(9) G. I. Goodfellow, D. Midgley, and H.M. Webber, Analyst(London), 101, 848 (1976). D. Midgley and K. Torrance, Analyst(London), 97, 626 (1972). D. Midaley, Analyst (London), 100, 386 (1975). D. Midgley, Anai. Chim. Acta, 87, 7 (1976). D. Midgley, Anal. Chim. Acta, 87, 19 (1976). G. J. Moody and J. D. R. Thomas, “Selective Ion Sensitive Electrodes”, Merrow Publishing, Watford, England, 1971. (15) K. Torrance, unpublished work. (16) H. M. Webber and E. A. Wheeler, unpublished work. (10) (11) (12) (13) (14)
(17) M. Mascini and A. Liberti, Anal. Chlm. Acta, 47, 339 (1969). (18) G. B. Marshall, unpubllshed work. (19) K. Garbett and K. Torrance, unpublished work.
RECEIVED for review Jgnuary 13, 1977. Accepted April 18, 1977. This work was carried out a t the Central Electricity Research Laboratories and is published by permission of the Central Electricity Generating Board.
Determination of Trace Acetylene in Oxygen with a Portable Acetylene Analyzer Marshall N. Cappelloni, Lowell G. Frederick, David R. Latshaw,” and John B. Wallace Air Products and Chemicals, Inc., P.O. Box 538, Allentown, Pennsylvania 18 105
The portable trace acetylene analyzer determines acotyJene concentrations in the 0.1 to 10 ppm range with an accuracy of *lo%. Acetylene in the sample gas is adsorbed and concentrated on a molecular sieve column. The column Is then desorbed by heating the molecular sieve and the released acetylene is purged by nitrogen through a modified Xlosvay solution. Upon contact with the Ilosvay solutlon, the acetylene reacts to form red copper acetyllde. The csncontratlsn of copper acetylide is determlned with a colorimeter and the concentration of acetylene in the gas sample Is determined by reference to a calibration curve. Use of two separate stock solutions that are mixed to form the Ilosvey solution when needed Increases the storage life of the Ilosvay solution and also enhances the portabllity of the analyzer.
Industries associated with the production and usage of oxygen are continuously aware of violent reactions that can be initiated if hydrocarbons are present in oxygen. One of the most feared situations is the presence of acetylene (1,2) in liquid oxygen (LOX). Acetylene presents a greater problem in LOX than other similar molecular weight hydrocarbons because it is much less soluble. A t -183 “C (-297 O F ) the solubility of acetylene in LOX is 5 ppm (3). At normal reboiler operating pressures of 8 to 10 psig, the solubility is slightly increased. Since acetylene has a low solubility in LOX, any acetylene in excess of the solubility limit will form solid acetylene crystals which will float on LOX. An interface between solid acetylene crystals and LOX is thus established. Therefore the need for monitoring acetylene in LOX is critical and the analysis for acetylene must be rapid and relatively accurate. A need existed for a portable instrument that could provide rapid and accurate analyses for acetylene in the 0.1to 10-ppm concentration range. The name of Ludwig Ilosvay is continuously brought into mind when speaking of trace acetylene analysis. However, Berthelot ( 4 ) is first credited with the method of precipitating copper acetylide with an ammoniacal solution of cuprous chloride. Ilosvay ( 5 ) in 1899 simplified and improved the method and his name has been carried with the acetylene absorbing solution ever since. The direct absorption of acetylene into Ilosvay solution is not sensitive enough for the acetylene concentration levels presently being determined in this paper. Because it is not possible to analyze directly for 1218
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acetylene concentrations in the low parts-per-million range with Ilosvay solution, scientists studied the possibility of concentrating the acetylene before it contacted the Ilosvay solution. Geissman, Kaufman, and Dollman (6) reported a satisfactory method for the determination of acetylene concentrations in the 1 to 15 ppm range. This method consisted of passing the gas sample containing acetylene through a cooled condensing coil in which the acetylene was condensed and thus concentrated. After gas sampling, the coil was warmed, and the concentrated acetylene was passed into a 300-mL gas-sample bottle where it was contacted with Ilosvay solution. Hughes and Gordon (7) adsorbed acetylene from a gas stream on a column of silica gel, 10 mm long and 2-mm diameter, cooled with dry ice to -78 “C. The gel was then warmed to room temperature and was treated with a solution of ammoniacal cuprous chloride. The presence and quantity of acetylene were indicated by the depth of color produced on the gel. The major problems encountered with their two methods were dry ice was not available at all analysis sites, and the methods did not have the precision and accuracy desired for the present test. The long term stability of Ilosvay reagent also has been a major problem. This is witnessed by the fact that numerous formulations exist for Ilovsay reagent (5-9). Additional information has been published on the formation of copper acetylides ( I 0) and on the copper-catalyzed oxidation of hydroxylamine (11). The present authors found the best stability by preparing the Ilosvay reagent in two parts. One solution consists of copper nitrate, ammonium hydroxide, and potassium chloride in water, while the other solution consists of hydroxylamine hydrochloride and gelatin in water. These individual solutions were stable for more than six months while stored in closed polyethylene bottles at temperatures between 0 and 30 “C. At an elevated temperature of 37 “C, the solution containing the gelatin was stable for three months.
