AnaLjst, September, 1974, Vol. 99, $9. 547-550 547 The Determination of Small Amounts of Water in Gases Using Karl Fischer Reagent BY E. E. ARCHER (British Petrolt-uni Limited, Group Rcsearch and Developnzent Department, Epsom Division, Epsom, Surrey) AND J. HIL~ON (BP Cheniicabs International Limited, Grangemouth, Stirlingshire, Scotland) Two methods are described for the determination of water in gases. In the first method, the gas being examined is led into a Karl Fischer titration cell, and water is titrated directly. In the second method, water from a gas stream is absorbed on a short column of a gas-chromatographic stationary phase. On heating the column, water is liberated and is carried over to the Karl Fischer cell in a stream of dry nitrogen and then titrated. The validity of the direct titration method has been checked by examining gas streams that had been passed over ice at various temperatures.THE direct determination of water in gases by absorption in a Karl Fischer cell followed by titration has been described in the literature.1-4 This method has been tried at the parts per million range in various laboratories with which we have been acquainted, but the validity of the results obtained was not generally accepted. Although in two of the papers referred to1y2 success at the parts per million level was claimed, we would not accept their validity as they are based on end-point detection techniques in which current differences are observed, and in our opinion such techniques are not sufficiently sensitive to determine water at the microgram level.I t is not generally recognised that in mainly methanolic titration mixtures reaction near the end-point is very slow, and consequently a true end-point cannot be achieved with any degree of certainty. In our work, a more sensitive end-point detection system based on voltage measureme~its~-~ and a titration medium in which reaction is rapid are used. The limit of detection of the direct method is of the order of 2 p.p.m. by mass. With the aim of increasing sensitivity, a methods in which water from a gas was absorbed on a gas- chromatographic column and subsequently eluted and determined with a gas-chromatographic detector was examined. At low levels of water inconsistent results were obtained and the errors appeared to be due to the absorption and desorption of water on the rather complex pipe-work involved.However, when water was absorbed from very large volumes of gas, and on desorption determined by Karl Fischer titration, consistent results were obtained. Sample gas at flow-rates up to 5 1 min -1 was passed through a short column of PEG 200 on Celite, which was cooled in a solid carbon dioxide - acetone bath. Water from the gas was absorbed on the column, which was then heated in boiling water. Water was liberated from the column and carried by a stream of dry nitrogen into the Karl Fischer cell, where it was tit rated. EXPERIMENTAL APPARATUS- Cell and titration assembly-Most of the details of this assembly are as previously de- scribed,' except that the syringe burette driven by an Agla micrometer movement has been replaced by a Metrohm 5-ml piston-type burette and the cell outlet marked D in the previous paper is taken over in the form of an inverted U.The apparatus has now been constructed to form one integral unit and will shortly be produced by Analysis Automation Ltd., Oxford. The stainless-steel column is in the form of a U-tube, Q inch 0.d. x 16 s.w.g. A Rulon-faced three-port valve is used as a two- way tap. The connection from the column to the titration cell is made of &inch stainless steel, terminating in &-inch stainless steel. The actual connection to the cell is made via a R7 polythene stopper, drilled so as to take the &-inch tubing in a tight fit. The column is packed with 30 per cent. PEG 200 on 60 to 80-mesh Celite, the packing extending to within about 4 inch of each end and being held in position with siliconised glass- @ SAC and the authors.Absorptioiz coZumn-This column is shown in Fig. 1.548 ARCHER AND HILTON: THE DETERMINATION OF SMALL AMOUNTS [Analyst, VOl. 99 wool. Before use, “condition” the column in order to remove trace amounts of water from the Celite support by passing dry nitrogen through it at 150 “C for 1 hour at the flow-rate of 60 ml min -l. Fig. 1. Absorption column Dry nitrogen suP$ly-The dry nitrogen supply is obtained by passing tank nitrogen first through a B24 gas scrubber jar containing 5A molecular sieve and then through a second jar containing an approximately 1 + 1 V/V mixture of 4-mm single-turn glass helices and phos- phorus(v) oxide.The