ISSN:0003-2654    

DOI:10.1039/AN97500FX009

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500BX011

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500FP029

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500BP035

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

MARCH, 1975 The Analyst Vol. 100 No. 11 88 Performance Characteristics of Gas-sensing Membrane Probes P. L. Bailey and M. Riley Electronic Instruments Ltd., Hanworth Lane, Chertsey, Surrey, KT16 9LF The construction and performance of gas-sensing membrane probes are des- cribed and discussed. Experimentally, probes t o measure ammonia, sulphur dioxide and nitrogen oxide in aqueous solution have been studied. In par- ticular, the effects of temperature and osmotic pressure were examined, the response speeds of the probes were determined and their use in continuous analysis was investigated. Selectivity is discussed. Consideration is given to the development of new methods of analysis and current applications of the probes are mentioned. Gas-sensing membrane probes have recently joined ion-selective electrodes in the expanding range of potentiometric sensors for species in aqueous solution, and the probes have been welcomed particularly because of their high selectivity.This paper aims to demonstrate the effect of several parameters on their performance, and to discuss some of the principles upon which they operate. In the light of the performance, consideration is given to the development of analytical procedures using such probes. Although the first gas-sensing membrane probe was developed for the measurement of carbon dioxide1S2 in 1957, it was not until more than a decade later that probes to measure ammonia were briefly Since then, more development has taken place and the range of dissolved gases for which probes are available has been extended.This range has been discussed by Ross et aZ.,5 who also proposed a model to explain the effect of some parameters on the performance of the probes. The probes are a relatively new type of device and hence the problem of nomenclature arises. They sense the partial pressure of gases in solution and thus it is appropriate to describe them as “gas-sensing.” As already adumbrated, it is proposed to describe the devices as probes instead of electrodes, as they are electrochemical cells and not simply electrodes in the conventional meaning of the word. Although the term “probe” does not give any information concerning the operation of the devices, it implies a complete system and is brief and not incorrect, as is the word “electrode.” It is necessary to add the further word “membrane” to the name in order to differentiate the probes from gas-sensing probes without membranes6 Hence the full name that is proposed for the devices is “gas-sensing membrane probes.” A recent paper,s in which gas-sensing probes without membranes are described, provokes consideration of the practical advantages conferred by a membrane.The advantages are : (a) The electrolyte layer on the probe tip does not need to be renewed after every measure- ment in order to maintain accuracy. The membrane serves to retain a bulk of in- ternal electrolyte, which stabilises the composition of the thin film by slow but continuous exchange. ( b ) As the probe can be immersed directly in the sample, no special equipment is necessary for sample presentation, and the probe can be used in continuous flow analysers.(c) Problems associated with the junction potential at the rim of the electrolyte film will be minimised by the slow but appreciable interchange of electrolyte from the bulk of the filling solution to the thin film. (d) The danger of the probe tip drying out and impairing the performance of the pH- sensitive glass is greatly reduced. 145146 BAILEY AND RILEY : PERFORMANCE CHARACTERISTICS A%Q$&, VOl. 100 (e) The thickness of the electrolyte film can be easily adjusted by alteration of the pressure of the glass electrode on the membrane; this allows simple optimisation of performance of the probe. (f) The membrane protects the internal electrolyte from attack by the air; this is par- ticularly important with the sulphur dioxide probe, in which the thin film of internal electrolyte is susceptible to oxidation.The flow of oxygen into the film is greatly reduced by a membrane. Gas-sensing membrane probes have found many applications and usually offer a much faster analysis than older methods. With the ammonia probe, satisfactory results have been reported, with respect to precision, accuracy, selectivity and lifetime in the analyses of boiler feed-water,’-g fresh waters,lO blood plasma11J2 and Kjeldahl d i g e ~ t s , l ~ ~ ~ * and also of river water, swimming-pool water, treated and untreated sewage and various trade wastes.15 In addition to these applications, the ammonia probe is currently used for the analysis of sea water, plating solutions and process liquors in fertiliser manufacture. The sulphur dioxide probe has many applications in the analysis of food and beverages, boiler water and effluents; in particular, “free” and “total” sulphur dioxide concentrations have been determined in fruit and vegetable products, sausage meat and sucrose and glucose products.Experimental Equipment The gas-sensing membrane probes used were an EIL Model 8002-200 ammonia probe, Model 8010-200 sulphur dioxide probe and a specially developed nitrogen oxide probe. This last probe was constructed from a probe body and glass electrode of the type common to all probes; the reference electrode was a silver wire coated with silver bromide, the internal filling solution was 0.4 M potassium nitrate plus 0.1 M sodium nitrite plus 0.1 M potassium bromide saturated with silver bromide, and the membrane was 0.025 mm thick polypropylene film.The internal filling solution was chosen so as to take account of the potential use of the probe for the determination of nitrates after reduction to nitrite, a process which causes the total concentration of dissolved species in the solutions finally presented to the probe to be relatively high. The reported performance of the nitrogen oxide probe does not neces- sarily represent the performance of any commercial product. The probes were used in conjunction with an EIL Model 7050 pIon meter, a Model 8000 automatic monitor and a Model 8981 ion-selective sampling unit. The sampler was a Hook and Tucker Ltd. Model A40 and the four-channel peristaltic pump was a Sage Instruments Inc.Model 371. Reagents and Standards All reagents used were of analytical-reagent grade. The water used throughout this work was distilled water, circulated through a mixed-bed de-ioniser. Ammonia standards were prepared from ammonium chloride, sulphur dioxide standards from potassium metabisulphite and nitrogen oxide standards from sodium nitrite. The buffers added for pH adjustment, in a 1 : 10 volume ratio unless otherwise stated, were 1 M sodium hydroxide solution, 2 M perchloric acid and 5.4 M sulphuric acid, respectively. The high concentration of sulphuric acid used in this work was unnecessary from the point of view of pH but necessary in order to maintain the correct osmotic pressure; a weaker acid with an inert electrolyte, such as potassium nitrate, added could have been used, e.g., 1 volume of 0.5 M sulphuric acid plus 2.1 M potassium nitrate solution per 5 volumes of sample.Construction and Operation The construction, shown in Fig. 1, was identical for the three probes used in this work. The slightly convex tip of the glass electrode is made of pH-sensitive glass. By screwing down the retaining sleeve, the glass electrode can be pressed on to the gas-permeable mem- brane, and thus a thin film of the internal electrolyte is sandwiched between the glass electrode and the gas-permeable membrane. When the probe is immersed in a sample, gas passes through the membrane until the partial pressures of gas are equal in the thin film and theMarch, 1975 OF GAS-SENSING MEMBRANE PROBES 147 sample.The equilibrium concentration of gas in the thin film determines its pH, which is measured by means of the glass electrode and the reference electrode in the bulk of the electrolyte. For ammonia, for example, the pH of the film is proportional to the logarithm of the partial pressure of ammonia in the sample. In the general case, the probe potential is related to the determinand concentration by the equation E = c (2*3RT/F) loglo [XI where X is the determinand, R is the gas constant, T is the temperature, F is the Faraday constant, E is the potential and c is a constant; the sign of the last term is positive for an acidic gas and negative for a basic gas. The derivation of this equation has been published previously. 6 9 7 Retaining Filling so I ut io I 1 Body seal Vf a s h er Membrane Thin film B C A Fig.1. Construction of probes : A, complete diagram ; B, enlarged diagram of end section; and C, flow-through cap. The fact that only gases in the sample pass through the membrane gives rise to the high selectivity of the devices, but usually also means that sample pre-treatment is required in order to convert the determinand into a form in which it can be measured. Thus, samples containing ammonium ions must be made alkaline and sulphite and nitrite samples acidified before measurement. As the devices sense partial pressure, any parameters that affect the Henry’s law constant of the systems will also affect the responses of the probes. In terms of the equilibria set up in the thin films, the ammonia probe is by far the simplest of the three probes.The nitrogen oxide probe is particularly complicated, with a complex set of equilibria presumably involving at least the five species nitrite ion, nitrate ion, nitrous acid, nitrogen dioxide and nitric oxide ; in addition, the various equilibrium constants have different temperature coefficients and some of the species are unstable. Also, in all experi- ments with the nitrogen oxide probe described in this paper, nitrite ion is considered to be the determinand because it is not known exactly what gaseous species passes from the treated148 BAILEY AND RILEY : PERFORMANCE CHARACTERISTICS Afialyst, VoZ. 100 sample, through the membrane, into the thin film. The imprecise term “nitrogen oxide” in the name of the probe reflects this uncertainty.Results and Discussion Calibration and Detection Limit The varying volatilities and abundances in the environment of the determinands affect the measurement technique, particularly at low concentrations. Thus, when the detection limits and Nernstian response ranges of the probes were measured, particular care had to be taken by use of closed systems. In practice, this situation can be achieved in several ways, some of which are described below. Normally, measurements with the ammonia probe could be made with the samples in open beakers because of the relatively low volatility of ammonia; however, for the calibration runs, a closed flow system (Model 8981 ion-selective sampling unit) was used in order to minimise contamination of the standard solutions with ammonia from the atmosphere.A different closed flow system (as shown in Fig. 4, but without the addition of air) was used with the nitrogen oxide probe. With the sulphur dioxide probe, measurements were made with samples in 100-ml conical flasks; when the probe was im- mersed in the sample, an O-ring on the probe body sealed the top of the flask. Sulphur dioxide has the highest volatility of the three determinands but the detection limit of the probe is high with respect to the concentration of sulphur dioxide that can normally be absorbed from the atmosphere. The calibration graphs obtained are shown in Fig. 2. The limit of detection of the ammonia probe has not been determined because the experimental limit is set by the purity of the available water.Similarly, for the nitrogen oxide probe, the limit of detection has not been Fig. 2. Calibration graphs: A, sulphur dioxide probe; B, nitrogen oxide probe; C, ammonia probe. determined because of interference from carbon dioxide (see Selectivity), which it is neither easy nor realistic to exclude. With the sulphur dioxide probe, the detection limit of 5 x 10-6 M (that concentration of sulphur dioxide which produces a shift of 1 mV from the potential of the probe in water) is set by the acidic characteristics of the internal electrolyte.March, 1975 Response Speed OF GAS-SENSING MEMBRANE PROBES 149 The response speed of a gas-sensing probe is dependent on several parameters, of which the following are the most important: (a) diffusion coefficient of the gas in the membrane; (b) partition coefficient of the gas between the membrane and sample; (c) membrane thick- ness; (d) geometry of the film of internal electrolyte; (e) response time of the glass electrode; (f) concentration of determinand ; and (g) temperature. A model has been proposed for the effect of parameters (a), (b), (c), (d) and (f) in the paper by Ross