EXPERIMENTAL Construction. A flow schematic of the acetylene analyzer is illustrated in Figure 1. The nitrogen flowmeter is capable of
measuring 10-50 cm3/min, and the oxygen flowmeter is capable of measuring 0.2-3 L/min. The connecting tubing is 0.32-cm 0.d. (7.1-mm wall) type 304 stainless steel, and it is connected by the use of stainless steel control valves and fittings. The sparger tube is 0.32-cm 0.d. (8.9-mm wall) type 316 stainless steel, and it is cut long enough to reach from the gas outlet to the bottom of the scrubber tube. The 30-cm long scrubber tube is constructed from
~. SCRUBBERTUBE
REGULATOR
YITROGEY CONTROL VALVE
OXIGEN CONTROL VALVE
SHUTOFF YA LV E
J
-
Figure 1. Flow schematic of the acetylene analyzer 1.59-cm 0.d. X 0.95-cm i.d. Lucite. A 100 psig relief valve is inserted between the pressure regulator and the shutoff valve. All tubing and components must be cleaned for oxygen service. The acetylene adsorber consists of a 0.635-cm 0.d. (7.1-mmwall) X 23 cm in length type 304 stainless steel column packed with 42/60 mesh Molecular Sieve 13X. The column is contained in a 1.91-cm i.d. X 15 cm in length heater rated at 340 W-115 V. Only the section of column inside the heater is packed with the molecular sieve. A proportional controller capable of sensing between 80-250 O C is used to control the heater. An elapsed time indicator is installed on the analyzer to measure time. The entire unit is contained in a metal cabinet. Apparatus. A colorimeter is required to determine the absorbance of the red copper acetylide solution at a wavelength of 550 nm. An inexpensive colorimeter may be purchased to accompany the acetylene analyzer in field use. Nominal 0.5 to 5 ppm acetylene gas standards are prepared for initial calibration of the analyzer. Reagents. Stock Solution A. Add 20 g of cupric nitrate to 58 g of potassium chloride contained in 400 mL of water. Add 120 mL concentrated ammonium hydroxide and dilute with water to 1 L. Stock Solution B. Add 150 g of hydroxylamine hydrochloride to 300 mL of water. Bring 600 mL of distilled water to a boil and, with constant stirring, slowly add 5 g of Knox gelatin. After the gelatin addition, the solution is cooled to room temperature, added to the hydroxylamine solution, and diluted to 1 L with water. Procedure. The Ilosvay reagent scrubbing solution is prepared at least 1h before it is going to be used. An equal volume of stock solution A and stock solution B are poured into a mixing bottle. The Ilosvay reagent is stable (colorless) for at least a day, provided the mixing bottle remains capped. If LOX is going to be sampled, it is collected in a cryogenic liquid sampler and completely vaporized. Gaseous samples are analyzed directly. The oxygen sample is connected to the analyzer and the oxygen control valve is adjusted so that the oxygen flowmeter indicates a flow rate on the order of 1-2 L/min. The time of flow is varied from 1-30 min depending upon the quantity of acetylene suspected to be in the oxygen. After the required adsorption time, the oxygen control valve is closed. The acetylene from the sample is now contained on the molecular sieve adsorption column. A nitrogen purge gas is connected to the gas inlet, and the oxygen control valve is opened so that a nitrogen flow rate of 0.2 L/min passes through the adsorber column. After this flow rate has continued for 5 min, the oxygen control valve is closed. The stainless steel sparger tube is placed in the empty scrubber tube and connected to the analyzer. The scrubber tube is held in place on the front panel of the analyzer by two spring clips. The nitrogen control valve is adjusted so the nitrogen flows through the adsorber column at a flow rate between 20-30 cm3/min. Add 12 mL of the Ilosvay reagent to the scrubber tube. Fine bubbles are present in the scrubbing solution at this time. The scrubber tube is pushed tightly against the bottom of the sparger to produce as small a bubble as possible. The Ilosvay reagent scrubbing solution will foam while the nitrogen purge gas is passing through. If the foam appears to be going to flow over the top of the scrubber tube, no more than 5 drops of isopropanol are added dropwise to reduce the excessive foaming. The heater is turned on, and after 5-6 min of purging the heated molecular sieve adsorber column with
07
1
O 056 I
P 0 3
3 2
I1-
PH
