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Determination of the ammonium content in waste waters by means of the air-gap electrode
/~w/yrico Cltithw rlcrtr. 72 ( 1974) 2 15-2 I3 Amsterdam I!‘.. Elscvicr Scientific Publishing Company.
SHORT
215 - Printed
in The
Ncthcrlands
COMMUNICATION
Determination electrode
of the ammonium
content
in waste
waters
by means
of the air-gap
Of the various methods for the determination of ammonia in, fDr example, sea. natural or waste waters, the indophenol method’ is the most widely used proccdurc. On the bilsis of an intcrcalibration of methods for ammonia, NOKDFOHSK‘S working committee on water analysis has recommended the indophenol method as the standard procedure2. In this method the water sample is treated with alkaline hypochlorite and phenol in the presence of sodium nitroprusside. yielding a blue indophenol dye which then is measured spectrophotometrically. Although the method is very sensitive and potentially is well suited for automation, there arc severnl limitations in making it applicable to routine analysis. The method is time-consuming as it takes 34 h to achieve complete colour development at room temperature, although a recently developed modified procedure has reduced the time required to cu. 30 min ‘. In automated operations (e.g., AutoAnalyzer), the time required is further decrcascd by allowing the renction to take .place at elevated temperature, and by measuring the absorbance before the reaction has reached equilibrium’. More important. however, is the filet that the analysis is subject to serious interference from. for example, mercury and iron ions, iis well as soluble sulphidcs2.3. Thus, NORDFORSK has recommended that the water samples should not be prcservcd by any means (such as mercury(II) or acid) and that the actual analysis ought lo be carried out as soon as possible after sampling. It should be added that the use of the many reagents needed for the procedure not only makes it complicated. but also may result in high reagent blynks. Ammonia can be determined very selectively by distillation and titration as described by Riley* who found that the pH value for optimal recovery of ammonia is 9.2. The procedure is very accurate above 1 p.p.m. nitrogen, but less suitable below this level owing to the possibility of titration errors. Howcvcr. the long time needed per assay makes the method inappropriate as ‘a standard procedure for series analysis. In addition, it has recently been reported that organic nitrogen compounds in waste water may decompose during distillation, thereby increasing t hc ammonia level’. In order to minimize sample handling and the likelihood of contamination, and to circumvent the interferences of the indophenol method. ammonia-specific
216
SHORT
COMMUNICATION
electrodes have been suggested for the determination of ammonia in aqueous solutions5.6. These gas-sensing electrodes consist in principle of ii hydrophobic gaspermeable mcmbranc which separates the alkaline test solution from an internal solution of ammonium chloride of fixed molnrity. A glass pH electrode and a reference clcctrodc are immersed in the internal solution. Thus. when the electrode is placed in an alkaline test solution. ammonia can difTuse ilcross the membrane. alter the ammonia concentration in the lilling solution and so cause a pH change which is monitored by the glass elcctrodc. However, shortcomings like clogging of the gas-permeable membrane and Icakage of the inner electrolyte around the gas membrane, us recently reported to occur with the Orion ammonia clectrodc’. may impair the use of such sensors. As the gas-permeable membrane must b’e hydrophobic, the siimplc solution must not contain any wetting agent*, and thcrefore measurements in, for example, effluent wilters. which may often contain surfactants, organic waste and particulate matter. hilve to bc ciirefully considered. Furthermore. i1 proviso for the iippliciition of such sensors in routine analysis is that the electrode not only gives a fast response, but also rapidly rccovcrs before the next measurement, that is, the ammonia must rediffusc eilsily out of the inner electrolyte solution. All these difficulties and limitations are eliminated in the recently developed air-gap electrode”, which has been used successfully for determinations of ammonium ion. hydrogencarbonnte and urea (oicr enzymatic dcgrodation) in full blood, plasma and serum’. lo. sulphur dioxide in wine’ ‘, and nitrate in foodstuffs. The air-gap electrode is based on the silme principle as other gas sensors. except that it does not have any gas-permeable membrane. The membrane is replaced by an air gap which separates the electrolyte layer from the sample solution, the entire system being contained in iI gas-tight measuring chamber. The electrolyte itself is adsorbed as :I very thin liiyer (film) on the surface of the indicator electrode (irl CUSII the pH clcctrode). The greatest advantage of this design is that the electrode never comes into direct physical contact with the sample solution, which can therefore contain components which normillly alTect the function of any electrode. In addition, simplicity of design, ease of renewill of electrolyte, and filst response are further advantages gained by replacing the gaspermeable membrane with the air gap. This gas sensor wits used for’ the determination of ammonium in wastewater samples collected at various points in a purificntion plant (including a nitrification unit) situated in Lundtofte. Denmark. The size of the plant is cquivalcnt to approximately 25.000 persons, the waste water originating equally from industry and households. Besides the potcntiometric assay. illI samples were also mensurcd by the indophenol and the distillation methods. The distillation method was carried out as described in Studad Mctltods ji)r the Escmimrior~ 01’ Water cud WCJSIL’ Wuter’, while the indophenol method wils used iis recommended by NORDFORSK’. Both methods were checked by ammonium chloride standards. During the application of the indophenol method, it was found advisable to use EDTA instead of citrate to mask the effect of the metal ions present. Furthermore. it was observed that as little as 1-2 p.p.m. sulphide gave a strong interference, and that sulphide absorbance of the measured contents above 15-20 p.p.m. decreased the relative dye to almost zero, thus seriously impairing the method.