et aL5 Parameters (a), (b) and (c) are controlled experimentally by the choice of membrane, which will be considered further in the section on osmotic effects.Parameter (d) is controlled by the pressure of the glass electrode on the membrane, which can be adjusted by means of the retaining sleeve, and by the shape of the tip of the glass electrode. The model of Ross et al. indicates that the thinner the film, the faster the response becomes.A practical limit is reached as a result of the mechanical resistance of the membrane to stretching and increase in the junction potentials round the rim of the glass electrode. The useful limit of membrane tension is reached appreciably before the membrane breaks. In order to produce a film that is very thin across the whole surface of the pH-sensitive glass, it is advantageous to have a slightly convex tip on the glass electrode. This shape has recently been introduced in place of the nominally flat tip used previously and has notably improved response times. Parameter (e) normally does not need consideration as the response speed of the glass electrode is fast with respect to the rate at which equilibrium is reached in the thin film.However, after several months in use, the response of the probe starts to become sluggish because of deterioration in the response of the glass electrode; the continuous use of such electrodes in a poorly buffered medium inevitably leads to a shorter than usual lifetime. The response can be restored by alternate treatment of the glass electrode with sodium hydroxide solution and hydrochloric acid. The effect of temperature, (g), on response speed has not been examined, although it has been shown that the response is faster at higher temperatures; no change in response speed was observed for temperature changes of less than 5 "C. The response times of the three probes were measured at room temperature (22-25 "C). The definition of response time adopted is the time taken for the probe response to reach a value 1 mV from the equilibrium value after a ten-fold increase or decrease in the concen- tration of determinand.The solutions, of volume 55 ml, were contained in 100-ml conical flasks and sealed by the probe as described previously. The solutions were stirred at approxi- mately 250 r.p.m. When the probe had reached an equilibrium value in one solution, it was removed from the solution, the flask containing the second solution placed on the stirrer, the end of the probe blotted with tissue-paper and the probe immersed in the second solution. A recording of the probe output was made on a chart recorder, on which the instant the probe was put into the second solution was noted and the final equilibrium value eventually determined.Hence, by chart measurement, the response times were calculated. The time taken to transfer the probe between the two solutions was about 6 s but the probe membrane was exposed to the air for only 1-2 s; the error produced from this source can be estimated, therefore, as a maximum of 2 s, which is negligible in the present context. The results of these response time measurements are presented in Table I. Using the probes in the flow system shown in Fig. 4, the response times for increases in concentration were similar to those recorded above, but the response times for decreases were substantially greater at low concentrations (less than lo4 M). In general, the results show that the response is faster at higher concentrations and for concentration increases, as predicted by Ross et aLs Also, the response times for decreases in concentration are less reproducible than for increases as they partly depend on the time of immersion of the probe in stronger solutions, which affects the uptake of sensed gas into the bulk of the internal electrolyte round the liquid junction.The response times found for the ammonia probe are shorter than those published pre- vi0usly,~~1* almost certainly because of the improved shape of the glass electrode mentioned above. The acceptability of long response times, such as are observed with the sulphur dioxide probe at low concentrations, must be assessed in the light of both the other performance150 BAILEY AND RILEY : PERFORMANCE CHARACTERISTICS Aaalyst, VoZ. IOU data and also the time taken for alternative methods of analysis. Even a response time of 5 min in a sulphur dioxide analysis is acceptable if the alternative is a distillation procedure that requires 1 h.The response can be accelerated by changing to a different membrane, but this procedure leads to osmotic problems, which outweigh the advantage. TABLE I RESULTS OF DUPLICATE DETERMINATIONS OF PROBE RESPONSE TIMES Timels Final concentration/M Ammonia probe Decadic concentration increases- 10-5 105, 110 10-4 32, 33 10-8 31,31 10-2 31, 30 Decadic concentration decreases- 10-6 120, 70 10-4 66, 34 10-8 32, 30 Sulphur dioxide probe Nitrogen o&de probe 370, 480 - 426, 336 95, 92 36,36 34,38 32,27 - I 156, 166 68, 42 460, 410 125, 66 Osmotic Effect It is inherent in the design of the probes that a semi-permeable membrane separates two solutions, the sample and the internal electrolyte film.If the total concentration of dissolved species differs on the two sides of the membrane, an osmotic pressure difference results and transfer of water vapour across the membrane occurs until the water activity is the same on each side. An osmotic pressure difference also arises when there is a temperature difference between the sample and thin film; as previously pointed out,S this difference can lead to very large rates of transfer of water compared with those produced by differences in the total concentrations of dissolved species. Transfer of water across the membrane will cause dilution or concentration of the elec- trolyte in the film, which in turn causes the probe potential to drift.Whether or not this drift is observed will depend on whether the change in concentration in the film is fast with respect to renewal of the film by interchange with the bulk of the internal electrolyte. With the ammonia and sulphur probes, drift is not observed if the sample is weaker than the internal electrolyte; for the ammonia probe, if the sample is stronger than the internal elec- trolyte, then drift does occur. In principle, equilibrium is eventually reached, but in practice this is seldom observed as the system (e.g., the ammonia probe) approaches equilibrium asymptotically over several hours. The magnitude of the drift will depend on both the osmotic pressure gradient across the membrane and also the permeability of the membrane to water.The nature of the membrane will substantially affect its permeability to water vapour. Two types of membrane, microporous and homogeneous, can be distinguished. In micro- porous membranes, transfer of gas takes place through the air in the pores of the membrane; the solid material of the membrane is in no sense selective as it merely serves to separate the electrolyte film from the sample. On theoretical grounds, it is not possible to distinguish between the response mechanism of a gas-sensing membrane probe with a microporous membrane and an “air-gap electrode.”6 The ammonia and nitrogen oxide probes contain microporous membranes, the former 0.1 mm thick PTFE and the latter 0.025 mm thick polypropylene. For homogeneous membranes, of which the 0.025 mm thick silicone rubber used in the sulphur dioxide probe is an example, different considerations apply.As suggested by the model of Ross et the important parameter is the product (Dk) of the diffusion coefficient of the diffusing species in the membrane (D) and the partition coefficient of the species between the membrane and the aqueous sample (k). The values of Dk quoted by Ross et aZ.5 show less variation between the different gases in silicone rubber than in air. However, by accepting a slower speed of probe response to the primary gas (sulphur dioxide) , the response of the probe to water vapour is made negligible.Maich, 1975 OF GAS-SENSING MEMBRANE PROBES 151 In order to determine the magnitude of the osmotic effect with the three probes, the drift rate of each probe was measured in a solution of the determinand in 3 M sodium chloride at a concentration in the middle of the working range of the probe.The values given in Table I1 are the initial drift rates. TABLE I1 RESULTS OF REPLICATE DETERMINATIONS OF INITIAL DRIFT RATES OF THE PROBES IN SOLUTIONS OF DETERMINAND I N 3 M SODIUM CHLORIDE Probe Concentration of determinand/M Initial drift rate/mV min-l Ammonia .. * . .. Sulphur dioxide , . .. Nitrogen oxide . . .. 6 x lo-' 1 x 10-8 1 x 10-4 - 12, - 19, - 14, -22 0.00, 0~00,0~01, 0.00 -2, -2 The drift rate of the ammonia probe decreased steadily over 2 h. If, after returning to samples that did not contain 3 M sodium chloride, the glass electrode was loosened and then re-tightened so as to renew the thin film of internal electrolyte, the probe behaviour reverted to normal immediately. In the experiments with the sulphur dioxide probe, there was no measurable drift (less than 0.1 mV) over 30 min in three experiments and only 0.2 mV in 20 min in the other experiment; the low drift rate of 0.01 mV min-l is not significant. The probe afterwards responded normally in samples that did not contain 3 M sodium chloride.The nitrogen oxide probe drifted at the quoted rate for about 5 min; the probe then became very insensitive to changes in the concentration of determinand and the membrane had to be replaced. There are two alternative means by which osmotic effects due to samples of high dissolved solids content can be eliminated. The first, utilised in the ammonia and nitrogen oxide probes, is to add sufficient inert electrolyte to the internal filling solution to balance the osmotic pressures on the two sides of the membrane.As osmotic pressure is a colligative property, sufficient of the inert electrolyte is added, as a first approximation, to equate the total concentration of dissolved species in the internal filling solution to that in the sample; such a procedure is essential if, for example, an ammonia probe is used to analyse sea water or Kjeldahl digests.13J4 The second means, utilised in the sulphur dioxide probe, is to select a membrane that is relatively insensitive to osmotic pressure; although this has the dis- advantage of reducing the response speed it is, in this instance, the better of the two alterna- tives because, in practice, the sulphur dioxide probe has many applications in which the osmotic pressure of the sample is not only high but also extremely difficult to assess.Selectivity A gas-sensing membrane probe suffers direct interference only from dissolved gaseous species in the treated sample that produce a change of pH in the thin film. That such species are not encountered in most applications is a measure of the high selectivity of the devices. The ammonia probe has been found to suffer interference only from volatile and filming amine.~.~J~ The sulphur dioxide probe suffers interference from concentrated hydrochloric acid and hydrofluoric acid, but the only analytically important interference is from acetic acid. Interference effects are shown in Fig.3. This interference prevents the use of the probe for the analysis of low concentrations of sulphur dioxide in pickle products; however, many chutnies and similar products contain sufficient sulphur dioxide to permit dilution of the sample to the level at which interference from acetic acid is negligible. Once severe inter- ference has occurred, the response of the probe becomes sluggish; soaking the probe in water for 15 min and renewal of the thin film are necessary in order to restore performance. Carbon dioxide does not interfere. Oxidising gases, e g . , chlorine, do not normally co-exist in solution with sulphur dioxide. Excess of chlorine in a sample will pass through the probe membrane and destroy the internal electrolyte. For the nitrogen oxide probe, carbon dioxide is, in practice, the most important interferent and limits the sensitivity of the probe.If the concentration of carbon dioxide in a sample is fixed at 1 0 - 3 ~ and the concentration of nitrogen oxide decreased in stages, there is no152 BAILEY AND RILEY : PERFORMANCE CHARACTERISTICS AnaEyst, VoZ. ZOO 380 I I I 0 1 2 3 4 ' - [CH,COOH] Fig. 3. Interference of acetic acid with the sulphur dioxide probe. [SO,] = lo-* M. interference down to lo4 M. However, at M the probe responds sluggishly and there is a positive potential shift of 8mV. This result accords with observations, made during the calibration experiments, that the probe drifts to more positive potentials in solutions with nitrogen oxide concentrations less than M prepared with carbon dioxide free water.Such drift is not observed with similar solutions in equilibrium with the air and, as the equilibrium concentration of carbon dioxide is about M, this implies a similar selectivity to that indicated above. Continuous Flow