Figure 2. Absorbance as a function of pH of Ilosvay reagent for a constant addition of acetylene nitrogen, the acetylene will have been desorbed and subsequently carried to, and reacted with the Ilosvay reagent scrubbing solution. This produces a red color. The heater is turned off and the nitrogen purge is allowed to continue for another 5 min. The scrubber is removed and the colored solution is transferred to the colorimeter sample vial. After standing for an additional 10 min, the absorbance of the solution is measured and the acetylene concentration in the gas sample is calculated from a calibration curve. The nitrogen purge is allowed to pass through the analyzer for an additional 30-60 min to cool the adsorber column to room temperature for the next run. After the nitrogen purge is disconnected, the analyzer is stored with all valves closed. RESULTS AND DISCUSSION
Instrument Startup. In order to activate the molecular sieve adsorber, on initial startup or after prolonged non-use, run the analyzer through a heated desorption cycle for 2 h at 200 "C. Dry nitrogen is purged through the adsorber during this time. Analysis Time. The Ilosvay reagent is prepared at least 1 h before it is going to be used. The time for sample adsorption can vary from 3-30 min. The time for purging, desorption, and color development is approximately 25 min. If duplicate analyses are to be performed, 30-60 min are required to cool the analyzer to ambient temperature. Stability of Ilosvay Reagent. The stability of the mixed reagent in a tightly closed container a t room temperature is limited to one day. Air oxidation, precipitation of copper metal by over-reduction, and flocculation of the cuprous acetylide due to instability of the gelatin in solution are all problems with the instability of the mixed reagent. By preparing individual stock solutions and preparing the Ilosvay reagent from these stock solutions at the time of use, it is possible to store the individual stock solutions at room temperature in closed containers for at least six months. The major factor limiting storage time is the instability of the gelatin in solution. Introduction of acetylene into an Ilosvay reagent in which the gelatin is no longer active is detected by the formation of large flocculated particles of copper acetylide. Storage of stock solution B at temperatures of approximately 35 "C for more than three months causes the gelatin to lose activity. Loss of ammonia is the biggest problem with instability of stock solution A. The loss of ammonia affected the final p H of the Ilosvay reagent and it was observed that the pH of the Ilosvay reagent directly affected the absorbance of the copper acetylide. A series of tests were performed with the release of a fixed quantity of acetylene into Ilosvay reagent adjusted to pH values from 4.8 to 8.0 by the addition of ammonium hydroxide or hydrochloric acid. The absorbance ANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
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Table I. Comparison of Acetylene Results between Trace Acetylene Analyzer and Gas Chromatograph Trace acetylene analyzer Gas chromatograph, Liters of Absolute Actual Acetylene acetylene concn, sample Relative error, ppm-liters concn, adsorbed error, % acetylene PPm PPm PPm 0.93 0.93 0.93 0.93 0.93 1.62 1.62 1.67 1.67 1.67
”
4.98 8.09 15.01 18.12 23.0 4.19 12.98 6.43 9.63 12.94
4.63 7.52 13.96 16.85 21.39 6.79 21.03 10.74 16.08 21.61
of the Ilosvay reagent was measured after the addition of the acetylene. Figure 2 shows the observed effect of the pH of the Ilosvay reagent on the absorbance of the copper acetylide produced. Since the pH of the Ilosvay reagent is critical, a buffer was added to the reagent to stabilize it between pH 6.0 and 6.5. However, addition of buffers produced Ilosvay reagents which had either a pale blue or green tint. As a compromise, it was decided to add enough ammonia to stock solution A so that when stock solutions A and B were mixed, the final pH of the Ilosvay reagent would be approximately pH 6.7. Thus a loss of ammonia causing the pH of the mixed reagent to change from pH 6.7 to 6.0 caused only an approximate 3% change in absorbance. It was also observed that as the pH of the Ilosvay reagent changed from pH 6.0 to a more acidic pH, the Ilosvay reagent began to acquire an increasingly blue tint. Thus a high blank was obtained which led to increasing uncertainty in the analysis. Time