SHORT
217
COMMUNICATION
The apparatus and the air-gap electrode were the same as previously described’.“. Both the larger and the smaller electrode chambers. having a total volume of 8.0 ml and 1.0 ml, respectively, were used. The electrolyte solutions ( l.0~10-3 M ammonium chloride for the larger electrode chamber, and 1.0. IO-’ M ammonium chloride for the smaller chamber) were saturated with a wetting agent (Victawet 12. Stauffcr Chemical Company. U.S.A.). The electrolyte layer on the surface of the glass electrode was rcncwed after each measurement, by letting the itir-gap electrode rest on it perspcx holder, containing a sponge soaked with the electrolyte solution 5). When not in use. the electrode was similarly stored in the electrode holder. When the larger (macro) chamber was used, the calibration curve was obtained by pipctting 2.0 ml of the standard solution of ammonium chloride into ii polyethylene beaker which fitted into the elcctrodc chamber. A Teflon-covered magnetic stirring bar wits placed into the beaker followed by I .O ml of 0.1 A4 sodium hydroxide. The chamber was then immediately closed by lneans of the body of the air-gap electrode and the mitgnetic stirrer was started. A steady reading of the electrode. pH,, was reached within 2-3 min and read to 0.001 pH,. For crrch standard ammonium chloride solution ( I. 2.5. 5. 7.5, 10. 13. and 15 p.p_m. nitrogen). two measurements were plotted uersus the pH, values obtained. The calibration curve was computed on a WANG 700 B Programmable Electronic Calculator provided with a WANG 702 Plotting Output Writer using a lincarregression itnalysis programmc. The calibrittion curve was then used in the subsequent evaluations of the waste-water samples. With the standard solutions. II straight line was obtained with a slope of 0.9790 log (p.p.m. nitrogen)/pH,. a standard deviation of O.O2’pH, (corresponding to S!>,), and it regression cocflicient of 0.998. When the waste-water samples were analyzed. the same procedure wits used. TABLE
I
DETERMINATION COLLECTED AT PLANT --.-
OF FIVE
THE AMMONIUM CONTENT DIFFERENT LOCATIONS IN ------
-
.I 3.ss 14.01 15.61 3.22 2.21
IN WASTE-WATER SAMPLES MUNICIPAL PURIFICATION -_--_-
___.--__-----------
N ~~~~~ttvr~r (p.p.wr.
Untrtxtcd W~SIC wutcr Scdimcntation basin lnlct 10 nitrilication unit Oullcl from nilrificuliot~ unit Oullct from purifkltion plant
A
A’If ,)
13.36 14.61 14.95 3.25 2.05
12.93 I3.71( 13.81 3.50 2.20
The calibration of the electrode and the actual waste water analysis performed with the smaller (micro) chamber were carried out by an identical procedure: 200 /tl of standard ammonium chloride solution (or 200 rtl of waste-water sample)
218
SHORT
COMMUNICATION
\viIs pipetteci by means of an MLA Precision pipette into the pcrspcx microchamber containing iI magnetic stirring bar. 200 ,tl of 0.2 M sodium hydroxide was added. and the chamber Wits immediately closed with the electrode body. Only then was the magnetic stirrer started. A steady pH, reading was reached within I-2 min and rend to 0.001 pH,. For each standard (0.05. 0. I. 0.5. 1. 5 and IO mmol I-‘). two measurements were plotted I:CI~.SU.S the pH, value obtilined. as outlined above. With the standard solutions. ;I straight line was obtained with iI slope of I.003 log[NH;k/pH,. a regression coefficient of 0.9999 and i1 standard deviation of 0.01 pH, (corresponding to 2.4’,!:,).
