Application Bulletin - Login
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Application Bulletin - Login
@Metrohm No. 133/2 e Application Bulletin General analytical laboratories Of interest for: A 1, 2, 7, 8, 11 Determination of ammonium with the ion-selective electrode Summary Although the known photometric methods for the determination of ammonia are accurate, they require a considerable amount of time (Nessler method 30 min, indophenol method 90 min reaction time). A further disadvantage of these methods is that only clear solutions can be processed. Cloudy solutions must first be clarified by time-consuming procedures. These problems do not exist with the ion-selective ammonia electrode. Measurements can be easily performed in drinking water, ground water and surface water as well as in waste water, soil extracts and Kjeldahl digestion solutions (without distillation). The determination of ammonia in ammonia salts, of the nitric acid content in nitrates and of the nitrogen content of organic compounds is based on the principle that the ammonia ion is released as ammonia gas upon addition of excess caustic soda: NH 4 + +OH--.. NH 3 t +H 2 0 The outer membrane of the electrode allows the ammonia to diffuse through. The change in the pH value of the inner electrolyte solution is monitored by a combined glass electrode. If the substance to be measured is not present in the form of an ammonia salt, it must first be converted into one. Organic nitrogen compounds, especially amino compounds are digested according to Kjeldahl by heating with concentrated sulphuric acid. The carbon is oxidized to carbon dioxide in the process while the organic nitrogen is transformed quantitatively into ammonium sulphate. Numerous examples are given. In the appendix, various sample preparations and an example for a Kjeldahl decomposition are described. Instruments and accessories • 1x2.692.0010 pH/Ion Meter • 1x2.665.0030 or 1x2.725.0010 Dosimat titration stand with 1x2.728.0040 Magnetic Stirrer and 1x6.3012.153 (5 ml) or 1x6.3012.213 Exchange Unit (10 ml) and 1x6.2124.000 Keypad (required for 665 only) • 1x6.2138.010 Connecting cable 692 - 665n28 @Metrohm Application Bulletin No. 133/2 e Determination of ammonium with the ion-selective electrode • Printers and connecting cables see 692 Instructions for Use, chapter «Connection possibilities» • 1 x6.0506.01 O NH3 electrode with fixed cable • 1 x6.1418.220 Titrating vessel with thermostatic sleeve and Page2 1x6.1414.010 Titrating head and thermostat or 1x6.1110.100 Temperature sensor Pt 1OOO with 1x6.2104.080 Cable Reagents • Caustic soda : c(NaOH) = 1O mol/L • Standard solution: 2.97 g pure NH4CI are dissolved in distilled water and the solution filled up to 1 L; p (NH 4) = 1OOO mg/L • Conditioning solution: c(NH 4CI) = 0.1 mol/L (can be kept during 1 to 5 days) Handling of the electrode To achieve optimal measurements, observe the following points: • Before the first use or after a longer storage in the dry state, the inner glass electrode must be soaked for 6 to 12 hours in a pH= 7 buffer solution. Rinse with distilled water afterwards and dry by dabbing with a soft paper towel. The electrode should be fitted with the membrane as described in the Instructions for Use and 2.5 ml of inner electrolyte solution added. After it is put together the electrode should be shaken like a fever thermometer to remove any air bubbles from the membrane. Then the electrode is placed for approximately 1O min in distilled water. • Rinse the electrode after every measurement with distilled water and remove any adhering water droplets by dabbing with a soft paper towel. Before every new measurement the electrode should be conditioned in distilled water for at least 1O min. For determining concentrations under 1 mg/L, the electrode must be conditioned for approximately 30 