Analysis The response of the probes is sufficiently rapid for them to be used for the continuous flow analysis of discrete samples. For this purpose, a flow-through cap, shown in Fig. 1 (C), has been designed to fit on to the probe body in place of the standard end-cap. Probes fitted with such flow-through caps were used in experiments with the flow system shown in Fig. 4; samples were separated by appropriate wash solutions in the usual way. Pump r--7 Probe \ Waste Sampler Fig. 4. Diagram of flow system.Examples of the traces obtained with the different probes are shown in Figs. 5 and 6, ammonia samples being analysed at the rate of 60 per hour and sulphur dioxide samples at 30 per hour. These traces suggest that it is possible to obtain good results at even higher sampling rates, which has been confirmed, e.g., by experiments with the sulphur dioxide probe involving repetition of the sequence of samples shown in Fig. 6 at the rate of 60 per hour. The ammonia probe has been used in this way in total nitrogen analysis by the Kjeldahl method14; after the digestion step, the acidic digest is made alkaline and its ammonia content determined directly with the probe. The sample becomes hot after the addition of alkali and the mixing coil and probe body were therefore immersed in a thermostatically controlled water-bath so as to ensure thermal stability; there is no danger of loss of ammonia because the addition of alkali is made in a closed system.March, 1975 OF GAS-SENSING MEMBRANE PROBES Glass 153 KH,PO, (0.176 M) N+HPO, (0.076 M) KC1 (0.08 M) Ag, AgCl AgCl (s) .Gelling agent 110 mV 200 s H 1 10 mg I-' mg :,$ I-' Time + Fig. 6. Response of an ammonia probe in the flow system. Buffer: 0.7 M sodium hydroxide. Wash solution : 0.5 mg 1-1 NH,+. Sampling time: 40 s. Wash time: 20s. Pumping rates: sample, 4.1 ml min-I; buffer, 0.7 ml min-1; air, 2.0 ml min-1; wash, 4.1 ml min-l. mV 300 s c-----l 50 mg I-' d 'I Time + Fig. 6. Response of a sulphur dioxide probe in the flow system. Buffer: 2 M perchloric acid. Wash solution : 25 mg 1-1 SO, (as K,S,O,).Sampling time: 70 s. Wash time: 60 s. Pumping rates: sample, 4.1 ml min-1; buffer, 0.6 nil min-I; air, 2.0 ml min-I; wash, 4.1 ml min-I. Temperature Effects Because of the complexity of the system, the effect of temperature on the probes is difficult to characterise. The only significant temperature coefficient that can be measured refers to the usually unrealistic state when the probe and sample are both in a thermostatically controlled environment of variable temperature ; such coefficients were determined in an EIL Model 8000 automatic analyser. In this analyser, both the buffer and sample are pumped by peristaltic pumps past the probe mounted in a flow cell in a thermostatically controlled cabinet; the temperature in the flow cell is constant to within &Om2 "C.The electrochemical cell of which the ammonia probe, for example, consists can be repre- sented by Bulk internal Thin film electrolyte Liquid junction The different parts of the cell have different temperature coefficients and thermal capacities, and thus the over-all temperature coefficient will apparently vary with time until thermal equilibrium is achieved. The output of the ammonia probe takes much longer than the other probes to reach equilibrium, presumably because an increase in the temperature of the sample induces osmosis, that is, transfer of water vapour into the thin film. As the tem- perature of the thin film slowly approaches the temperature of the sample, the transfer of water vapour decreases and then reverses in direction until the original electrolyte concen- tration in the thin film is restored.The second osmotic step is slow and is responsible for the long equilibration time of the ammonia probe. A further problem with the ammonia system is that the equilibrium constant, K , has a high temperature coefficient (about -0.28 1 mol-1 "C-l) and thus the probe has a high temperature coefficient. If the internal electrolyte of the glass electrode is changed to an ammonia - ammonium chloride buffer, the temperature coefficient is halved, but the drift caused by osmosis remains the predominant temperature effect. The temperature coefficients for the probes in a Model 8000 automatic analyser are listed in Table 111.154 BAILEY AND RILEY : PERFORMANCE CHARACTERISTICS Analyst, VoZ.100 TABLE I11 RESULTS OF REPLICATE DETERMINATIONS OF THE PROBE TEMPERATURE COEFFICIENTS Concentration of Temperature Temperature Probe determinand/M coefficientlmv "C-l range/"C Ammonia .. .. .. 6 x lo-' 1.6, 1.0 28-36 Sulphur dioxide .. .. 1 x 10-8 0-5, 0.6, 0.6, 0.6 26-43 Nitrogen oxide . . . . . . 1 x 10-4 0.2 25-3 1 0.0, 0.0, 0.0, 0.1, 0.0 31-46 The curves obtained in experiments with the ammonia and sulphur dioxide probes are shown in Fig. 7. The shape of the curve for the ammonia probe was different from the shapes of those for the sulphur dioxide and nitrogen oxide probes, which were similar. The trough in the curve for the ammonia probe can be attributed to osmosis occurring before the probe has reached thermal equilibrium, as discussed above. This result again highlights the advan- tage of using a membrane that is less permeable to water vapour on the sulphur dioxide and nitrogen oxide probes.0 25 50 75 100 125 Ti melm in Fig. 7. Curves of potential variation with time obtained after a temperature change for the ammonia and sulphur dioxide probes. A, Ammonia probe: [NHJ = 6 x 1 0 - 4 ~ ; 31.3-34.6"C. B, Sulphur dioxide probe: [SO,] = lo-* M; 31.0-36.7 "C. If a probe is used under laboratory conditions, with the bulk of the probe at room tempera- ture and the samples at varying temperatures, a different behaviour is observed. The apparent temperature coefficient of the ammonia probe increases to about 2 mV "C-1 and the sulphur dioxide probe becomes virtually insensitive to temperature. Thus, in the use of an ammonia probe, care must be taken to ensure that standards and samples are kept at the same tem- perature and that the probe is not subjected to rapid temperature fluctuations, for example by exposure to direct sunlight.Development of Method In the development of an analytical procedure involving the use of a gas-sensing membrane probe, there are a number of factors that require consideration; many of these problems have been discussed in previous sections but, for convenience, all of the important factors can be listed as follows: (a) sample pH; (b) sample temperature; (c) sample osmotic pressure; (d) state of determinand; (e) potential interferents; (f) volatility of determinand; (g) stirring of sample; and (h) standardisation. (a) The pH of samples must be adjusted, if necessary, in order to convert virtually all of the determinand into the state of dissolved gas.For the determination of ammonia, the sample, when presented to the probe, should have a pH above 12. In sulphur dioxide deter- minations, the pH should be below 0.7; the buffering is achieved by the addition of a non- volatile acid such as sulphuric or perchloric acid. Buffering at a higher pH, at which only a fixed proportion of the sulphur dioxide is present as the dissolved gas, reduces the sensitivity of the procedure and requires precise pH adjustment if accuracy is to be maintained. The same argument applies to the ammonia probe. For the determination of nitrogen oxide, the sample must also be acidified.March, 1975 OF GAS-SENSING MEMBRANE PROBES 155 (b) The temperatures of samples and standards should be as closely similar as possible, ideally within 0.5 “C, so that the Henry’s law coefficient is constant throughout the experiment, and so that the probe remains at constant temperature.(c) The osmotic pressure of the sample as presented to the probe is an important con- sideration in the use of the ammonia and nitrogen oxide probes. In the application of the ammonia probe, the osmotic pressure of the internal electrolyte must be adjusted by addition of inert electrolyte (e.g., potassium sulphate or sodium chloride) if the total concentration of dissolved species in the sample exceeds 0.3 M. As a first approximation, sufficient elec- trolyte should be added to increase the total concentration of dissolved species from 0-2 M to that of the sample.Often, such adjustment can be avoided by dilution of the sample if the concentration of the determinand permits. Dilution of the sample is advantageous even if it is not possible to make the osmotic effect negligible, as the lower the osmotic pressure the less precisely does the adjustment have to be made. With the nitrogen oxide probe, all samples must be adjusted so as to have a total concentration of dissolved species of 1.20 & 0-06 M by the addition of an appropriate buffer, as in the experiments described in this paper. (a) In many ammonia samples, a significant proportion of the ammonia is complexed with metals such as copper and zinc. If it is required to measure this complexed ammonia also, EDTA must be added to the buffer; a buffer containing 50 g 1-1 of sodium hydroxide and 15 g 1-1 of EDTA added in the ratio of one volume to ten volumes of sample may be used.The use of such a buffer also has the advantage of preventing precipitation of many metal hydroxides that could absorb ammonia and block flow systems. Sulphur dioxide in many foods and beverages is partly present bound in aldehyde - hydrogen sulphite addition compounds, which are not decomposed by simple acidification. Hence “free” sulphur dioxide can be measured by acidification. “Total” sulphur dioxide can be measured after a preliminary treatment with alkali in order to decompose the addition compounds; decomposition is very rapid at pH values above 12.5. The sample is then acidified, as before, to a pH below 0.7 and the reading of “total” sulphur dioxide made.Recombination of the sulphur dioxide with the aldehydes is very slow compared with the response speed of the probe. (e) The possibility of chemical interference with the probe response must be considered. If possible, interferents should be eliminated. (f) Measurements with the sulphur dioxide and nitrogen oxide probes should, as far as possible, be taken in closed systems. The conical flask arrangement described previously has been found to be satisfactory; alternatively, the use of a closed flow system eliminates any problem due to the volatility of the determinand. (g) All samples and standards must be stirred. In a few instances, such as in the analysis of sausage-meat slurries with the sulphur dioxide probe, vigorous stirring is recommended so as to prevent particles of the sample from adhering to the probe.(h) The accuracy of measurements with a probe is limited by the accuracy of the standards. This is a particularly important consideration with the sulphur dioxide probe, as solutions of potassium metabisulphite or sodium sulphite deteriorate rapidly; hence, for the most accurate measurements, the stock solution should be standardised by an iodine - thiosulphate tit rat ion. The treatment of samples and standards should be identical. It is particularly important to keep the total dilution of samples and standards by buffers the same. If a preliminary treatment is necessary, as in the decomposition of aldehyde - hydrogen sulphite addition compounds in foods, allowance must be made for the increased dilution of the sample. Although all of these points must be considered in the development of a new method, the resultant procedure will usually be straightforward and accomplished in a much shorter time than by conventional methods. The authors thank Electronic Instruments Ltd. for permission to publish this paper and gratefully acknowledge the assistance of their colleague Mr. C. L. Jamson. References 1. 2. Stow, R. W., Baer, R. F., and Randall, B. F., Archs. Phys. Med. Rehabil., 1957, 38, 046. Severinghaus, W., and Bradley, A. F., J. Ap$l. Physiol., 1958, 13, 615.BAILEY AND RILEY Briggs, R., Paper No. 12, Proceedings of the Water Resources Board Conference on Data Retrieval Okada, M., and Matsusita, H., J . Chem. SOC. Japan, Ind. Chem. Sect., 1969, 72, 1407. Ross, J , W., Riseman, J. H., and Krueger, J. A., Pure A+pl. Chem., 1973, 36, 473. RbiiCka, J., and Hansen, E. H., Analytica Chim. Acta, 1974, 69, 129. Midgley, D., and Torrance, K., Analyst, 1972, 97, 626. Midgley, D., and Torrance, K., Analyst, 1973, 98, 217. Mertens, J., Van den Winkel, P., and Massart, D. L., Bull. SOC. Chim. Belg., 1974, 83, 19. Beckett, M. J., and Wilson, A. L., Wat. Res., 1974, 8, 333. Sanders, G. T. B., and Thornton, W., Clinica Chim. Acta, 1973, 46, 465. Park, N. J., and Fenton, J. C . B., J . Clin. Path., 1973, 26, 802. Todd, P. M., J . Sci. Fd Agric., 1973, 24, 488. Buckee, G. K., J . Inst. Brew., 1974, 80, 291. Evans, W. H., and Partridge, B. F., Analyst, 1974, 99, 367. and Processing, Reading University, January, 1969. Received August lst, 1974 Accepted November llth, 1974 156 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 16.

