INDUSTRIAL AND ENGINEERING CHEMISTRY P U B L I S H E D BY T H E A M E R I C A N C H E M I C A L SOCIETY W A L T E R J. M U R P H Y , EDITOR
Sampling and Analysis of Anhydrous Hydrogen Fluoride C A R L FRANCIS SWINEHART AND H A R R Y FRANK FLlSlK Research Liboratories, The Harshaw Chemical Company, Cleveland, Ohio pounds of the hydrogen fluoride run into the HE production and use of anhydrous An accurate and Procedure tank car. Caution! A cylinder must never Thydrogen fluoride in tank quantities for the sampling and analysis of be filled to more than of its water have required the development of an inhydrous hydrogen fluoride from capacity. accurate . . procedure for its sampling and tank cars describe& APPARATUS.From H-cylinder , samples analysis. On account of the volatility of anhydrous hydrogen fluoride (boiling point 19.4" C.), theserious danger of burns, and the tendency of impurities to segregate, a correct and safe sampling procedure is absolutely WSentid. Strict adherence to the procedure described here yields excellent results. SAMPLING
Bleed a proportional sample from the pipeline leading from the storage tank to the tank car into an evacuated, or vented, H-type cylinder fitted with valve and coupling suitable for anhydrous hydrogen fluoride. To accomplish this the cylinder must be placed on a scale and weighed, all connections made gastight, and the valves opeded slightly to bleed off 1 pound for every lo00
Figure 1. Assembly of Acid Container Mrbl par( to be #old-plated Intide and out
draw a smaller sample from the liquid phase only into an evacuated or cooled E-type cylinder and fill to 85% of its water capacity. This is done by tilting the H-cylinder sufficiently to ensure the drawing of liquid hydrogen fluoride only, connecting to an evacuated Ecylinder, and allowing about 3 kg. (6 pounds) to flow in, This is the sample taken to the laboratory.' On1 the liquid phase is sampled, since it constitutes about 99.966, of the net weight - of the shi ment. J r a w a sample for analysis from the 6-pound sample cylinder; in order to do this fairly the following special equipment 1s necessary: 1. A rubber weighing tube (Figure 1) is machined from a 5-cm. (2-inch) (iron pipe size) extra-heavy-wall hard-rubber pipe. A narrow ledge on the inside is machined 12 cm. (4.875 inches) above the bottom. The lower end is plugged with a hard-rubber disk and the entire inside is coated with a heavy layer of high melting point wax (Quaker State M wax 165 dark amber). The top end is stbppered with a removable No. lo1/* solid rubber stopper which has been wax-treated. The removable metal part consists of a metal tube with coupling at the upper end, a metal hook silver-soldered to the tube just below the coupling, a per. forated metal grid which slides on the tube, and a grid stop, silversoldered at the end to prevent it from sliding off, The whole metal assembly of the sampling device must be goldplated or, better, made of platinum. On account of contact with ice and hydrofluoric acid, the metal assembly cannot be of cadmium, brass, copper, iron, or steel, since corrosion of these metals contaminates the sample, with the possibility of giving high fluosilicate and sulfuric acid results. Monel metal i s passable but preferably it should be gold-plated. The metal assembly should not be left in contact with the acid any longer than necessary. 2. A torsion balance with a sensitivity of 15 mg. and a capacity of 500 grams. 3. A sturdy support for the E-type cylinder to hold it horizontally, as shown in Figure 2. Before drawing a sample for analysis, set up the sample cylinder on the support in a sufficiently inclined position to ensure withdrawal of the liquid phase only. Cylinders filled to 85% of water capacity can be placed in a horizontal position, but if many sam les are to be taken the cylinder should be tilted slightly. $elow place a Harvard trip balance (sensitivity need be no greater than * 1.0 gram), so that the left pan is in line with the cylinder valve outlet. Set the rubber weighing tube on the left pan and add enough weights to the other pan to overbalance, in order to hold the weighing tube a t its highest position on the balance. Loosely couple the metal tube assembly to the cylinder adapter, then raise or lower the balance, so that the hook on the metal tube is in line with the top of the weighing tube. Caution! This adjustment is necessary to prevent suckback of liquid into cylinder valve. Make certain the cylinder clamps are secure in order to prevent slipping when the cylinder valve is opened. The entire setup must be under a Rood hood. SAMPLING PROCEDURE. The metal part and rubber stopper may be dried in the oven but the weighing tube must be dried a t room temperature by torcing dry air into it until globules of water or moist areas are no longer visible. Rubber weighing tubes after repeated use should be rewaxed when there are any bare spots in the coating or when any odor of hydrogen fluoride is detected in the dry tube.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Weigh the entire dried weighing tube (metal tube and rubber