The results obtained by the macro-chamber proccdurc. compared with those indophenol and the titration methods. ;Irc summarized in Table I. In order to cvuluatc the st:\ndilrd deviiltion on the individual determinations by the micro-chamber proccdurc. two waste-water SiImplcs (silmpled at il different date from those used previously) were tneilsurcd 4 and 8 times. rcspcctivcly. The lirst sample (untrcatcd waste wiltcr) was mcnsurcd twice. each series consisting of 4 dcterminntions. In the ftrst series. the stnndard deviation was 0.0031 pH, and in the second one 0.065 pH,. i.e.. corresponding to 0.75~-1.56’,‘$ (the silmplc contained. according to the standard curve, 37.3 p_p.m. nitrogen). The second sample (water from the outlet of the purification plant) wils measured 8 times, resulting in a stand;lrd deviation of O.CKI81 pH,. corresponding to 1.93’>;, (the sample containcd 1.90 p_p.m. nitrogen). To cheek the method further. two waste-water samples which previously had been measured. were spiked with known amounts of atnmonium~chloride and then each measured twice ilgi\in with the micro-chamber ilssembly. The first sample (untreated witste wiltcr), containing 28.6 p.p.m. nitrogen. was spiked with 21.4 p.p.m. nitrogen (i.e.. ;l tOtill of 50.0 p.p.m.): it was found to contain 50.6 p.p.m. nitrogen. The second sample (outlet from the nitrification plant: the sample Svi\s collected during i1 cold period) WiIS originally found ‘to contain 5.72 p.p.m. nitrogen ils ilmmonia; after being spiked with 2. I4 p.p.m. nitrogen. the content was found to be 7.80 p.p.m.. compared to the calculated value of 7.86 p.p.m. nitrogen. Thus. within cxperimcntal error. very ilccurate results were obtilincd. . of the
It takes only II few minutes to make it determination with the ilir-gap elcctrodc, and thus the time of analysis is drastically reduced compared to the conventional methods. Furthcrmorc. the method is highly selective and is not affected by any of the interferences to which the other methods are subject. The potcntiomctric method should equai)y well be applicable to the analysis of such as drinking water and sea wilter. AS only iL ~CCW less polluted waters. waste-water samples were an;llyzed. this communication does not pretend to be ;I thorough investigation. but is rather intcndcd to s&e ~1s a demonstration of the capacity of the electrode for this type of mcasurcmcnt.’ The authors extend their sincere thanks to J. P. Jorgensen of the Department of Sanitary Engineering of this University for critical discussions i\nd for his help
SHORT
COMMUNICATION
in providing the wnstc-water samples; technical assistance. This work was Rcsenrch Council.
219
and to I. M. Johansen for her conscientious in part supported by the Danish Natural
REFERENCES of 1Vfircr o~itl M’osrp I Slcorclartl Mrrliotls jar llrc Escoilimtrio~r Wurw. American Public tic;~lth Associalion. American Water Works Association. and Water Pollution Control Fcdcration. 12th cdn.. 1965. 2 NORDFORSK. Iritcrccrlihrtrriotl of rh Ittrlopl~et~ol Mvtltocl Ji)r rlrc Drfrrttlittariott of Amrtrcmitr. Working Committee on Warcr Analysis. Miljiiu;irdssckrcl;Iri;llct. Publ. 1973: I. 3 V. W. TrucsJ;llc. AfwfJ~sr (Lo~irlr~~i). 96 ( 1971 ) 584. 4 J. P. Riley. A~cct/. ‘Chim. Acru. 9 (1953) 575. 5 P. J. LcBlrtnc i111d J. F. Sliwinski. Amr. L.ah.. 5 (1973) 51. 6 T. R. Gilbert and A. M. Clay. Awl. Cfwn.. 45 ( 1973) 1757. 7 H. F. Proclss and B. W. Wright. Ch. Cl~tw.. I9 (1973) 1162. 8 J. W. Ross and J. H. Hiscmun. Plenary Lccturc ;II The Inrcrnalional Symposium on Sclcctirc Ion-Scnsitivo Elcclrodcs. Curdiff. Wales. 1973. 9 J. RG%i?k;c and E. H. Hansen. Awl. Chi~m Acftc. 69 ( 1974) 129. IO E. H. Hansen and J. RliZiPka. Awl. Cbiw. Arrtr. in press. I I E. H. Hansen. H. Bcrgamin Filho and J. RJZitka, Awl. Cllittr. Acre. in press.