ISSN:0003-2654    

DOI:10.1039/AN9750000145

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Analyst, March, 1975, Vol. 100, pp. 157-162 A Flow-through Electrode Unit for Measurement 157 of Particulate Atmospheric Nitrate Larry J. Forney and John F. McCoy Department of Civil Engineering, University of Illinois, Urbana, Ill. 61801 , U.S.A. Walden Research Division of Abcor Inc., Cambridge, Mass. 02139, U.S.A. The design and performance of a flow-through electrode unit for use in the measurement of particulate atmospheric nitrate is described. The unit, housing ion-selective nitrate indicator and fluoride reference electrodes, pro- vides temperature control (f0.5 "C), a sensitive regulation of solution content (44 f 1.6 ml) and optimum liquid circulation, while minimising liquid volume. Preliminary tests conducted at 38.5 f 0.6 "C indicate that the unit behaves as an ideal well stirred vessel with a time constant T = V/Q.The nitrate-selective electrode has been used successfully to measure nitrate levels in soils,lp2 plants,5 wateI.4 and microbiological media,5 and to determine the concentration of nitrogen oxides in flows of combustion effluents.6~7 More recently, the nitrate electrode has been introduced into a continuous flow system designed to monitor the atmospheric particulate nitrate as described by Driscoll and Forney.8 In this application, a fixed volume of de-ionised water is recirculated through an aerosol impaction device (ERC LEAP sampler, Model 3440) in which nitrate particulate is collected and dissolved. If ideal aerosol collection is assumed, the nitrate concentration, C, within the monitor recirculation volume is given by .... .. * * (1) CQt VT C (moll-l) = 1.61 x - where pg rn+ is a time-average atmospheric nitrate concentration, Q 1 min-l is the air sampling rate, t min is the sampling time and VT ml is the recirculation volume. As the nitrate liquid ion exchange is only partially selective for nitrate ions and responds to a number of other dissolved species, as indicated in Table I, it was necessary to determine which of the species were potential interferents in atmospheric particulate. As described TABLE I POTENTIAL INTERFERENCES IN THE NITRATE ELECTRODE METHOD OF ANALYSIS~ K, = 1.0 for NO,-. Selectivity constants Present in atmospheric Species (Kz) particulate c10, 103 No I- 20 Y e s C103- 2 N o Br- 0.13 Y e s HS- 0.04 Yes NO,- 0.04 Y e s CN- 0.01 N o HC0,- 9 x 10-3 Y e s c1- 4 x 10-3 Y e s CH,COO- 4 x 10-4 No C03a- 2 x 10-4 Yes by Driscoll and Forney,* two of the most serious interferents, perchlorate and chlorate, are not present in atmospheric particulate while anticipated levels of bromide, iodide and sulphide were eliminated by using a 1 0 - 2 ~ silver fluoride collection solution.Thus the silver ion eliminated the potential halide and sulphide interferences while a source of fluoride was provided for a fluoride reference electrode (see Fig. 1). Preliminary atmospheric tests using the monitor indicated that the device yields nitrate levels comparable with those from standard sampling techniques (e.g., high volume sampling with wet-chemical filter analysis8) with substantial savings in time.158 AlzaZyst, VoZ.100 This application of the nitrate electrode necessitated the design and evaluation of a continuous flow-through electrode unit that provides several simultaneous functions. Briefly, these functions are reproducible stirring of the test solution, a constant temperature, a steady liquid level or recirculation volume, a minimal solution volume, provision for solution inflow and outflow and housing for the necessary instrumentation. After a critical review of existing flow-through units, in particular those described by RdiiEka and TjellO and Milham,lo it was found that none adequately provided all of the necessary functions. It is the purpose of this paper to describe in detail the design and evaluation of the flow-through electrode unit. FORNEY AND MCCOY: A FLOW-THROUGH ELECTRODE UNIT FOR Instrument Design A schematic diagram of the nitrate monitor as discussed in detail in a Walden Research Division Report,ll including the flow-through unit, is shown in Fig.1. The electrodes used were the Orion Model 92-07 nitrate electrode, used as indicator, and Model 94-09 fluoride electrode, used as reference, with the voltage difference indicated by an Orion 701 digital pH/mV meter. Housing lucite. adapt t o the dimensions of the probe. The housing of the flow-through unit was machined from a solid cylindrical block of This arrangement provided a sturdy, transparent dielectric cell that was easy to I unit J Fig. 1. Schematic diagram of flow-through electrode unit and associated nitrate monitor instrumentation. As seen in equation (l), the response time of the monitor is inversely proportional to the recirculation volume.As a large fraction of the fluid resides within the flow-through electrode unit, a small solution volume was sought, which, at the same time, would provide adequate circulation. An optimum design, indicated in Fig. 1, was found to be a solution cavity in the shape of an inverted cone. This design provided both solution depth and sensitivity to small changes in liquid level, while allowing for free circulation at the baseMarch, 1975 MEASUREMENT OF PARTICULATE ATMOSPHERIC NITRATE 159 of the cavity. The solution volume was determined to be V m 44 ml with probes, electrodes and stirring bar inserted. Temperature Control The nitrate monitor is intended for use in the field under ambient weather conditions. As the voltage response of the nitrate indicator electrode and fluoride reference electrode is temperature dependent, it was necessary to maintain the temperature of the solution cavity at a fixed value outside the normal range of ambient temperatures.Our method was to heat and control the temperature of the nitrate solution at 38.5 "C with an immersion heater (Pyrex, No. 33847426) coupled with a temperature probe (Versa Therm, Model 8446), all of which are shown in Fig. 1. In order to minimise non-uniform temperatures within the solution and to ensure adequate circulation, the immersion heater was located at the centre of the solution cavity at a depth such that no appreciable direct heating of the lucite housing was encountered.Stirring Elimination of inhomogeneities within the solution cavity and a reproducible stirring rate were provided by the magnetic stirring bar unit indicated in Fig. 1. In order to achieve a slow, steady stirring rate, it was necessary to adapt a high-torque, slow-revolution (240 rev min-l) synchronous motor to a revolving magnet removed from a commercial stirring unit. This arrangement provided adequate circulation and minimised bubble formation. These results are consistent with the flow-through unit of Milhamlo under similar conditions. Liquid Level Control Collection fluid in the nitrate monitor was recirculated from the flow-through unit through the LEAP sampler: as indicated in Fig. 1. Briefly, a peristaltic pump within the LEAP sampler recirculates the collection fluid from the flow-through unit at &lo ml min-l to a revolving disc adjacent to an air flow of 600 1 min-l within the LEAP sampler, where the solution spreads in a thin layer over the aerosol collection disc.Our laboratory tests indicated that the fluid evaporated from the disc at a rate of about 1 ml min-l, depending on ambient temperature and the LEAP air flow-rate. In order to replace the evaporative losses and to maintain a constant recirculation volume, V,, in equation (l), it was necessary to add make-up solution to the system automatically. This addition was accomplished by adapting three liquid-level sensing probes (Dyna-Sense, No. 7186-2) to the lucite housing, as indicated in Fig. 1, in such a way that the liquid level in the solution cavity was maintained between the level of two platinum probe tips.The position of the inverted-cone solution cavity below the probe level was found to provide a geometry extremely sensitive to small changes in solution volume. Evaluation of Instrumentation and Results A series of tests were conducted on the flow-through electrode unit in order to evaluate its performance and the results are indicated below. Liquid Level Control The monitor was filled with collection solution and allowed to recirculate for several minutes until the liquid-level controller triggered the addition of make-up solution. When a solenoid valve closed,'l indicating a full volume of collection solution, the system was drained manually until the level controller was re-activated. In preliminary tests, with a rough setting of the sensing probe positions, the liquid volume was maintained to within 3 ml, limiting the error in recirculation volume (V, plus liquid volume in tubing and sampler) to less than 5 per cent.This error could be decreased by making further adjustments. For the purpose of this nitrate measurement, it was not necessary to control the volume closely, as the collection solution was doped with fluoride ions and a fluoride-selective electrode was used as reference. However, it was necessary to re-fill the system accurately with a known volume of collection solution at the beginning of a run. Temperature Response The transient response of the unit to temperature changes was determined by measuring the time necessary to bring the entire stirred solution volume from room temperature to160 FORNEY AND MCCOY: A FLOW-THROUGH ELECTRODE UNIT FOR Analyst, T/'Ol.100 48 "C. This measurement was accomplished by recording thermometer readings of the solution temperature after activation of the immersion heater. The results are indicated in Fig. 2. Although this test is considered to be far more severe than that anticipated under normal conditions in which only the incoming fluid need be heated, the solution volume 2 Heater Illrlllllr 0 1 2 3 4 5 6 7 8 9 ' Time/min Fig. 2. Transient temperature re- sponse of flow-through unit. Flow- rate = 6 mlmin-1; cell volume = 44 ml; and immersion heater voltage = 40V. reached 48 6 0.6 "C in about 4 min with the immersion heater operating at 40 V (about 11 W) while maintaining a flow-rate through the cell of 6 ml min-l.The response time of 4 min is much shorter than the characteristic time associated with ambient temperature changes. Electrode Response In order to minimise the response time of the electrodes to changes in concentration within the solution cavity, the mixing characteristics of the cavity should approach that of an ideal stirred vessel in which the feed is dispersed instantaneously throughout. Thus, assuming an ideal stirred vessel of volume V through which a solution is flowing at a rate Q, one has1% t [NO3-] = ([NO3-], - [N03-li) e-; + [N03-Ji . . .. - * (2) where p03-],, is the initial nitrate concentration within the solution cavity at f = 0, [NO3-Ii is the inlet nitrate concentration and r = V/Q is the solution residence time.Moreover, if the concentration of nitrate within the vessel is measured with a nitrate indicator electrode and fluoride reference electrode operating within the Nernstian region (about 4 x 10-6-10-2~ NO3-) where the electrode voltage response is given by .. * * (3) .. . . RT [NO -3 AE = AE, - - I n 2 nF [F-] one can combine equations (2) and (3) to yield where AE is the relative potential, AE, is the standard potential, R is the gas constant, T is the absolute temperature, n is the number of electrons transferred and F is the Faraday const ant. In order to check the validity of equation (4), two tests were conducted using a gravity-fed nitrate solution to the flow-through unit. The results of both tests are shown in Fig. 3, for which the parameters of the system were as follows: AE, = 73 mV; RT/nF = 26.9; T = 38.5 "C; n = 1; [F-] = As can be seen in Fig.3, the response of the flow-through unit conformed closely to that of a well stirred vessel. The small drift in the results relative to the theoretical results was M; and V = 44 ml.March, 1975 MEASUREMENT OF PARTICULATE ATMOSPHERIC NITRATE 161 caused by a decrease in head above the unit as the experiment progressed. Bubble Formation With a cell operating temperature of 38.5 "C, it was found that small bubbles formed on the inside surfaces of the solution cavity and that these bubbles occasionally lodged in a position adjacent to the liquid membrane of the nitrate electrode, causing an unstable electrode potential. This problem has previously been reported as one of the greatest diffi- culties encountered in the use of flow-through cells.B However, we found that the use of 1.0 2.0 3.0 t/r 4.0 5.0 Fig. 3.Transient voltage response at flow-through unit using test parameters of Table I. Solid lines M, M, predicted by equation (4). M, [NO,-], = 3 X Q = 17.6 ml min-l, 7 = 2.5 min; 0 indicates response with [NO,-], = 3 x lo-$ M, [NO,-], = 3 X Q = 15.7 ml min-l, T = 2.8 min. A indicates response with [NO,-], = 3 x a stirring rate of 240 rev min-l a t 38 "C normally allowed a 4-h sampling period without interference from bubbles. This problem might be eliminated either by maintaining the solution at a temperature below ambient so that gas is not desolved from the solution, or by directing a jet of fluid at the liquid membrane of the nitrate electrode.Work is currently under way to eliminate this problem. Conclusions The flow-through electrode unit incorporating an inverted-cone solution cavity as des- cribed in this paper provides reproducible stirring of the test solution, a constant solution temperature above ambient, a steady liquid-level recirculation volume and housing for the necessary instrumentation, while minimising the solution volume. The unit was found to behave as an ideal well stirred vessel with a time constant r = V/Q. Preliminary tests indicated that the unit is suitable for use in atmospheric nitrate particulate analysis in conjunction with a commercially available aerosol collection device. The work described in this paper was performed by the Walden Research Division of Abcor Inc., pursuant to EPA Contract No. 68-02-0591. References 1. 2. 3. 4. 5. 0ien. A., and Selmer-Olsen, A. R., Analyst, 1969, 94, 888. Mahendrappa, M. K., Science, N.Y., 1969, 108, 132. Paul, J. L., and Carlson, R. M., J. Agric. Fd Chsm., 1968, 16, 766. Orion Research Inc. Catalogue, Cambridge, Mass., 1969, p. 961. Manahan, S. E., Appl. Microbiol., 1969, 18, 479.162 FORNEY AND MCCOY DiMartini, R., Analyt. Chem., 1970, 42, 1102. Driscoll, J. N., Bergen. A. W., Funkhouser, J. T., and Sommers, C. S., J . Air. Polluf. Colztrol Driscoll, J. N., and Forney, L. J., in Stevens, R. K., Editor, “Analytical Methods Applied to Air RfiZiCka, J., and Tjell, J. C., Analytica Chim. Ada, 1969, 47, 475. Milham, P. J., Afialyst, 1970, 95, 758. Walden Research Division of Abcor Inc., Final Report for EPA Contract 68-02-0591, Cambridge, Perry, J. H., Editor, “Chemical Engineers’ Handbook,” McGraw-Hill Book Co., New York, 1963. Received May 16fh, 1974 Accepted July 2294 1974 0. 7. 8. 9. 10. 11. 12. Ass., 1972, 22, 119. Pollution Measurements,” Ann Arbor Science Publishers, Ann Arbor, 1974. Mass., 1974.