stopper included but not assemb!ed) on the torsion balance, using raugh weights, or make u an ap roximate tare weight
and balance exactly with the sli8ng weigkt. Test the accuracy of this torsion balance for such factors as e uality of arms positioning of weights, etc., and make suitabqe corrections 'if the errors are in excess of 20 mg. After thus establishing the weight of the entire weighing tube, do not disturb the tare weights, and from this point on make all succeeding weighing5 by adding anslytical weights. Complete the following operations as quickly as possible: Place 80 grams of chopped ice in the bottom part of the weighing tube, and properly insert the metal tube, so that the perforated grid rests flat on the narrow ledge and the hanger rests over the rim of the rubber tube. Add 50 grams more of cho ped ice to the top part of the tube above the grid. Wipe off any jroplets of water on the outside of the weighing tube or on the coupling. Weigh the entire assembly exactly including rubber stop er, adding only analytical weights, and record the total weigit of ice added. The 80 grams of ice in the bottom serve to absorb the heat of dilution of the hydrogen fluoride and the 50 grams in the top serve to trap any vapors formed through local concentration of heat. The entire weight of ice must not greatly exceed 130 grams, as this amount when melted plus about 40 grams of sam le will not bring the liquid level above the outlet of the metal d e . If the setup is exactly as described above, a clearance of about 5 cm. (2 inches) is assured. Caution! Never allow the metal tube outlet to be submerged during addition of the sample, for a very rapid suckback whirh surely will follow will ruin the sample and may cause an explosion. C a u t i o d y open the valve (always wear rubber gloves) of the sample cylinder already set up and allow a few milliliters of the hydrogen fluopide t o flow out into a Monel waste beaker to sweep the outlet free from condensed water that may have lodged there in. Place the weighing tube (without the rubber stopper) on the Harvard trip balance and immediately couple the metal tube to
Figure 2.
Assembly of Sampling Apparatus
Vol. 16, No. 7
the cylinder adapter, tightening with a wrench (Figure 2). Balance with the rough weights, make certain that the metal tube does not hinder the balance swing, then add 40 grams more. Carefully o n the cylinder valve slightly and adjust the flow of hydrogen g o r i d e to about 10 grams per minute. As the h drogen fluoride strikes the ice a sizzling sound may be fain& heard and through this guidance the rate of flow may be varied, During the flow, watch the top of the weighing tube for esca ing vapors and as soon as any are seen cut down the flow, geep testing the balance swing, to make certain shifting ice in the upper part does not cause sufficient friction against the metal tube to hinder the swing. Kee running in the sample until the 40 grams are approximately baknced, close the valve, and wait 15 seconds for drainage. Disconnect the metal tube and drop carefully into the weighing tube, then stopper tightly without delay: Weigh 'exactly on the torsion balance, adding only analytical weights, and record the additional weight over the ice weight as the sample weight. Mix thoroughly by careful inversion until all the ice melts, being certain to kee the tube tightly stoppered, so that none of the solution is lost lefore the ice melts and the solution becomes homogeneous. Remove the metal tube and restopper without delay to prevent escape of sulfur dioxide. The ivell-mixed diluted acid. clinging to the metal tube will be of no consequence, since aliquot weiqhts will be taken for analysis. Clean and dry the metal tube a t once. Proceed with the analysis as directed below without delay. ANALYSIS
SULFURDIOXIDE. This must be the first constituent deter-
mined, because opening the weighing tube for taking the other aliquot samples msy result in loss of sulfur dioxide. Provide a well-waxed 250-ml. beaker and a Bakelite stirring rod. To this beaker add 50 ml. of w a t q and exactly 10 ml. of standard 0.1 N iodide-iodate solution (3). Weigh on R torsion balance. Place a 50-gram weight on the balance pan, then pour
HARVARD TR/P
8ALAUCE
ANALYTICAL EDITION
July, 1944
an aliquot portion of the sam le into the beaker carefully until slightry overbalanced. Weigh accurately to *0.5 gram. Back-titrate the excess liberated iodine with standard 0.1 N thiosulfate, using starch solution as indicator. If no color appears upon adding starch, titrate with iodide-iodate to a blue color. Standardize the iodide-iodate solution against the thiosulfate under like conditions, substituting 50 ml. of water for the sample and making slightly acid with pure hydrofluoric acid. Calculate to sulfur dioxide and report to two significant figures. 1 ml. of N thiosulfate = 0.032 gram of SO2