for Color Development. Studies were performed to determine the amount of time required for the copper acetylide red color to develop and fully stabilize in the Ilosvay reagent. Figure 3 shows the results of adding three different quantities of acetylene to the Ilosvay reagent and then determining the absorbance of the copper acetylide solution at various times. Interferences. Studies were performed to determine possible interference materials with the Ilosvay reagent. Direct injection of small volumes of interference gases such as nitrogen dioxide, sulfur dioxide, carbon dioxide, and methyl acetylene into the line between the adsorber and the scrubber during the acetylene desorption step produced no detrimental effect on the acetylene analysis. Oxygen alone purging through the Ilosvay reagent will oxidize the copper and thus turn the reagent blue. In order to overcome this problem when oxygen remains on the molecular sieve after a sample of acetylene in oxygen is passed through the adsorber column, an additional step was placed in the operating procedure in which the adsorber column is first purged with nitrogen for five minutes before the desorption into the Ilosvay reagent is started. Another type of interferent is one which prevents adsorption of acetylene on the molecular sieve by being a more strongly adsorbed species and displacing the more weakly held acetylene. Water is a substance that causes such a difficulty. If water is collected on the adsorber, it can be removed by regeneration of the adsorber. Nitrogen dioxide is also in this category. It is more strongly adsorbed than acetylene, and it is extremely difficult to remove completely during the desorption cycle. Reference 12 indicates that nitrogen dioxide reacts with unsaturates such as butadiene, acetylene, or cyclopentadiene. In earlier experiments, this laboratory observed a removal of butadiene that was passed over a molecular sieve bed containing a small quantity of nitrogen 1220
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1.02 0.89 0.98 0.97 0.92 1.67 1.63 1.63 1.61 1.67
0.09 0.04 0.05 0.04 0.01 0.05 0.01 0.04 0.06 0
9.7 4.3 5.4 4.3 1.1 3.1 0.62 2.4 3.6 0
O 012 I
001 0
I IO
8
20
MINUTES A f T E R
I
I
30
40
50
INITlbiL C O L O R DEVELOPMENT
Figure 3. Absorbance decrease as a function of time after initial formation of copper acetylide
dioxide. Because of the problems associated with nitrogen dioxide, it is recommended that no nitrogen dioxide be present in the gas being analyzed. Concentration Range. Low parts per million concentrations of acetylene were placed in gas cylinders. The bulk gases were nitrogen, oxygen-nitrogen or oxygen-nitrogen-trace carbon dioxide. The acetylene in all the prepared gas standards was analyzed by gas chromatography by comparison against a weighed acetylene standard. Since it was desired to have the analyzer capable of determining acetylene in the 0.1 to 10 ppm range, the volume of scrubbing solution was varied until a volume was found which would produce a measurable absorbance with gas standards in the 0.1 to 10 ppm range. Experiments showed this volume of scrubbing solution to be 12 mL. With the establishment of the volume of scrubbing solution at 12 mL, various quantities of the acetylene standards were adsorbed on the molecular sieve adsorber, then desorbed into the Ilosvay reagent, and the absorbance of the Ilosvay reagent was measured in a 1-cm cell. The data were plotted as absorbance vs. ppm-liters of acetylene. A straight line calibration was obtained. This calibration line had an absorbance of 0.1 for 4.2 ppm-liters of acetylene and an absorbance of 0.7 for 29.3 ppm-liters of acetylene. Reproducibility and Accuracy. In order to evaluate the reproducibility of the analyzer, 9 L of a standard containing 0.93 ppm acetylene in 10% nitrogen-90% oxygen were passed into the analyzer at a flow rate of 1 L/min. The reproducibility of this analysis was 4 5 % . To evaluate the accuracy, after the calibration curve was established, standards containing trace quantities of acetylene were analyzed by the trace acetylene analyzer and these results were compared with the