ISSN:0003-2654    

DOI:10.1039/AN9750000157

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Analyst, March, 1975, Vol. 100, PP. 163-167 163 Natural The Determination of Water in Modified Karl Fischer Titration Apparatus Gas Using a R. J. Davies British Gas Corporation, Research and Develofiment Division, Midlands Research Station, Wharf LaNe, Solihull, West Midlands, B91 2 J W The Karl Fischer titration procedure has been extensively applied in the determination of water in solids and liquids, but not in gases. In this paper a simple method for the determination of water in natural gas by use of a modified version of a commercially available Karl Fischer titration apparatus is described. Sample gas is passed at a known flow-rate through an involatile solvent containing a measured volume of standardised Karl Fischer reagent. Complete absorption and reaction of the water in the gas in and with the Karl Fischer reagent occurs.The time taken for complete neutralisation of the Karl Fischer reagent is measured and the water content of the gas deduced. The procedure has been successfully applied to the analysis of gases con- taining from 40 to 200 v.p.m. of water with the results showing a standard deviation of 3-4 per cent. Where the composition of the gas has allowed comparisons, results agree with those obtained by gravimetric, phosphorus(V) oxide cell monitor and dew-point measurement procedures. The distribution of natural gas is now extensive and in order to provide reserves to satisfy peak demand, natural gas is being stored in the liquid state at various plants in the United Kingdom and elsewhere. Before liquefaction, easily condensable trace components, such as methanol, water, carbon dioxide and hydrogen sulphide, must be removed from the gas in order to avoid problems associated with blockage caused by freezing out, for example, in heat exchangers. The concentration of these constituents that can be tolerated in the raw gas and in the various sections of the cooling plant needs to be known, and a method for their accurate measurement at trace levels is highly desirable.Methods are well established for three of the substances mentioned, but not for water. Hygrometers and other instruments, such as those based on the phosphorus(V) oxide cell, can be used for the determination of water in some gases but they are affected by condensable hydrocarbons and methanol in natural gas, so that incorrect values are obtained.The Karl Fischer procedure for the determination of water, which is free from interference by these components, was attractive as an alternative and reference method. The technique described in this paper, using a robust, commercially available apparatus, which suits the needs of on-site analysis, facilitates the determination of water in the gas by absorption in a cell where direct reaction with Karl Fischer reagent occurs. The procedure for the deter- mination of water in solid and liquid samples is well documented (British Standard 2511 : 1970) but only a few workers have reported its application to gase~.l-~ All of these workers absorbed the water from the gas in a solvent which was predominantly methanol and used specially designed apparatus. In our application the loss of methanol from the titration cell, owing to its volatility, created procedural difficulties and consequently the much less volatile solvent mixture, ethane-1,2-diol - pyridine (4 + l ) , was used; use of this solvent mixture also minimised spurious ammeter deflections, which are caused before the end-point when methanol is used.* In order to minimise problems associated with the decrease in strength of Karl Fischer reagent with time the more stable modified reagent proposed by Peters and JungnickeP was used.Experimental Apparatus Cell and titration assembly An electrometric apparatus for Karl Fischer titrations, marketed by Baird and Tatlock (London) Ltd., was suitably modified for the determination of water in gas.The inlet tube,164 DAVIES: THE DETERMINATION OF WATER IN NATURAL GAS Analyst, VoZ. IOU designed to carry inert gas over the liquid in the titration cell, was extended, by means of a glass tube fitted with a sintered glass disc (porosity grade l ) , to a point near to the bottom of the cell in order to enable gas to be bubbled efficiently through the titration solvent. The titration cell was provided with a side arm that was capped with a septum, and a wide bore drain tap was incorporated to facilitate adjustment of the level of liquid in the cell without the need to dismantle the apparatus. The cell assembly is shown in Fig. 1. The right-hand 25-ml burette of the Baird and Tatlock apparatus was replaced by a 10-ml microburette, which was connected to the reagent reservoir containing modified Karl Fischer reagent that had a water equivalent of about 0.5 mg ml-l.Desiccant guard tubes were filled with anhydrous calcium sulphate. To dead-stop ammeter ,Tip of microburette Tip of 25-mI burette . 0 - - - Solvent liquid level Electrode 1% Porosity 1 glass Fig. 1. Modified cell for Karl Fischer titration apparatus. Gas sampling The pressure of the natural gas or test gas was reduced to a gauge pressure of 14 kN m-2 (2 lb in-2) and was passed to the gas inlet of the cell using &in 0.d. stainless-steel tubing, which minimised water absorption and desorption effects. The gas flow-rate through the cell was regulated by using a stainless-steel fine control valve inserted in the stainless-steel sample line. Reagents supplied by Hopkin and Williams Ltd. is suitable.of 0.04 per cent.) can be used. Karl Fischer reagent (equivalent to 5 mg ml-l of water). The modified Karl Fischer reagent Pyridine. General Purpose Reagent for Karl Fischer titration (maximum water content Ethane-1 ,Z-diol. Analytical-reagent grade. Ethane-1,2-diol- pyridine solvent. To 800 ml of ethane-1,Z-diol add 200 ml of pyridine. Dry ethane-1,2-dioZ - pyridine solvent. By use of Karl Fischer reagent (equivalent to 5 mg ml-1 of water) determine the water content of the solvent mixture. Deduce the volume of the reagent that would be necessary to neutralise the water in 1 1 of solvent, and add this volume of Karl Fischer reagent to 11 of solvent. Karl Fischer working reagent. To 10ml of the Karl Fischer reagent add 90 ml of dry solvent .Procedure Standardisation of Karl Fischer reagent (water equivalent, 0-5 mg ml-l) Fill one of the reservoirs of the apparatus with Karl Fischer reagent (equivalent to 5 mg ml-f of water) and the other with working reagent. Fill the titration cell with the 1 ml of working reagent w 0.5 mg of water.March , 19 75 165 ethane-1,Z-diol - pyridine solvent to a level just below the junction of the cell and its side arm. Next, fill the burettes with the two Karl Fischer reagents, discard the contents to eliminate residual moisture in the burettes, then re-fill the burettes. Pass high purity nitrogen or dried sample gas through the cell at a flow-rate of 1 1 min-l to act as an electrode depolariser. Bring the ammeter to its neutral point by the addition of Karl Fischer reagent (5 mg ml-1) to the cell and then re-adjust the liquid level in the cell.Inject 2p1 of water into the cell, from a micro-syringe via the septum, and return the ammeter to its neutral position by using the Karl Fischer working reagent from the micro- burette. Repeat the determination until consecutive results duplicate satisfactorily, then calculate the water equivalent of the Karl Fischer working reagent used. USING A MODIFIED KARL FISCHER TITRATION APPARATUS Determination of water in sample gas Replace the dry gas with the sample gas and allow the latter to by-pass the cell and flow through a soap bubble flow meter. Adjust the sample gas flow-rate to 1 1 min-l. Return the gas flow through the cell for 30 min to ensure the complete elimination of dry gas from the system and record the ambient temperature and air pressure.Next, return the ammeter to its neutral point by using Karl Fischer reagent (5 mg ml-I), thus ensuring that the titration solvent is momentarily completely dry. Immediately start a stop-clock and add about 3 ml of Karl Fischer working reagent from its burette to the titration cell. Record the volume of Karl Fischer reagent added only when the burette reading becomes constant, about 10 min being allowed for the relatively viscous reagent to drain from the burette walls. Stop the clock when the ammeter indicator returns to its neutral point, in order to record the time required for the water in the timed gas flow to neutralise the Karl Fischer reagent added to the titration cell.Calculation Calculate the water content of the sample from the results obtained. I' 22a4 lo3 v.p.m. of water Water content of gas, Q = T x R x Y x 18 where I' is the volume of modified Karl Fischer reagent added to the cell (ml); W is the water equivalent of the modified Karl Fischer reagent (mg ml-l) ; T is the time of flow of the sample gas (min) ; R is the sample gas flow-rate (1 min-l) ; Y is the factor to reduce volume of gas passed to standard conditions; and v.p.m. are the volume parts of water vapour per million volume parts of gas. Results Water Absorption Efficiency in the Titration Cell A reliable measure of water content will be obtained only if the water in the sample gas is removed completely by the solvent and reacts immediately with the Karl Fischer reagent in the cell.The absorption efficiency of the solvent and Karl Fischer reagent in the cell was tested in two ways. The volume of 0.5 mg ml-l Karl Fischer reagent added to the cell containing dried solvent was kept constant at 3 ml and the sample gas flow-rate was varied from 1 to 5lmin-l. The results obtained are shown in Table I. In addition, the sample gas flow-rate was maintained constant at 4 lmin-l and the volume of 05mgml-l Karl Fischer reagent added to the cell was varied from 1 to 5 ml [see Table I1 (a)]. A second natural gas sample was employed while maintaining the gas flow-rate at a lower value of 1 1 min-1 [see Table I1 (b)]. The results [Tables I and I1 (a)] that refer to the same gas indicate a mean water content of 49.6 and 48-0 v.p.m.for the two techniques, which were shown not to be statistically different. It is evident, therefore, that the indicated values were independent of the time taken to carry out a determination, the volume of the sample gas and its flow-rate when this was not greater than 5 1 min-l. The efficiency of absorption of water by the cell liquid was inferred to be virtually 100 per cent., confirming the findings of Muroi2 and Archer and H i l t ~ n . ~ The results in Tables I1 (a) and (b) show that a maximum value for the water content of the sample is obtained when the cell contains 2.5-3 ml of 0.5 mg ml-1 reagent. Although no explanation can be offered at present for this observation it is apparent, from166 DAVIES: THE DETERMINATION OF WATER IN NATURAL GAS Analyst, VoZ.100 TABLE I WATER CONTENT OF NATURAL GAS FOUND BY KARL FISCHER TITRATION Constant Karl Fischer reagent addition, 3 ml; gas flow-rate, variable. Gas flow-rate/l min-l 1-35 1.35 2.0 2.0 2.6 2.6 3.25 3-25 4.0 4.0 4.6 4.6 5.3 5.3 Time of testlmin 41 44 29 28 22 23 17 16 14 14 12 12 11 10 Water content at N.T.P., v.p.m. 50.2 46.7 47.8 50.4 48.1 45.9 60.1 53.3 48.8 50.2 50.2 50-2 49.6 53.4 Mean .. 49.6 results of the determination of water in nitrogen mixtures, that the maximum value observed is the true water content of the sample. The experimental procedure described was based upon these findings, and involved the use of a steady flow-rate of gas (1 1 min-l) and the addition of 3.0 ml of 0.5 mg ml-l Karl Fischer reagent. TABLE I1 WATER CONTENT O F TWO NATURAL GAS SAMPLES BY KARL FISCHER TITRATION Gas flow-rate, constant ; Karl Fischer reagent addition, variable.(a) Gas flow 4 1 min-l (b) Gas flow 1 1 min-1 1 '-----h- Karl Fischer Time of Water content a t Time of Water content at reagent addition/ml testlmin N.T.P., v.p.m. testlmin N.T.P., v.p.m. 1.0 4.8 46.8 10.5 74-1 1.6 - - 15.0 77.6 2.0 9.5 49.9 19.3 80-6 2.5 11.5 51.5 23.3 83.4 3-0 14.0 49.5 28.3 82.3 3.5 17.25 46.3 33.5 80.6 4.0 19.8 46-9 44.0 68.0 4.5 - - 62.5 65.5 5-0 26.8 45.6 - - Mean . . 48-0 Mean .. 76.5 Determination of Water in Moist Nitrogen Samples Mixtures containing water vapour in nitrogen were prepared in 5.4-1 aluminium cylinders at a gauge pressure of 7000 kN m-2 (1000 lb i r 2 ) and were analysed by the method described, with a moisture monitor based on the technique of absorption by phosphorus(V) oxide, and by absorption in magnesium perchlorate.The results are shown in Table 111. The agreement between the methods showed that the recovery of water in the gas by Karl Fischer titration was satisfactory and quantitative. The results also show that the maximum value observed previously was the true value for the water content of the sample. TABLE I11 DETERMINATION OF WATER IN NITROGEN Water content, v.p.m. Technique c y m e = 2 Karl Fischer titration . . .. .. .. 85, 83 83, 81, 84 Moisture monitor . . .. * . .. .. 82 83 Magnesium perchlorate absorption .. * . 80 87 In this instance the moisture monitor provided a result more quickly, but it cannot be used with natural gas because of the presence of interfering compounds. The methodMarch, 1975 USING A MODIFIED KARL FISCHER TITRATION APPARATUS 167 involving absorption in magnesium perchlorate is tedious and cannot be used for natural gas as methanol is also absorbed.These two methods were, however, suitable for the deter- mination of water in nitrogen, and as reference procedures against which to compare the Karl Fischer method. Application of the Procedure to Natural Gas The procedure described has been used to measure the concentration of water in a natural gas during liquefaction at pilot scale and the results have been compared with those obtained by use of a UGC Dewscope hygrometer, fitted with a bubbler containing paraffin in order to remove easily condensable hydrocarbons. Values for four different samples are given in Table IV; these results show that the procedure was applied successfully to natural gas analysis.TABLE IV DETERMINATION OF WATER IN NATURAL GAS Water content a t N.T.P., v.p.m. f A \ Sample Karl Fischer procedure Dewscope 1 43 2 72 3 94 4 49 48 69 89 45 Precision of the Procedure The over-all precision of the method was evaluated from a number of determinations of the water content of a natural gas sample by using the procedure described. A mean value of 95.8 v.p.m. of water was obtained with a standard deviation of 3.2 v.p.m. Discussion The method was required essentially as an absolute standard against which other instru- mental procedures could be checked. Water sensors relying on hygrometry and chemical cells [e.g., the phosphorus(V) oxide cell] are subject to interference from trace constituents in natural gas, such as hydrocarbons and methanol, and for their successful application separation procedures prior to water determination would be required.The Karl Fischer titration procedure is specific for water and the procedure described has been shown to be applicable to natural gas. Water contents of between 40 and 200 v.p.m. in natural gas are readily determined with a standard deviation of 3 4 per cent. It may be possible to extend the technique to other gases containing more than 200 v.p.m. of water, but this need has not yet arisen. Conclusions A commercially available Karl Fischer titration apparatus that is suitable for the deter- mination of water in liquids and solids, has, after slight modification, been applied successfully to gases. The efficiency of water absorption from the gas was shown to be virtually complete and consequently a procedure for water determination in at least nitrogen and natural gas samples was established. Results obtained on a natural gas sample containing 96 v.p.m. of water when using the procedure had a standard deviation of 3 4 per cent. The method has been successfully applied to natural gas samples containing up to 200 v.p.m. of water, but the standard deviation is greater than 3 per cent. for samples containing less than 40 v.p.m. This paper is published by permission of British Gas. The author also thanks K. R. Compson (present address : AEI , Scientific Apparatus, 1 Dock Road, Urmston, Manchester) for assistance with the experimental work. References 1. 2. 3. 4. Roman, W., and Hirst, A., Analyst, 1961, 76, 10. Muroi, K., Japan Analyst, 1961, 10, 847. Archer, E. E., and Hilton, J., Analyst, 1974, 99, 547. Peters, E. D., and Jungnickel, J. L., Analyt. Chem., 1955, 27, 460. Received June 24th, 1974 Accepted October 30th, 1974