SHORT
215 - Printed
in The
Ncthcrlands
COMMUNICATION
Determination electrode
of the ammonium
content
in waste
waters
by means
of the air-gap
Of the various methods for the determination of ammonia in, fDr example, sea. natural or waste waters, the indophenol method’ is the most widely used proccdurc. On the bilsis of an intcrcalibration of methods for ammonia, NOKDFOHSK‘S working committee on water analysis has recommended the indophenol method as the standard procedure2. In this method the water sample is treated with alkaline hypochlorite and phenol in the presence of sodium nitroprusside. yielding a blue indophenol dye which then is measured spectrophotometrically. Although the method is very sensitive and potentially is well suited for automation, there arc severnl limitations in making it applicable to routine analysis. The method is time-consuming as it takes 34 h to achieve complete colour development at room temperature, although a recently developed modified procedure has reduced the time required to cu. 30 min ‘. In automated operations (e.g., AutoAnalyzer), the time required is further decrcascd by allowing the renction to take .place at elevated temperature, and by measuring the absorbance before the reaction has reached equilibrium’. More important. however, is the filet that the analysis is subject to serious interference from. for example, mercury and iron ions, iis well as soluble sulphidcs2.3. Thus, NORDFORSK has recommended that the water samples should not be prcservcd by any means (such as mercury(II) or acid) and that the actual analysis ought lo be carried out as soon as possible after sampling. It should be added that the use of the many reagents needed for the procedure not only makes it complicated. but also may result in high reagent blynks. Ammonia can be determined very selectively by distillation and titration as described by Riley* who found that the pH value for optimal recovery of ammonia is 9.2. The procedure is very accurate above 1 p.p.m. nitrogen, but less suitable below this level owing to the possibility of titration errors. Howcvcr. the long time needed per assay makes the method inappropriate as ‘a standard procedure for series analysis. In addition, it has recently been reported that organic nitrogen compounds in waste water may decompose during distillation, thereby increasing t hc ammonia level’. In order to minimize sample handling and the likelihood of contamination, and to circumvent the interferences of the indophenol method. ammonia-specific
216
SHORT
COMMUNICATION
electrodes have been suggested for the determination of ammonia in aqueous solutions5.6. These gas-sensing electrodes consist in principle of ii hydrophobic gaspermeable mcmbranc which separates the alkaline test solution from an internal solution of ammonium chloride of fixed molnrity. A glass pH electrode and a reference clcctrodc are immersed in the internal solution. Thus. when the electrode is placed in an alkaline test solution. ammonia can difTuse ilcross the membrane. alter the ammonia concentration in the lilling solution and so cause a pH change which is monitored by the glass elcctrodc. However, shortcomings like clogging of the gas-permeable membrane and Icakage of the inner electrolyte around the gas membrane, us recently reported to occur with the Orion ammonia clectrodc’. may impair the use of such sensors. As the gas-permeable membrane must b’e hydrophobic, the siimplc solution must not contain any wetting agent*, and thcrefore measurements in, for example, effluent wilters. which may often contain surfactants, organic waste and particulate matter. hilve to bc ciirefully considered. Furthermore. i1 proviso for the iippliciition of such sensors in routine analysis is that the electrode not only gives a fast response, but also rapidly rccovcrs before the next measurement, that is, the ammonia must rediffusc eilsily out of the inner electrolyte solution. All these difficulties and limitations are eliminated in the recently developed air-gap electrode”, which has been used successfully for determinations of ammonium ion. hydrogencarbonnte and urea (oicr enzymatic dcgrodation) in full blood, plasma and serum’. lo. sulphur dioxide in wine’ ‘, and nitrate in foodstuffs. The air-gap electrode is based on the silme principle as other gas sensors. except that it does not have any gas-permeable membrane. The membrane is replaced by an air gap which separates the electrolyte layer from the sample solution, the entire system being contained in iI gas-tight measuring chamber. The electrolyte itself is adsorbed as :I very thin liiyer (film) on the surface of the indicator electrode (irl CUSII the pH clcctrode). The greatest advantage of this design is that the electrode never comes into direct physical contact with the sample solution, which can therefore contain components which normillly alTect the function of any electrode. In addition, simplicity of design, ease of renewill of electrolyte, and filst response are further advantages gained by replacing the gaspermeable membrane with the air gap. This gas sensor wits used for’ the determination of ammonium in wastewater samples collected at various points in a purificntion plant (including a nitrification unit) situated in Lundtofte. Denmark. The size of the plant is cquivalcnt to approximately 25.000 persons, the waste water originating equally from industry and households. Besides the potcntiometric assay. illI samples were also mensurcd by the indophenol and the distillation methods. The distillation method was carried out as described in Studad Mctltods ji)r the Escmimrior~ 01’ Water cud WCJSIL’ Wuter’, while the indophenol method wils used iis recommended by NORDFORSK’. Both methods were checked by ammonium chloride standards. During the application of the indophenol method, it was found advisable to use EDTA instead of citrate to mask the effect of the metal ions present. Furthermore. it was observed that as little as 1-2 p.p.m. sulphide gave a strong interference, and that sulphide absorbance of the measured contents above 15-20 p.p.m. decreased the relative dye to almost zero, thus seriously impairing the method.