Table 11. Comparison of Instrument Features between Trace Acetylene Analyzer and Portable Gas Chromatograph Features Trace acetylene analyzer Portable gas chromatograph costs Basic instrument $475 for parts, 4 5 h construction time $1330 for Carle Model 9500 with FID and gas sampling valve Data readout $225 for colorimeter $600 for Recorder Additional preparation Prepare scrubbing solutions in lab None Additional equipment None Carrier gas Shipping regulations None Hydrogen cannot be transported on a passenger plane Repairs Few simple electrical components Trained personnel required to repair instrument Analysis time None 30 min Warmup time 30 rnin None. Initial calibration holds. Calibration No need to calibrate the instrument each time it is used. 15 rnin First sample 50 rnin Preparation for second sample 60 min (cool-down) None Sample size (for 1.5 ppm C,H,) 3L 1 cm3 0.1 ppm-1% (without changing Range 0.1-10 ppm sample loops) k 5% Accuracy (for 1 . 5 ppm C,H,) * 5% ?: 5% * 5% Reproducibility (for 1.5 ppm C,H,) Versatility Can be used for other hydrocarbons Specific for C,H, results obtained with a gas chromatograph. The standards were adsorbed on the analyzer a t various flow rates with different total volumes of gas. The results obtained with three acetylene standards are shown in Table I. Comparison with Portable Gas Chromatograph. Table I1 shows a comparison of the instrument features between the trace acetylene analyzer and a typical portable gas chromatograph. Both instruments have unique and favorable features. Basically the trace acetylene analyzer was designed to be truly portable, capable of being operated by personnel with no technical background, and capable of producing results for acetylene which cannot be confused with other hydrocarbons such as ethane or ethylene. Areas of Caution. Care must be taken to ensure that the gas passed into the analyzer does not introduce any gases that would contaminate the analyzer for use with LOX. No copper fittings or tubing should be used with any gas that is suspected to contain acetylene. Copper acetylide solids form in the Ilosvay reagent if the gelatin in the reagent is no longer active or if the gas contains too high an acetylene concentration. Experiments show that solids will form on the inside of the sparger tube if the desorbed concentrated “slug” of acetylene contacts Ilosvay reagent which is contained on the inside walls of the sparger tube. Therefore it is recommended in the procedure that the nitrogen purge gas be started through the dry sparger tube contained in the scrubber tube before the 1 2 mL of Ilosvay reagent are added to the scrubber tube. In this manner, the flowing purge gas will not allow the Ilosvay reagent to wet the inside wall of the sparger tube during the desorption cycle.
in February 1975. With the proven success of this instrument, an additional forty-four analyzers have been built and are now in use. They are capable of being operated by nontechnical personnel. Although the analyzers are capable of determining acetylene in the 0.1 to 10 ppm concentration range, they have primarily been used to monitor the acetylene concentration in LOX in the 0.1 to 2.5 ppm range. The analyzer provides a portable instrument that is specific for acetylene a t trace levels, and yet does not lose accuracy because of its portability. Use of two separate stock solutions that are mixed to form the Ilosvay solution when needed, has increased the storage life of the Ilosvay solution and has enhanced the portability of the analyzer.
CONCLUSIONS The original trace acetylene analyzer was put into service
RECEIVED for review January 24, 1977. Accepted April 26, 1977.
LITERATURE CITED (1) H. P. McKoon and H. D. Eddy, I d . Eng. C h m . , Anal. Ed., 18, 133-136 (1946). (2) R. W. Rotzler, J. A. Glass, W. E. Gordon, and W. R. Heslop, Chem. Eng. Prog., 56, 66-73 (1960). (3) M. F. Fedorova, Russ. J. phys. Chem. (Engl. Trans/.), 14, 422-426 (1940). (4) M. Berthelot, Compt. Rend., 54, 107 (1862). (5) L. Ilosvay, Ber., 32, 2697 (1899). (6) T. A. Geissman, S. Kaufman, and D. Y. Dollman, Anal. Chem., 19, 919-921 (1947). (7) E. E. Hughes and R. Gordon, Jr., Anal. Chem., 31, 94-98 (1959). (8) M. 8. Jacobs, “The Chemical Analysis of Air Pollutants”, Interscience Publishers, Inc , New York, p 363. (9) E. W. Hobart, R. G. Bbrk, and R. Katz, Anal. Cbem., 39, 224-226 (1967). (10) V . F. Brameld, M. T. Clark, and A. P. Seyfang, J. Soc. Chem. Ind., 346-353 (1947). (11) J. H. Anderson, Analyst (London), 89, 357-362 (1964). (12) S. Haseba, T. Shlmose, N. Kubo, and T. Kltagawa, Chem. Eng. Progr., 62, 92-96 (1966).
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