ISSN:0003-2654    

DOI:10.1039/AN9750000163

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

168 Analyst, March, 1975, Vol. 100, pp. 168-172 A Simple Reaction - rate Method for the Determination of Biuret M. 1. Karayannis and E. V. Kordi Laboratory of Analytical Chemistry, The University of Athens, Athens, Greece The phenol - hypochlorite reaction is employed for the determination of biuret in aqueous solutions. The recommended reaction-rate method is fast, simple, sensitive (0.018 A min-1 per micromole of biuret), accurate (to 2 per cent.) and precise (3 per cent. relative standard deviation). The useful analytical range of concentrations of biuret is 1-6 x to 1.3 x 1 0 - 3 ~ . The effect of the phenol concentration on the reaction rate has been studied. The effect of interfering species such as ammonium chloride, urea and cyanurate has also been investigated. It is known that ammonia and urea react with hypochlorite and phenol to give coloured ind~phenols.l-~ Mechanisms of the above reactions have been proposed by various investi- gators,234 and the kinetics as well as the analytical data have been given in the literature.6-7 We have investigated the urea - hypochlorite - phenol reaction both in acidic and alkaline solutions,&* and, in addition, have found that many organic substances that react with hypochlorite to give chloramine (NH,Cl) also yield coloured indophenols in the presence of phenol, the colours given depending on the pH of the reaction system.Thiourea, formamide and biuret give the same reaction, with the formation of an intermediate yellow product that absorbs light at 465nm. With biuret the reaction takes place in alkaline solutions and the yellow product is slowly converted into a final bluish-green indophenol, which is characterised by its absorption bands at 640 and 355 nm.The various steps of the reaction are as follows: the formation of (1) chloramine from biuret and hypochlorite ions, (2) quinone chloroimine from the chloramine and phenol and (3) the final indophenol product with the excess of phenol. The formation of an intermediate quinone chloroimine gains support from the fact that the same phenolindophenol is obtained by allowing p-aminophenol to react with hypochlorite first, and then adding phenol to the mixture. When the same reaction takes place in acidic solution no product absorbing a t 640nm is obtained, although the yellow intermediate compound appears as with alkaline solutions.On standing, the colour of this product changes from yellow to light pink; a similar effect is observed when the solution of the bluish green indophenol is acidified. Experimental Apparatus The investigations reported in this work were performed with a modified Heath 701 single- beam spectrophotometer. The modification consists in replacing the original cell-basket and its support with a home-made cuvette placed in a double-walled brass housing, which is connected to a Sargent heater and circulator equipped with a temperature monitoring unit. The brass housing of the cuvette is mounted on a magnetic stirrer, a plastic plate with a magnetic rotor inside, which is powered by tap water, compressed air or a vacuum (G. F.Smith Chemical Co., Columbus, Ohio, U.S.A.). The observation cell is a Vycor-glass tube, 8-3 cm long, with a flat bottom. The internal and external diameters of this cuvette are 1.53 and 1.8 cm, respectively. The photocurrent from the photomultiplier output is fed to the log-lin unit attached to a chart recorder (Heath EU-20-28). Depending on the setting on the log - lin module, the time course of the absorbance or transmittance of the reacting system can be recorded.KARAYANNIS AND KORDI 169 Reagents All reagents were of analytical-reagent grade and were used as supplied. Bizwet solutions. All the solutions of biuret were prepared from an aqueous stock solution (1.67 x M) by dilution with a borate - sodium hydroxide buffer (pH lo). Phenol solutions. All the phenol solutions employed for the experiments were prepared from an 8.0 per cent.aqueous solution of phenol (0.851 M) by dilution with double-distilled water. HyPochZorite solutions. The hypochlorite reagent used was Klinex bleach liquid (a locally prepared commercial sodium hypochlorite solution, with pH 12), with an original concen- tration of free chlorine of about 5 per cent. Spectral Study For the spectral study the following procedure was followed: 20ml of a 1.67 x 1 0 4 ~ solution of biuret in a buffer of pH 10 were mixed with 0.5 ml of bleach liquid (1.38 M in hypochlorite ions) in a beaker with stirring. The mixture was left to react for 30 s and then 10.0 ml of a 4.25 x M aqueous solution of phenol were added. Immediately after mixing, an aliquot of the final mixture was placed in a l-cm quartz cuvette and its absorption spectrum was recorded with a double-beam spectrophotometer (Beckman DK), against water, at different times after the addition of the phenol.The first spectrum (shown in Fig. 1) was taken 1 min after the addition of the phenol. 1 0.8 0.6 a, c m 2 2 $ 0-4 0.2 0 Wave I eng t h/nm Fig. 1. Absorption spectra of reaction system biuret - hypochlorite - phenol at different times after the addition of the phenol reagent. Curves: 1, 1 min; 2, 9 min; 3,19 min; 4,34 min; 6, 66 min; 6, 79 min; and 7, 120 min. Initially, the 465-nm band predominates over the bands at 355 and 640 nm. A kinetic study of the reaction system showed that the 465-nm band is due to a yellow intermediate product, and that the bands at 355 and 640nm are attributable to the same species, i.e., the bluish green indophenol formed as the final product of the reaction (the results of these experiments will be published elsewhere). Analytical Application For the determination of biuret in aqueous solutions by means of the above reaction, the following procedure is recommended: switch on the instrument and the electronics at least170 KARAYANNIS AND KORDI: A SIMPLE REACTION-RATE METHOD Analyst, 'Vd.100 30 min before taking any measurements. Set the wavelength at 465 nm on the monochromator and calibrate the spectrophotometer for zero absorbance with the cuvette filled with distilled water. For optimisation of the instrument variables reference should be made to the appro- priate instruction manual.Measurement of Standards Because of the absence of a blank value and the good precisionof the method, theanalytical curve can easily be constructed with only two or three different standards in the range of the unknowns; 4 ml of the buffered standard solution of biuret (pH 10) were pipetted into the observation cell and then 100 p1 of the hypochlorite reagent (1.38 M) were added, while stirring, with a Hamilton syringe. After an interval of 30 s, 2.0 ml of the phenol reagent (4.25 x Two seconds after the addition of the phenolreagent the reaction can befollowedbyrecording against time the absorbance of the reaction system. Fig. 2 shows the curves obtained for three different concentrations of biuret. In each instance, the initial reaction rate is calculated graphically.The slope of the linear portion of the curve at the beginning of the reaction, expressed in absorbance units ( A ) per minute, is taken as the initial reaction rate. By plotting this value against the concentration of biuret, a straight line is obtained with a slope of 0.018 A min-l per micromole of biuret. Fig. 3 shows such a graph, which is linear in the concentration range 1.6 x M of biuret. M in phenol) were added with a 2-ml hypodermic syringe. to 1.3 x Time/s Fig. 2. Reaction curves (absorbance v e m a time a t A = 466 nm). The reaction mixture contained NaOCl at 2.3 x M; phenol at 1.4 x M; and biuret: 1, a t Concentration of biuret/M X lo5 3-32 x 10-4 M ; 2, at 1-67 x M; and Fig. 3. Working graph for biuret 3, at 4.98 x l o - 5 ~ concentration.determination. Effect of Phenol Concentration on the Reaction Rate The above experimental procedure was applied in order to study the effect of the concen- tration of phenol on the reaction rate. The final concentrations of biuret and hypochlorite in the reaction mixture were kept constant at 2.180 x 10"' and 2.300 x M, respectively, and the concentration of phenol reagent was varied from 2.125 x M (final concentration in the mixture 0.70 x The initial reaction rate, expressed in A min-l, has been determined for different concentrations of phenol and the results obtained are illustrated in Fig. 4. M, which corre- sponds to a final concentration of 1.3 x M of phenol in the reaction system. The ratio of the concentration of hypochlorite to that of phenol is then about 2.The decrease in the initial reaction rate for ratios of hypochlorite to phenol of less than 2 is due to the formation of trichlorophenols in addition to mono- and dichlorophenols, which are formed when this ratio is below 2. Phenol that is substituted in the $ara-position cannot react with biuret according M to 19-125 x to 6.30 x 1 W 2 ~ ) . The maximum reaction rate occurs at a concentration of about 4.0 xMarch, 1975 FOR THE DETERMINATION OF BIURET 171 to the above scheme. The original phenol concentration of 4-25 x M (0.4 per cent. of phenol) was selected for our experiments because it gives nearly the maximum initial reaction rate for the concentration of hypochlorite employed, and thus ensures maximum sensitivity. Interference Study As stated above, many compounds that react with hypochlorite to form chloramine give the same final product with phenol.Unfortunately, the rates of reaction of different species such as urea, ammonium salts and cyanurates, which commonly occur in admixtures with biuret, do not differ sufficiently to enable multi-component analysis to be attempted by a differential rate method. By applying the procedure described above, we studied the interference effect of ammonium chloride, urea and cyanurate on the reaction rate. The results for two starting concentrations of biuret are presented in Fig. 5. The ratio of the concentration of interfering species to the concentration of biuret (Cint/Cb) was varied from 0 to 10. The effect of ammonium salts does not seem to be significant for Cfnt/Cb ratios of up to 3 with low concentrations of biuret and for higher ratios the effect is negative, probably due to the much faster rate of formation of chloramine from ammonia, which is available initially in the reaction mixture. During the waiting time of 30 s the chloramine decomposesg and has no effect on the reaction rate.Cyanurates form chloramine with hypochlorite at a slower rate than NH, and at a faster rate than biuret. Both ammonium chloride and cyanurates consume hypochlorite ions to give chloramine or other unspecified products, which, during the 30-s waiting time, partially or completely decompose, depending on their starting concentrations. At higher concentrations these compounds drastically decrease the available concentration of hypochlorite ions, thus decreasing the rate of formation of chlor- amine from biuret, which is shown in Fig.5, by the negative effect that they have on the reaction rate when the ratio Cint/Cb or the absolute values for their concentrations are high. The rate of consumption of hypochlorite from urea is comparable with that from biuret and consequently the effect of urea on the reaction rate is positive. I I I I 8 16 Concentration of phenol/M X 1 o2 Fig. 4. Effect of phenol concentration on the reaction rate. Fig, 6 . Interference effect of urea (a), cyanurate ( x ) and NH4C1 (0) on the reaction rate. Concentration of biuret: -, 0.98 x lo-' M and - - -, 3.32 x lo-* M. Discussion As shown in Fig. 3, the dependence of the initial reaction rate on the concentration of biuret is linear, with a useful analytical concentration range from 1-6 x low5 to 1-3 x M.The slope of the straight line is 0.018 A min-l per micromole of biuret, thus indicating that the above method is very sensitive and that it can be used for the determination of biuret in aqueous solutions. The precision of the method, measured for a standard sample in the above concentration range, is shown by the 3 per cent. relative standard deviation (rt = 7,95 per cent. confidence level). The accuracy, calculated from the results given in Table I, was172 KARAYANNIS AND KORDI found to be about 2 per cent. The method is very simple and anyrecording spectrophotometer can be used following a minor modification to the cell compartment. The time for one deter- mination is 3 to 5 min, including the time required for calculating the slope of the recorded curve.In the absence of a blank value, the working curve can beconstructed withonly two or three points. All measurements reported in this work were performed at 25.0 & 0.1 "C. The activation energy for the reaction, as calculated from Arrhenius graphs (log Kobs. veysus l/T), was found to be 26-7 kcal. Because this value is relatively high, good thermal stabilisation of the observation cell is necessary. The standard solutions of biuret are stable for more than 2 weeks in aqueous solution, as well as in buffered solutions of pH 10, and consequently there is no need to prepare fresh standards. TABLE I RESULTS FOR AQUEOUS BIURET SOLUTIONS Concentration of biuret/M X lo6* -7 Taken Found 1-09 1-10 3.27 3.30 10.90 10.40 21.80 21.30 32.70 32.80 43-60 44.70 64.60 66.10 66.40 66.60 87-20 86.90 Error, per cent. 0.9 0.9 4.5 2.3 0.3 2.6 3.0 0.2 0-3 Mean . . 1-7 Initial reaction rate/A min-l 0.27 0.80 2.80 6.86 9.07 12-32 16.62 18.08 24.00 * The concentrations given are those in the reaction mixture. References 1, 2. 3. 4. 6. 6. 7. 8. 9. Berthelot, M., Rep. Chim. Appl., 1859, 1, 284. Bolleter, W. T., Bushman, C . J., and Tidwell, P. W., Analyt. Chem., 1961, 33, 692. Weatherburn, M. W., Analyt. Chem., 1967, 39, 971. Horn, D. B., and Squire, C. R., Clinica Chim. Ada, 1967, 17, 99. Weichselbaum, T. E., and Hagerty, J. C . , Analyt. Chem., 1969, 41, 848. Karayannis, M. I., and Malmstadt, H. V., unpublished results. Karayannis, M. I., Analyt. Lett., 1973, 6 (7), 629. Karayannis, M. I., Chimica Chronica, New Series, 1973, 2, 86. Rashing, F., Chemikerzeitung, 1907, 31, 926. Received April 22nd, 1974 Accepted July 16th, 1974