outlet of the second jar is terminated by B-inch 0.d. glass tubing so that connection can be made directly via a Drallim connector to the column inlet. Lead-in tube for direct sampling method-This tube is similar to the connection from the column to the cell used in the collection method; an + to &inch Drallim connector is fitted so that connection can be made directly to plant pipe-work. REAGENTS- Cell base liquid-Previous work7 showed that there was a slight end-point drift when a base liquid that contained N-ethylpiperidine was used. A base liquid in which reaction is rapid, but which is free from end-point drift, is prepared as follows. Mix 300 ml of dry methanol and 110 ml of anhydrous pyridine in a 750-ml conical flask and slowly pass sulphur dioxide into this solution, mixing carefully, until the increase in mass is 32 g.Cool the solution in a freezing mixture and, when c,ool, add sufficient AnalaR-grade iodine to give a permanent brown colour. Add 63 g of iodine and swirl the mixture until it has dissolved, then make the volume up to 500 ml with dry methanol (solution A). Mixing carefully, slowly add 20 g of sulphur dioxide to 180 ml of anhydrous pyridine (solution B). To prepare the base liquid, add 55 ml of dry methanol and 75 ml of solution B to 55 ml of solution A in a round-bottomed flask. Boil the mixture under reflux for 10 minutes, then cool it. Just before use, add sufficient water to the base liquid in the titration cell to bring the cell contents to the null-point. Karl Fischer reagent-This reagent is prepared exactly as solution A referred to above.METHODS Add 20 ml of base liquid to the titration cell. DIRECT SAMPLING METHOD- Connect the lead-in tube to the plant stream, and purge with a rapid flow of gas so as to remove water from the connections. Pass gas at the rate of about 500 ml min-l into the titration cell, measuring the flow with a gas meter connected to the cell outlet. Continuously titrate the solution so as to maintain the cell contents at the null-point. After 10 minutes, note the burette and meter readings and, after a further 30 minutes, again note these readings. COLLECTION METHOD- First “condition” the PEG - Celite column in order to remove any traces of water. Immerse the column in a beaker of boiling water and pass dry nitrogen through the column at the rate of 500 ml min-1 for 15 minutes.Move the port over to the vent position and couple it to the stream to be examined by means of Drallim connectors. Vent a suitableSeptember, 19743 OF WATER IN GASES USING KARL FISCHER REAGENT 549 amount of gas so as to sweep the connecting pipes and port assembly; no fixed recommenda- tion can be made for this amount as it will depend on the length of the connecting pipes. Immerse the column in a beaker containing a solid carbon dioxide - acetone mixture and connect a 0 to 5 1 min-l rotameter to the outlet via the B7 polythene stopper. By using a beaker surrounded by cotton-wool as a bath, and adding some large pieces of solid carbon dioxide, the bath can be maintained at -70 "C for over 1 hour without attention.Change the port over so that the gas passes through the column and adjust the flow-rate to about 5 1 min-l. Allow the gas to flow for up to 1 hour, depending on the anticipated water content of the sample, then return the port to the vent position and turn off the sample gas stream. Wipe off any water that has condensed around the top of the polythene stopper (frost condenses for most of the length of the stainless-steel tube connection). Remove the tube leading to the rotameter and close the end with a B7 tube pushed on to the polythene stopper. Disconnect the apparatus from the plant and remove it to the laboratory, leaving the U-tube in the solid carbon dioxide - acetone bath. Connect the apparatus to the dry nitrogen supply and, with the port still in the vent position, pass dry nitrogen through the connections at the flow-rate of 500 ml min-l for 10 minutes.Charge the Karl Fischer cell and titrate its contents to the null-point. Insert the delivery tube from the column into the titration cell via the polythene stopper and move the port so that dry nitrogen is passed through the cell. Water picked up on the connections will be carried into the cell. Titrate back to the null-point, and continue to titrate until a steady state has been reached. When the system is in a steady state, remove the solid carbon dioxide - acetone bath and surround the column with a beaker containing boiling water. Titrate the water evolved, recording the titration at 1-minute intervals. The pattern of titration to be expected is illus- trated