minutes. For storage times of 1 to 5 days the electrode is placed in c(NH 4CI) = 0.1 mol/L. For longer periods, store dry. • An excessive slope in a standard addition is usually due to conditioning times that are too short or to an error in the preparation of the electrode. However, slopes that are not more than 3 mV above the theoretical value can be tolerated. • The measurements should be performed under uniform stirring conditions and at constant temperature. Stirring should be continuous (stirring parameter 692: «ON»), because bubbles can form on the membrane during an interruption in stirring, which can cause false readings. Make sure that the stirrer is switched on and that a stirring rod is in the measuring vessel. If additions are made without stirring, erroneous results are obtained! • Interferences can have the following causes: Volatile amines: Simulate an elevated concentration of ammonia. Ethylene amine gives almost the same difference in potential as ammonia. Longer C chains cause less interference. @Metrohm Application Bulletin Determination of ammonium with the ion-selective electrode No. 133/2 e Page 3 Hydrazine: Causes the same interference as described above. Complexing ions: Ag, Hg, Cu, and Ni among others simulate a lower ammonia concentration. The interference caused by 20 ppm Hg can be eliminated by using a solution with an additional 0.46 g Nal per 100 ml NaOH instead of the nonnal NaOH solution. Concentration: Should not exceed 1 mol/L. At higher concentrations the sample must be diluted with distilled water (Kjeldahl digestion). However, in the lower concentration ranges the electrode becomes less sensitive and requires a longer time to reach a stable reading (up to 15 min). Proteins: A high protein concentration (1 g/L) simulates higher ammonia concentrations. The calibration curve becomes linear only above 10 mg/L of ammonium. • If more than 5 to 6 minutes are required to obtain a stable reading, the membrane and inner electrolyte must be replaced. • If samples with greatly varying ammonia concentrations are measured frequently, the time required to reach a stable reading becomes longer for the samples with lower concentrations. • After the addition of NaOH the pH-value must be at least 12. Acidic samples must be neutralized before the addition of NaOH. Add the NaOH and begin the measurement immediately. • Choice of Procedure: General hints The standard addition is a quick and reliable method especially for undefined sample matrices. A direct measurement is indicated when many measurements of the same solutions are to be carried in a defined and unproblematic sample matrix. • Choice of Standard Concentrations: In order to ensure an accurate evaluation of the standard addition by the 692 pH-lion Meter, the standard concentrations (Cstd) for the different burette volumes (Vburettefml) must be chosen as a function of the sample concentration (Csmp1) according to the table below Vburettefml Cstd : Csmp1 5 40: 1 10 20: 1 20 10: 1 50 5: 1 If the sample has been greatly diluted with ISA or TISAB, this ratio must be considered. @Metrohm Application Bulletin No. 133/2 e Determination of ammonium with the ion-selective electrode Example: sample concentration (Csmp1) burette volume CVburette) sample size ISAITISAB total volume (Vtotal) Factor from table (Cstd : Csmpt) Page 4 5 mg/L 10 ml 10 ml 10 ml 20ml 20 The result is a sample concentration of 2.5 mg/L in the initial solution. The optimal concentration of the standard is therefore 2.5 mg/L x 20 = so mg/L. Note, however, that this is only a general guide for the standard concentration. Exact measurements are also possible if the above recommendations are not strictly adhered to. • The following fonnulae are used to calculate the results (see also the Instructions for use, paragraphs 3.1 and 3.2): For csample size OFF ml»: Result = ci x factor; for csample size XX.X ml»: Result= ci x cVtotal»/«sample size»; for «sample size XXX.X g»: Result= ci x «Vtotal»/«sample size» x 1 g/ml Example using «sample size X.XXX g»: A precisely weighed amount of substance, e.g. 5.153 g, is dissolved and the solution filled up to 100 ml. This solution is diluted 100 times by taking 1O ml and topping it off to 1OOO ml, for example. 