ISSN:0003-2654    

DOI:10.1039/AN9750000168

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Analyst, March, 1975, Vol. 100, pp. 173-177 I 173 Determination of Biphenyl and 2-Phenylphenol in Citrus Fruits by Gas - Liquid Chromatography Gunnel Westoo and Arne Andersson The National Food Administration, Stockholm, Swedelz Biphenyl and 2-phenylphenol are steam distilled from a citrus fruit homo- genate into two portions of cyclohexane. An aliquot of the first extract is analysed for biphenyl by gas - liquid chromatography. Aliquots of the first and second extracts are combined and then cleaned up and concentrated by means of further extractions prior to the gas - liquid chromatographic deter- mination of 2-phenylphenol. Post-harvest treatment of citrus fruit with 2-phenylphenol and the maintenance of a certain biphenyl concentration in the air around the fruit (by, for instance, using wrappings im- pregnated with biphenyl) prevent the growth of moulds on the skin of the fruit during transport and storage.Several spectrophotometric methods (with ultraviolet or visible light) have been described for the determination in citrus fruit of both biphenyll-s and 2-phenylphen0l.~+~ Gas - liquid chromatographic methods have also been applied.4p7-9 In most of these methods the fungicides are separated from the fruit by steam distillation before their extraction with an organic solvent. The distillation is omitted from some methods. A clean-up of the extract is always performed before the spectrophotometric determination and sometimes before the application of gas - liquid chromatography. I I la) Biphenyl J Fig. 1. Gas chromatograms of orange extracts.(a), No biphenyl added; ( b ) , 20 mg of biphenyl added per kilogram of fruit. Since 1969, a method for the determination of biphenyl in citrus fruits by steam distillation into cyclohexane and subsequent spectrophotometry of the purified extractsJO has been used in our laboratory, in combination with a simple gas - liquid chromatographic determination of biphenyl on the dried, crude extract from the same steam distillationl1J2 (Fig. 1). The values obtained by the spectrophotometric determinations were approximately 102 per cent. of those174 WESTOO AND ANDERSSON : DETERMINATION OF BIPHENYL Analyst, VoZ. 100 values obtained with the gas - liquid chromatographic procedure described below, with a standard deviation of 3-6 per cent. for 109 samples.By using the gas - liquid chromatographic method the recovery of biphenyl added to citrus fruits (40.0-100.0 mg kg-1) was 97 per cent. (standard deviation 2 per cent.; four samples). The limit of detection of biphenyl when 1 p1 of the extract is chromatographed is 1 mg k g l . For several years, we have used the spectrophotometric method of the Nordic Committee on Food Analysis5 for the determination of 2-phenylphenol. Although the 2-phenylphenol in the cyclohexane extract obtained after steam distillation is cleaned up and a colour reaction is performed, the natural compounds extracted from the citrus fruits often contribute to the light absorption, with the result that the 2-phenylphenol values found are slightly higher than the actual levels. Such interference was also observed by other authors using other rnethod~.~1*~6~9 In order to avoid this interference, a method involving a clean-up step followed by a gas - liquid chromatographic determination of 2-phenylphenol has now been elaborated.The 2-phenylphenol is extracted with small volumes of 1 N sodium hydroxide solution from the cyclohexane extract obtained in the application of the Nordic Committee on Food Analysis m e t h ~ d , ~ the alkaline extract is acidified, and the 2-phenylphenol is re- extracted into a small volume of cyclohexane. The volume of cyclohexane in which the 2-phenylphenol is contained is thus decreased ten-fold in order to increase the sensitivity of the method. Fig. 2 shows the great difference between the gas chromatograms obtained from the cyclohexane extract of the distillate of lemon after ten-fold concentration (a), by evaporating the solvent and (b), by using the clean-up procedure described here.Fig. 3 shows chromato- grams of cleaned up orange extracts. The chromatograms in Figs. 2 and 3 should be com- pared with the chromatograms published by Beernaert: in which interferences from plant co-extractives with the same retention time as 2-phenylphenol are observed. 10 5 0 Tim e/m i n Fig. 2. Gas chromatograms of com- bined. dried cvclohexane extracts obtained 2-P heny I p heno I : J J 2-Phen ylphenol Fig. 3. Gas chromatograms of orange extracts. (& No 2-phenylphenol added; (b), 4 mg of 2-phenylphenol added per kilogram of fruit. from steam distillation of a lemon sample. The combined extracts were concentrated ten-fold by (a), evaporation of the solvent before injection and ( b ) , the clean-up procedure.The efficiency of the clean-up procedure has been examined in analyses of 31 samples of citrus fruits, selected because of their low 2-phenylphenol content. Table I shows the amount of 2-phenylphenol found in orange, lemon and grapefruit using the spectrophotometric6 and the gas - liquid chromatographic methods. The spectrophotometric method gave results which were 0.2-1.5 (average 0.9) mg k g l higher than the results obtained with the gas -March, 1975 AND 2-PHENYLPHENOL IN CITRUS FRUITS BY GLC 175 liquid chromatographic method. In many of these samples no absorption maximum was observed at the expected wavelength, which indicates that the spectrophotometric results were incorrect.The low levels of 2-phenylphenol shown by gas - liquid chromatography (Table I) were not artifacts. This was verified both qualitatively and quantitatively by using thin-layer chromatography,13 which was performed both with and without the colour reaction on the cleaned up sample extracts and on 2-phenylphenol standards. When the colour reaction was omitted, the 2-phenylphenol area of the chromatogram of the sample was extracted by shaking with 1 N hydrochloric acid and cyclohexane. The dried cyclohexane extract was injected into the gas chromatograph before and after silylation with N-methyl-N-trimethylsilyltri- fluoroacetamide. The R, value for the compound extracted from the fruit was identical with the Rp value for the 2-phenylphenol standard and the retention times for the original and silylated compound from the fruit were identical with the corresponding figures for the original and silylated 2-phenylphenol. The 2-phenylphenol concentration levels found were in all instances in good agreement with the levels obtained when determinations were carried out as described under Procedure (Table I).TABLE I COMPARISON O F CONCENTRATIONS OF 2-PHENYLPHENOL FOUND I N CITRUS FRUIT BY USING THE GAS - LIQUID CHROMATOGRAPHIC METHOD AND A SPECTROPHOTOMETRIC METHOD' Concentration of 2-phenylphenol/mg kg-1 Sample Orange Lemon Grapefruit Gas - liquid chromatography 0.6 0.2 0 0-1 0-4 0.8 0 0 0 0 0.07 0.06 0.06 0 0.07 0 0 0 0.08 1.0 0 0.3 0 0.1 0.8 0.05 0.2 0.06 0.5 0.4 1.1 Spectrophotome'try 1.3 1.0* 0*8* 1*0* 1.4* 1.9* 1*3* 0*8* 1*4* 1.3" 1*6* 1.2* 1*1* 1.2* 1*4* 1.1" 1.2* 1*0* 0.6* 1-2 1.4 1*2* 1-1 1.4 1.4 0-7* 0.8* 0-7* 1.2* 1.0 1.7 * No absorption maximum was observed.The recovery, by using the steam distillation pro~ess,~ of 2-phenylphenol added to citrus fruits was 90-100 per cent. When 0.160-0.960 mg of 2-phenylphenol was taken through the above clean-up steps, 99 per cent. (standard deviation 2 per cent.; 4 samples) was recovered. When 2-phenylphenol was added to citrus fruits (4.00-10.00 mg k g l ) and taken through the whole procedure, the recovery was 94 per cent. (standard deviation 3 per cent.; 4 samples).176 WESTOO AND ANDERSSON : DETERMINATION OF BIPHENYL Analyst, Vol. 108 The limit of detection of 2-phenylphenol when 1 p1 of the final extract is chromatographed is 0.1 mg kg-1 (cf., the limit of 5 mg kg-1 for the procedure described by Morriess).Method Apparatus Mincer. With 4-mm holes in grinding plate. Waring blender. Heating mantle. Capacity for 2-1 flask; with rheostat control, 500 to 550 W. Clevenger trap. Modified, see Fig. 1, reference 3. Gas chromatograph with jclame-ionisation detector. Column for biphenyl determination. Glass column (5 ft x + in) with 5 per cent. Carbowax 20M on Chromosorb W, AW-DMCS, 60-80 mesh. Gas flow-rates: nitrogen, about 25 ml min-1; hydrogen, about 30 ml min-l; and air, about 280 ml min-1. Column temperature, 155- 160 "C. Retention time, about 4 min. Column for 2-phenylphenol determination. Glass column (6 ft x & in) with a mixture (1 + 1) of 10 per cent.DC 200 and 15 per cent. QF1 on Chromosorb W, AW-DMCS, HP, 80-100 mesh. Gas flow-rates: nitrogen, about 35 ml min-l; hydrogen, about 30 ml min-1; and air, about 280 ml min-1. Column temperature, 180 "C. Retention time, about 6 min. Reagents All reagents must be of analytical-reagent grade. Sulphuric acid, concentrated. Antqoam agent. Dow Corning Antifoam A. Cyclohexane. Sodium sulphate, anhydrous. Sodium hydroxide solution, 1.0 N. Hydrochloric acid, 6.0 N. Standard solutions of biphenyl. (a) Stock solution. Dissolve 100.0 mg of biphenyl in cyclo- hexane and make up to 100.0ml. ( b ) Dilute the stock solution so as to obtain standard solutions containing 10.00-800 pg ml-l of biphenyl. Standard solutions of 2-phenylphenol. (a) Stock solution. Dissolve 100.0 mg of Z-phenyl- phenol in cyclohexane and make up to 100.0 ml. (b) Dilute the stock solution so as to obtain standard solutions containing 5.00-300 pg ml-l of 2-phenylphenol.Procedure Prepare the first extract in the manner described under Procedure in reference 3. Reconnect the Clevenger trap, introduce 20ml of water and 40ml of cyclohexane into it and prepare the second extract in exactly the same way. Determination of biehenyl In order to determine the biphenyl content of the extract, inject a 1-pl aliquot of the first cyclohexane extract and a 1-p1 volume of a standard solution with about the same biphenyl concentration into the gas chromatograph and compare the peak heights. (The standard graph is a straight line passing through the origin in the concentration range 0-1000 pg ml-l of biphenyl.) Determination of 2-fihenylphenol Shake a mixture of 20.0 ml of each of the two 50-0-ml cyclohexane extracts and 5.0 ml of 1 N sodium hydroxide solution in a separating funnel for 3 min. Separate and centrifuge the turbid, alkaline phase.Transfer the clear alkaline extract so obtained into another separating funnel with a pipette, and add the cyclohexane phase from the centrifuge tube to the remaining cyclohexane phase in the original separating funnel. Re-extract the cyclo- hexane solution with two 5.0-ml volumes of sodium hydroxide solution. Acidify the combined alkaline phases with 6-0 ml of 6 N hydrochloric acid. Add 4.00 ml of cyclohexane and shake for 3 min, separate the layers and dry the cyclohexane extract with anhydrous sodiumMarch, 1975 AND 2-PHENYLPHENOL I N CITRUS FRUITS BY GLC 177 sulphate. Inject a 1-p1 aliquot of the dried extract and a l-pl volume of a standard solution with about the same 2-phenylphenol concentration into the gas chromatograph and compare the heights of the peaks obtained. (The standard graph is a straight line passing through the origin in the concentration range 0-300 pg ml-1 of 2-phenylphenol.) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Rajzman, A., AIzalyst, 1963, 88, 117. Gunther, F. A., Blinn, R. C., and Barkley, J. H., Analyst, 1963, 88, 36. Westoo, G., Analyst, 1969, 94, 406. Vogel, J., and Deshusses, J., Mitt. Geb. Lebensmittelunters. u. Hyg., 1963, 54, 330. Nordic Committee on Food Analysis, No. 73, 1970. Rajzman, A., Analyst, 1972, 97, 271. Thomas, R., Analyst, 1960, 85, 551. Momes, P., J . Ass. Publ. Analysts, 1973, 11, 44. Beernaert, H., J . Chromat., 1973, 77, 331. Nordic Committee on Food Analysis, No. 72, 1970. Westoo, G., V& Foda, 1969, 21, 113. Westoo, G., Andersson, A., and Mattsson, S., V& Foda, 1973, 25, Suppl. 2, 46. Davenport, J . E., J . Ass. Off. Analyt. Chem., 1971, 54, 976. Received June 17th, 1974 Accepted October Sth, 1974