Figure 3. Platinum Calculation to per cent sulfur dioxide Weighing Bottle is based on the following equation Approximate wrigh!, which also applies to succeeding calcu99 grams lations: (Grams of ice grams of anhydrous HF) X (factor X)x (ml. of standard solution) X 100 = % X (weight of aliquot) X (grams of anhydrous HF) Factor X = normality of standard solution X normal equivalent for X
+
TOTALACIDITY. Weigh a 12-ml. platinum weighing bottle (Figure 3) on the analytical balance. Add as quickly as possible 35 to 45 drops of the sample solution by means of a Bakelite dropping pipet, cover the weighing bottle promptly, and reweigh. I n a 250-ml. waxed beaker place 100 ml. of water and 1 ml. of phenolphthalein indicator and make faintly pink with 0.1 hf alkali. In this submerge the weighing bottle, knock off the cover, and titrate with standard 0.5 N alkali to a permanent pink color. Approach the end point slowly by adding a fraction of a drop of alkali at a time. If the ink color fades, wash the solution into a plain beaker, heat to ,\out 60" C., and continue the titration to a permanent pink. A fading end point is due to fluosilicate, which may be in the acid or formed during the titration from silica in the standard alkali. Silica in the standard alkali does not affect a total acidity determination if the heating precaution is taken near the end point but does cause erroneous values in determination of fluosilicic &Ad. Calculate the total acidity to hydrogen fluoride and report to three significant figures. Corrections for the impurities will be made later. 1 ml. of N alkali = 0.01999 gram of H F SULFURIC ACID. Weigh on the torsion balance a 50-gram aliquot of the sample into a 75-ml. platinum dish and evaporate to apparent dryness on the water bath. Add 10 ml. of water, evaporate again to dryness on the water bath, and note as evaporation progresses whether any odor of hydrofluoric acid can be detected. Repeat as often as hydrofluoric acid could be detected in the previous evaporation. Usually two evaporations with water are sufficient when the sulfuric acid content is below 0.1%. When all the hydrofluoric has been expelled, add 25 ml. of water and titrate with 0.1 N. alkali, using phenolphthalein as indicator. The titration is eqmvalent to sulfuric acid and fluosulfonic acid. The latter decomposes to sulfuric acid upon evaporation with water. Calculate to sulfuric acid and report to two significant figures (sulfuric acid plus fluosulfonic acid) as sulfuric acid,
Jf heavy metals, such as copper, lead, nickel, etc., are present, add 1gram of neutral potassium oxalate before titration. FLUOSILICIC ACID. Weigh on a torsion balance a 50-gram aliquot Sam le into a 75-ml. platinum dish. Add 0.2 gram of potassium cgloride, stir with a platinum rod until the salt dissolves, and evaporate to dryness on the water bath. Dissolve the residue in 25 ml. of water. Add 2 grams of potassium chloride, and cool to below 10" C. (4). Add 1 ml. of phenolphthalein indicator and titrate with silica- and carbonate-free 0.5 N alkali. This titration should preferably be carried to just short of the end point and finished with s,ilica- and carbonatefree 0.1 N alkali. During the entire titration do not allow the temperature to rise above 10" C.; keep the dish surrounded with chopped ice. Disregard the amount of alkali required, for the acidity is due to acid fluoride with which we have no concern except to neutralize it. Heat the dish to at least 60" C. and titrate the hot solution with 0.1 N alkali to the first faint permanent pink color. If means of cooling for the first titration is not conveniently available, add 35 ml. of ethyl or methyl alcohol, stir somewhat, let stand 15 minutes for potassium fluosilioate to precipitate, and titrate at room temperature as above. After reaching the end point wash the solution into a beaker, boil 10 minutes, and titrate the hot solution with 0.1 N alkali as above. The hot titration is equivalent to four of the six fluoride atoms in the fluosilicic acid. Multiply the hot titration by 3/2 to obtain the total milliliters of alkali for fluosilicic acid. Calculate to two significant figures. 1 ml. of N alkali = 0.024 gram of HtSiFs
The equations involved in fluosilicic acid determinations are as follows: HzSiFe 2HF KHFp KzSiFs
Table 1.
Table Total acidity a8 HF ImpuritieaasHF True H F SOX
+
1
2
1
99.9 99.9 99.75 0.11 0.11 0.17 99.8 99.8 99.6 0.096 0.098 0.17
(HFSOI H2S04) 0.059 0 . 0 6 0 as HxSOd HxSIFs 0.027 0.027 M Analyst M
2
99.79 0.16 99.6 0.16
0.027 0.026 0.021 0.022 JI M
2KC1 KCl KOH 4KOH
+
+ -* -+
KzSiFa KHFn 2KF 6KF
+ + + +
2 HCl7
(1)
HCIP HzO Si(OH),
(2)
Recovery
(3) (4)
of Silica A d d e d to Hydrofluoric A c i d SiOr
HSSiFsin 1007 HF Added flound
48% HF
Added
GTame
Gram
Qram
%
50 50 50 22 22
None 0,0369 0.0335 0.0059 0.0044