in the results given below.From the titration obtained and the amount of gas passed, calculate the water content of the gas. RESULTS CHECK OF EFFICIENCY OF ABSORPTION OF WATER INTO KARL FISCHER REAGENT- Using two sets of apparatus, the outlet of the first cell was connected to the inlet of the second cell. A stream of moist nitrogen was passed at the flow-rate of 500 ml min-1 through the system and both cells were titrated to the null-points. Constant small additions of reagent were necessary in order to maintain the first cell at the null-point, but after the first adjustment no additions to the second cell were necessary. SPECIMEN TITRATION PATTERN- The titration pattern obtained on an actual sample of ethylene is given in Table I. Ethylene was passed at the rate of 5 1 min -l for 30 minutes.The starting point was about 3 minutes after the delivery tube had been inserted in the cell; water picked up from the delivery tube had been titrated out and an almost steady state had been reached. TABLE I TITRATION RESULTS OBTAINED ON A SAMPLE OF ETHYLENE Titre/ml of Karl Fischer Time/minutes reagent Time/minutes 0 0-000 6 1 0.00 1 7 2 0.002 8 3 0.002 9 4 0.002 10 5 0.002* * Boiling water added. Titre/ml of Karl E'ischer reagent 0.026 0.040 0.042 0.042 0.042 The dry nitrogen supply, which was examined in the same way, was found to be com- pletely free from water.550 ARCHER AND HILTON EXAMINATION OF GAS STREAMS IN EQUILIBRIUM WITH ICE- Two copper U-tubes were constructed from Q-inch 0.d. tubing and packed with copper turnings.About 5 ml of water were added to each tube and they were connected in series with each other. A connector from one of the tubes led into a Karl Fischer cell and a stream of dry nitrogen was connected to the other tube. The two tubes were immersed in a beaker containing acetone, which was cooled to various temperatures by the addition of solid carbon dioxide, while a stream of nitrogen was passed through the apparatus at the rate of 500mlmin-1, Water in the gas stream was measured as described under Direct sampling method. The theoretical water content was calculated from the following equation : Calculated water content, p.p.m. by mass = M w - x- fiw x 106 M , p-pw where &Iw is the relative molecular mass of water, M , is the relative molecular m a s of the gas, P is the total pressure of the gas stream and 9, is the saturated vapour pressure at the temperature of the U-tubes.Zimmerman and Lavines stated that the vapour pressure of supercooled water should be taken, but our results, given in Table 11, are more consistent with the vapour pressure of ice. TABLE I1 WATER CONTENT OF NITROGEN TemperaturelOC - 16 - 20 -21 - 28 - 34 - 36 - 37 - 42 - 49 - 50 - 58 Water, p.p.m. by mass Measured Calculated 700 960 490 655 460 690 250 297 150 158 115 127 120 116 70 66 27 28 26 26 8 9 A I 1 DISCUSSION The methods described are applicable to any gas that does not react with Karl Fischer reagent. Although no rigid proof of the validity of the methods has been attempted, it was demon- strated that water is completely removed fIom a gas stream by bubbling the gas through Karl Fischer reagent, which is continuously kept at the null-point.In addition, substantially correct results were obtained when dry gas was passed over ice at low temperatures. To give an indication of the sensitivities of the two methods, if 0.010 ml is the smallest titre that can be regarded as significant, it will correspond to 2 p.p.m. by mass in the direct method and 0.2 p.p.m. by mass in the collection method. It is assumed in obtaining these figures that the direct method is run for 30 minutes with a gas flow-rate of 500 ml min-1 and the collection method is run for 30 minutes with a gas flow-rate of 5 1 min-1. The collection method is particularly useful in plant areas where only apparatus that is “intrinsically safe” is permitted. They are absolute and hence no calibration is necessary. REFERENCES 1. 2. 3. 4. 5. 6. 7 . 8. 9 . Roman, W., and Hirst, A., Analyst, 1951, 76, 10. hluroi, K., Japan AnaZyst, 1961, 10, 847. Griies, H., and Heincke, W., Erdiil Kohle, 1661, 14, 714. Capitani, C., and Milani, E., Chim. I n d . , Milaizo, 1954, 36, 177. Hawkins, A. E., Analyst, 1964, 89, 432. Brown, J . F., and Volume, W. F., Ibid., 1956, 81, 308. Archer, 1;. E., Jeater, H. W., and Martin, J., Ibid., 1967, 92, 524. Carlstrom, A. .\., Spencer, C. F., and Johnson, J. F., Analyt. Chem., 1060, 32, 1056. Zimmerman, 0. T., and L,avine, I., “Psychrometric Tables and Charts,” Industrial Research Service Received November 15th, 1973 Accepted Apvil 8th, 1974 Inc., Dover, New Hampshire, 1964.