SHORT
217
COMMUNICATION
The apparatus and the air-gap electrode were the same as previously described’.“. Both the larger and the smaller electrode chambers. having a total volume of 8.0 ml and 1.0 ml, respectively, were used. The electrolyte solutions ( l.0~10-3 M ammonium chloride for the larger electrode chamber, and 1.0. IO-’ M ammonium chloride for the smaller chamber) were saturated with a wetting agent (Victawet 12. Stauffcr Chemical Company. U.S.A.). The electrolyte layer on the surface of the glass electrode was rcncwed after each measurement, by letting the itir-gap electrode rest on it perspcx holder, containing a sponge soaked with the electrolyte solution 5). When not in use. the electrode was similarly stored in the electrode holder. When the larger (macro) chamber was used, the calibration curve was obtained by pipctting 2.0 ml of the standard solution of ammonium chloride into ii polyethylene beaker which fitted into the elcctrodc chamber. A Teflon-covered magnetic stirring bar wits placed into the beaker followed by I .O ml of 0.1 A4 sodium hydroxide. The chamber was then immediately closed by lneans of the body of the air-gap electrode and the mitgnetic stirrer was started. A steady reading of the electrode. pH,, was reached within 2-3 min and read to 0.001 pH,. For crrch standard ammonium chloride solution ( I. 2.5. 5. 7.5, 10. 13. and 15 p.p_m. nitrogen). two measurements were plotted uersus the pH, values obtained. The calibration curve was computed on a WANG 700 B Programmable Electronic Calculator provided with a WANG 702 Plotting Output Writer using a lincarregression itnalysis programmc. The calibrittion curve was then used in the subsequent evaluations of the waste-water samples. With the standard solutions. II straight line was obtained with a slope of 0.9790 log (p.p.m. nitrogen)/pH,. a standard deviation of O.O2’pH, (corresponding to S!>,), and it regression cocflicient of 0.998. When the waste-water samples were analyzed. the same procedure wits used. TABLE
I
DETERMINATION COLLECTED AT PLANT --.-
OF FIVE
THE AMMONIUM CONTENT DIFFERENT LOCATIONS IN ------
-
.I 3.ss 14.01 15.61 3.22 2.21
IN WASTE-WATER SAMPLES MUNICIPAL PURIFICATION -_--_-
___.--__-----------
N ~~~~~ttvr~r (p.p.wr.
Untrtxtcd W~SIC wutcr Scdimcntation basin lnlct 10 nitrilication unit Oullcl from nilrificuliot~ unit Oullct from purifkltion plant
A
A’If ,)
13.36 14.61 14.95 3.25 2.05
12.93 I3.71( 13.81 3.50 2.20
The calibration of the electrode and the actual waste water analysis performed with the smaller (micro) chamber were carried out by an identical procedure: 200 /tl of standard ammonium chloride solution (or 200 rtl of waste-water sample)
218
SHORT
COMMUNICATION
\viIs pipetteci by means of an MLA Precision pipette into the pcrspcx microchamber containing iI magnetic stirring bar. 200 ,tl of 0.2 M sodium hydroxide was added. and the chamber Wits immediately closed with the electrode body. Only then was the magnetic stirrer started. A steady pH, reading was reached within I-2 min and rend to 0.001 pH,. For each standard (0.05. 0. I. 0.5. 1. 5 and IO mmol I-‘). two measurements were plotted I:CI~.SU.S the pH, value obtilined. as outlined above. With the standard solutions. ;I straight line was obtained with iI slope of I.003 log[NH;k/pH,. a regression coefficient of 0.9999 and i1 standard deviation of 0.01 pH, (corresponding to 2.4’,!:,).
