling off carbon dioxide, mixing with pure sodium hydroxide solution, warming, and filtering. The silica content of these standard alkali solutions was accurately determined gravimetrically. T O ADIOUNTS OF STANDARD ALKALI SOLUTIONS
TABLE I-RELATION O F ANALYTICAL RESULTS
Si02
Grams per Liter NaOH 0.036 0.180 0 252 0 396 0 536
Per cent H F Found 51 01 49 78 48 84 47 10 45 89
SILICA I N
Per cent HzSlFe Found 2 36 4.05 4 90 6 58 8.12
The importance of the effect of silica will be understood when it is realized how rapidly silica is taken up from a glass bottle by a sodium hydroxide solution. As an example, a standard alkali solution freshly prepared showed 0.020 gram of silica per liter. After standing less than four months this solution showed 0.092 gram of silica per liter, which would make a percentage error of nearly 40 per cent in the amount of hydrofluosilicic acid reported in that particular sample. It will be seen from Table I1 that the silica in the standard alkali added to the solution up to the cold end point is all converted to hydrofluosilicic acid, and is so registered in the analysis--that is, the silicon tetrafluoride first formed by the action of the hydrofluoric acid on the silica is completely hydrolyzed to fluosilicic acid. h’aturally, any silica in the alkali added after the cold end point has been passed would not count, because there would be no free hydrofluoric acid left to react with it. In Table I1 all samples are calculated to a I-gram basis for ease of comparison. The column c - b in Table 11, when multiplied by 100, gives the percentages of hydrofluosilicic found in the analyses after deducting the amount of hydrofluosilicic acid
1081 TABLE II
(a) Si02 Added
G. 0.00182 0.00883 0.01227 0.01898 0.02542
(b) HaSiFs Equivalent (cl (C-67 to Si02 Added HaSiFs Found Corrected HnSiFi‘ G. G. G. 0.00436 0.0236 0.0192 0.02113 0.0405 0.0194 0.02936 0.0490 0.0196 0.04543 0.0658 0,0204 0.06082 0.0812 0,0204
equivalent to the silica added in the form of impurity in the standard alkali. These percentages run from 1.92 to 2.04, which for this determination may be considered a satisfactory degree of concordance. The gradual increase in the percentage of (corrected) hydrofluosilicic acid found with increasing amounts of silica introduced as impurity cannot be definitely explained a t this time, but it is doubtless connected with the increasing difficulty of detecting the cold end point as the amount of silica increases. It should be noted, however, that with the lower amounts of silica, such as would be found in solutions of reasonable freshness, the deviation is very slight, and it is well within the limit of accuracy of the method.
CONCLUSION In conclusion, if accurate results are desired by this method, it is suggested that a determination of silica be made in even freshly prepared solutions of standard alkali, and that corrections be applied to the figures found for the two acids. One molecule of silica will react with six molecules of hydrofluoric acid to form one molecule of hydrofluosilicic acid. Therefore, the weight of silica added during the titration up to the cold end point should be multiplied by the factor H*SiF&iOZ, or 2.393, to find the weight of hydrofluosilicic acid to be deducted from the weight of hydrofluosilicic acid found; and by the factor 6HF/Si02, or 1.991, to find the weight of hydrofluoric acid to be added to the weight of hydrofluoric acid found.
Explosion a t Bureau of Standards On the afternoon of September 20, a violent explosion followed by fire occurred i n the dynamometer laboratory of the Bureau of Standards. One man was killed instantly, three others were injured so seriously t h a t they died during the night, and four others seriously burned or cut. ‘ T h e heroism of the survivors of the staff in rescuing t h e injured from t h e furiously burning wreckage and in shutting off the electric circuits and t h e ammonia valves, minimized the loss of life and property. The explosion occurred in the altitude chamber which is used in testing t h e performance of aircraft engines under the conditions of low pressure and temperature obtaining at high altitudes. At the time of the accident the room was being used in investigating the performance of a n automobile engine, a t temperatures corresponding t o winter operation, using various grades of gasoline. The work was intended t o determine the possible increase in gasoline production per barrel of crude oil, with the accompanying conservation of our natural resources, by the use of gasoline of lower volatility. The explosion was due t o the ignition of a n explosive mixture in the chamber. The dead are: Logan L. Lauer Urban J . Cook Stephen N.Lee Joseph Kendig
The injured are: Henry K . Cummings Frank E. Richardson Roger Birdsell George W. Elliott C. N. Smith R F. Kohr
Most of these men were college graduates with experience and skill in research work, and a grave blow t o science and engineering must be added t o the human loss to their families and colleagues. Thus grows the long list of those who have given their lives for the increase of human knowledge and welfare.