ISSN:0003-2654    

DOI:10.1039/AN9750000173

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

178 Analyst, March, 1975, Vol. 100, @. 178-181 Spectrophotometric Determination of Thiambutosine M. 6. Devani, C. J. Shishoo and Hema J. Mody Department of Pharmaceutical Chemistry, Lallubhai Motilal College of Pharmacy, Ahmedabad-9, India N-( 4-Butoxyphenyl)-N’- (4-dimethylaminophenyl) thiourea (thiambutosine) is made to react with 2,3-dichloro- 1,4-naphthoquinone in an ethanolic medium and on rendering the reaction mixture alkaline with ethanolic ammonia solution a purple colour is developed with an absorption maximum a t 540 nm. A procedure based on this reaction is described for the assay of thiambutosine in micro-amounts. The method is applied to its determination in tablets. The results are in agreement with those obtained by the official method. In continuation of the previous work,lJ a spectrophotometric method is described for the determination of thiambutosine, a well known anti-leprosy drug.Various procedures for its determination include non-aqueous titrimetric3 and spectrophotometric3p4 methods. Both of these methods are non-specific for the thiourea moiety in the drug. In view of the sensitivity and specificity of the reaction between thiourea and 2,3-dichloro-l,4-naphthoquinoneJ~~~ it was thought to be of interest to employ this reagent in the spectrophotometric determination of thiambutosine. In the present work, suitable reaction conditions were established. The procedure is applied successfully to the assay of thiambutosine in pure samples and in tablets containing it. The results compare favourably with those obtained by the official procedure.Experimental Apparatus & Lomb) equipped with four matched 10-ml cells that have a I-cm light path. All spectral measurements were carried out on a Spectronic 20 spectrophotometer (Bausch Reagents Thiambutosine B.P., absolute ethanol (Indian Pharmacopoeia) and copper acetate (Riedel de Haen) were used. Ethanolic ammonia solution (2.5 per cent. m/V) and 2,3-dichloro-1,4- naphthoquinone reagent solution (0.026 per cent. m/ V ) were prepared as described previously. All other reagents were of analytical-reagent grade. Standard solution of thiambutosine, 0.05 per cent. m/V, in absolute ethanol. Procedure Determination of Thiambutosine A standard solution containing 0.5 to 2-0 mg of thiambutosine was mixed with 3-0 ml of ethanolic ammonia solution and 12.0 ml of 2,3-dichloro-l,4-naphthoquinone reagent solution in a 25-ml calibrated flask.The final volume was adjusted to the mark with absolute ethanol and the reaction mixture was allowed to stand for 10 min at room temperature. The absor- bance was measured at 540 nm against the blank. The blank consisted of 12.0 ml of 2,3-dichloro-lJ4-naphthoquinone reagent solution plus 3.0 ml of ethanolic ammonia solution diluted to 25 ml with absolute ethanol. Determination of Thiambutosine in Tablets Twenty tablets were weighed and powdered. An aliquot of the powder equivalent to 50 mg of thiambutosine was weighed accurately. Four equal portions of hot ethanol (20.0 ml) were used to extract thiambutosine from the powder. Each time, the extracts were filtered through Whatman No.40 filter-paper. The residue on the filter-paper was then washed with 10-Om1 of warm ethanol. The filtrate and the washings were combined in a 100-ml calibrated flask and, after cooling, the volume was adjusted to the mark with the solvent. The solution was analysed by the above procedure.DEVANI, SHISHOO AND MODY Factors that Affect the Reaction of Thiambwtosine with 2,3-Dichloro-1,4-naphthoquinone Reagent E 0-3 ; I= 0 d In + 8 0-2 -E 53 n a 0.1 179 I -i’ I I 1 1 1 1 Concentration of 2,3-Dichloro-l,4-naphthoquinone Reagent The absorbance at 540 nm of the coloured product formed by the reaction of thiambutosine with 2,3-dichloro-l,4-naphthoquinone increased as the concentration of the reagent increased. The maximum absorbance was obtained in the presence of 12.0ml of the reagent solution and decreased slightly on further increase in the volume of reagent solution (Fig.1). 0.1 0 1.0 2.0 3.0 4.0 5.0 Amount of 2.5 per cent. m/V ethanolic ammonia solutiodml Fig. 2. Effect of the concen- tration of ethanolic ammonia solu- tion in a 25-0-ml reaction mixture on the absorption at 640 nm of the product formed on reactionof 1.6 mg of thiambutosine with 12.0 ml of the reagent solution. Concentration of Ammonia The typical purple colour developed after the reaction of the ethanolic ammonia solution with the thiambutosine and the 2,3-dichloro-1,4-naphthoquinone reagent. Maximum colour intensity was obtained in the presence of 3.0 ml of ethanolic ammonia solution (Fig. 2). 0.4 I 0 5 10 15 20 25 30 35 Time/min Fig.3. Rate of development of purple coloration after reaction of 1.5 mg of thi- ambutosine with 12.0 ml of reagent solution when using 3.0 ml of ethanolic ammonia solution in a total reaction volume of 25.0 ml. Concentration of Thiambutosine B.P./jtg ml-’ Fig. 4. Effect of the amount of thiambutosine in a 26.0-ml reaction mixture on the absorp- tion at 640nm of the product formed on reaction with 12.0 ml of the reagent solution when using 3.0 ml of ethanolic am- monia solution.180 DEVANI, SHISHOO AND MODY : SPECTROPHOTOMETRIC Analyst, VoZ. 100 Time of Reaction Maximum colour intensity developed on keeping the reaction mixture for 10 min at room temperature, and then decreased slightly on further standing (Fig. 3). Concentration of Thiambutosine 20-80 pg per millilitre of reaction mixture (Fig.4). The absorbance at 540 nm was proportional to the amount of thiambutosine in the range Results and Discussion Thiambutosine was made to react with 2,3-dichloro-1 ,knaphthoquinone in the presence of ammonia to give a typical purple-coloured product with an absorption maximum at 540 nm (Fig. 5). Om' t 1 1 1 1 . 1 1 ' 460 480 500 520 540 560 580 6 Wavelengt h/n m Fig. 6. The absorption spectrum of the product of the reaction between 1.6 mg of thiambutosine and 12.0ml of the reagent solution when using 3.0 ml of ethanolic am- monia solution in a total reaction volume of 26.0 ml. Samples of thiambutosine were obtained from commercial sources and analysed by the The percentage recovery and the standard proposed as well as the official procedures.deviation calculated from a series of experiments are given in Table I. TABLE I DETERMINATION OF THIAMBUTOSINE IN SAMPLES Sample r A I Recovery, per cent., obtained by- No. official method proposed method 1 99-63 f 0-486* 99-98 f 0*473* 2 99.80 f 0.487 99-76 f 0.471 * Standard deviation calculated from ten determinations. The proposed procedure was also applied to the analysis of thiambutosine tablets. The results are in good agreement with those obtained by the official method (Table 11). The usual TABLE I1 DETERMINATION OF THIAMBUTOSINE IN TABLETS Recovery per tablet obtained by- Sample Labelled amount r A 1 No. per tablet/mg official method/mg proposed method/mg 1 600 499-23 f 1*269* 600.37 & 2*077* 2 600 499.26 f 0.8299 499.37 & 0.7864 * Standard deviation calculated from ten determinations.March, 1975 DETERMINATION OF THIAMBUTOSINE 181 tablet diluents, lubricants and excipients do not interfere in the analysis by the proposed method (Table 111).The method is simple, rapid and accurate. TABLE I11 RECOVERY OF THIAMBUTOSINE FROM VARIOUS EXCIPIENTS BY THE PROPOSED METHOD Sample Excipient 26 mg of thiambutosine: Stearic acid . . .. .. .. .. 200 mg of excipient Magnesium stearate . . .. .. .. Calcium sulphate . . .. . . .. 25 mg of thiambutosine: Citric acid . . . . .. .. .. 10 g of excipient Potassium hydrogen carbonate . , .. Talc . . .. . . .. .. .. Sucrose and maize starch . . .. .. Potassium hydrogen carbonate, citric acid and sucrose (1 + 1 + 1) . . .. .. * Average recovery of three experiments. Recovery, per cent.* 100.46 100.47 100.38 100.78 100-69 99-84 99.95 99-70 Conclusion Suitable reaction conditions have been established for the assay of thiambutosine in the range 20 to 80 pg per millilitre of reaction mixture. The method is rapid, has a reproducibility of &O-473 per cent. and the results compare favourably with those obtained by the official method. The authors express their sincere thanks to Dr. C. S. Shah, Principal, L.M. College of Pharmacy, Ahmedabad-9, India, for providing the facilities to carry out this work. References 1. 2. 3. 4. Devani, M. B., Shishoo, C. J., and Shah, M. G., Analyst, 1973, 98, 759. Shishoo, C. J., Devani, M. B., and Shah, M. G., Analyst, 1973, 98, 762. “British Pharmacopoeia 1968,” The Pharmaceutical Press, London, 1968. p. 1008. Smith, R. L., and Williams, R. T., J . Mednl Pharm. Chem., 1961, 4, 163. Received May 2nd. 1974 Accepted July 22nd, 1974

ISSN:0003-2654    

DOI:10.1039/AN9750000178

出版商:RSC    

年代:1975

数据来源: RSC