The results obtained by the macro-chamber proccdurc. compared with those indophenol and the titration methods. ;Irc summarized in Table I. In order to cvuluatc the st:\ndilrd deviiltion on the individual determinations by the micro-chamber proccdurc. two waste-water SiImplcs (silmpled at il different date from those used previously) were tneilsurcd 4 and 8 times. rcspcctivcly. The lirst sample (untrcatcd waste wiltcr) was mcnsurcd twice. each series consisting of 4 dcterminntions. In the ftrst series. the stnndard deviation was 0.0031 pH, and in the second one 0.065 pH,. i.e.. corresponding to 0.75~-1.56’,‘$ (the silmplc contained. according to the standard curve, 37.3 p_p.m. nitrogen). The second sample (water from the outlet of the purification plant) wils measured 8 times, resulting in a stand;lrd deviation of O.CKI81 pH,. corresponding to 1.93’>;, (the sample containcd 1.90 p_p.m. nitrogen). To cheek the method further. two waste-water samples which previously had been measured. were spiked with known amounts of atnmonium~chloride and then each measured twice ilgi\in with the micro-chamber ilssembly. The first sample (untreated witste wiltcr), containing 28.6 p.p.m. nitrogen. was spiked with 21.4 p.p.m. nitrogen (i.e.. ;l tOtill of 50.0 p.p.m.): it was found to contain 50.6 p.p.m. nitrogen. The second sample (outlet from the nitrification plant: the sample Svi\s collected during i1 cold period) WiIS originally found ‘to contain 5.72 p.p.m. nitrogen ils ilmmonia; after being spiked with 2. I4 p.p.m. nitrogen. the content was found to be 7.80 p.p.m.. compared to the calculated value of 7.86 p.p.m. nitrogen. Thus. within cxperimcntal error. very ilccurate results were obtilincd. . of the
It takes only II few minutes to make it determination with the ilir-gap elcctrodc, and thus the time of analysis is drastically reduced compared to the conventional methods. Furthcrmorc. the method is highly selective and is not affected by any of the interferences to which the other methods are subject. The potcntiomctric method should equai)y well be applicable to the analysis of such as drinking water and sea wilter. AS only iL ~CCW less polluted waters. waste-water samples were an;llyzed. this communication does not pretend to be ;I thorough investigation. but is rather intcndcd to s&e ~1s a demonstration of the capacity of the electrode for this type of mcasurcmcnt.’ The authors extend their sincere thanks to J. P. Jorgensen of the Department of Sanitary Engineering of this University for critical discussions i\nd for his help
SHORT
COMMUNICATION
in providing the wnstc-water samples; technical assistance. This work was Rcsenrch Council.
219
and to I. M. Johansen for her conscientious in part supported by the Danish Natural
REFERENCES of 1Vfircr o~itl M’osrp I Slcorclartl Mrrliotls jar llrc Escoilimtrio~r Wurw. American Public tic;~lth Associalion. American Water Works Association. and Water Pollution Control Fcdcration. 12th cdn.. 1965. 2 NORDFORSK. Iritcrccrlihrtrriotl of rh Ittrlopl~et~ol Mvtltocl Ji)r rlrc Drfrrttlittariott of Amrtrcmitr. Working Committee on Warcr Analysis. Miljiiu;irdssckrcl;Iri;llct. Publ. 1973: I. 3 V. W. TrucsJ;llc. AfwfJ~sr (Lo~irlr~~i). 96 ( 1971 ) 584. 4 J. P. Riley. A~cct/. ‘Chim. Acru. 9 (1953) 575. 5 P. J. LcBlrtnc i111d J. F. Sliwinski. Amr. L.ah.. 5 (1973) 51. 6 T. R. Gilbert and A. M. Clay. Awl. Cfwn.. 45 ( 1973) 1757. 7 H. F. Proclss and B. W. Wright. Ch. Cl~tw.. I9 (1973) 1162. 8 J. W. Ross and J. H. Hiscmun. Plenary Lccturc ;II The Inrcrnalional Symposium on Sclcctirc Ion-Scnsitivo Elcclrodcs. Curdiff. Wales. 1973. 9 J. RG%i?k;c and E. H. Hansen. Awl. Chi~m Acftc. 69 ( 1974) 129. IO E. H. Hansen and J. RliZiPka. Awl. Cbiw. Arrtr. in press. I I E. H. Hansen. H. Bcrgamin Filho and J. RJZitka, Awl. Cllittr. Acre. in press.