20 ml of this dilute solution plus ISAITISAB, etc. are put into the measuring vessel. These 20 ml contain 500 times (100 mU20 ml x 100) less substance than the original sample. The corresponding portion (aliquot) is entered as «factor 500». With every new weighing, the exact sample weight is entered as «sample size X.XXX g». If the result is expressed in g/1 OOg (percent) or mg/kg (ppm) there is no correction for specific gravity. If, however, a volume-related unit is used, such as mol/L, g/L, µg/L, etc., the specific gravity of the original sample must be taken into account. If the specific gravity is, for example, 2.01 g/ml, the factor referred to above must be multiplied by 2.01 (500 x 2.01 = 1005). • Low ammonia concentrations: For concentrations < 1 mg/L the standard solutions must have the same ionic background as the sample solutions. The adjustment time becomes distinctly longer. The blank values of the chemicals are to be taken into account. • All measurements should be perfonned at constant temperature (for example, at 25 °C). • Refer to the hints given in the Instructions for Use of the 692 pH/Ion Meter. No. 133/2 e Application Bulletin Page 5 Determination of ammonium with the ion-selective electrode Direct Measurement: Please consider the hints given in the Instructions for Use of the 692 pH/Ion Meter. The following calibration is an example for a sample with an unproblematic matrix: Initial solution: 20ml ISA solution: 1 ml Standard: 1OOO mg/l in 1O ml Exchange Unit Concentration range: 1 ... 200 mg/l No. of calibration points: 6 93-11-17 time 1e:2'3:25 · idl ff>p!ik.Nr.13311.::1 id2 ltl3-!.~ date 93-11-16. time 16:23:34 cone.calibration date Eas. inPUt: · t1 HH4(+1) ion tlft lfl3. electr. id teweraturi Rl. 25.0 C cal.date 93-11-16 16:23 Ol.V cone/PPS 93-11-17 10:29:25 OConcl'~ sid. 1 9.99E-81 -73.6 0.2 std. 2 std. 3 2.89£+98 -85.2 S.l3Eiillll .,.183.5 2.48E'**l1 -126.8 6.93E+01 -152.6 2.08:+92 -179.2 -e.s std. 4 std. 5 :;td. 6 8.3 e.1 -9.1 e.e U/..U -68r l -Sir -1-~ -121l ~ -1411 -16~ -111t r - 1 - - - 1 - - . 1 " - - . . . . J 'log c -288'-. -1 variance slOPe EC8> c<blank) I 1 2 8.883 -59.2 .v -42.711.J 2.33E+fle PPI === 109 PPI 3 H!i4(+1) @Metrohm No. 133/2 e Application Bulletin Page6 Determination of ammonium with the ion-selective electrode Direct measurement in an aqueous ammonium standard: date 93-11-16 t.i.me 16:28:42 Parameter 93-11-16 tilt 17;56:52 config direct INS.\fi: }dU)(iHaries >leasuriM Parwters ion t':IPt: cone.mi i: date ON emiish last disit: lfi4(+1) diaiM: PPll 1eas.il'lf".Jt: date 93-it-16 t.i.riie 17:56:57 electr. id: NHS tell'. ti'i~ i: c drift twtrature 8.5 .v11in !"l.(\ l'IUllber t..we of Dosi.a,: 1 25.0 tfi4(+) aetMd id stirrer: X:alculation Par.•ters s.l size IJ total f.letor wl size 111W >calibration paraiiiters drift no.of standards addition: ain. conc. aax.conc. Vinit ;jeiJ h:e {ff delta aedSUretent: WlftratiJre c Off ral 21.0 Ii 1.9 state: >Plot Paraeters left..,.. ri9"1t .ar. left aan.T riiht rwg. T ?rint header: alwaYS DH da~t.:ue: !ff Uk. tir .133/ld tlH3-ISE 25.0 c 8.5 1\.llmin Ml 6 auto 1. 9£+00 PPS 2.0E-t02 PPil date&ti1e: >RS232 seuir.95 baud rate: data bit: StOP Di t: 28.0 11 har1dshake: HWs RS control: OH l.0E+03 10 v Exchne lkli.t 1: >anal0t ouiPUt Hi•i ts Cone state: >l11its i Ci tlzen id2 )print mtas. value no.oi Exchamt lklits s~ie: 692 /t>rinter ::h<ir.acter set: ldl 11 665 692.0010 Pfil9ra&l 01~ conc.1 select: label PPll Cone (fF OFF (fF 8. 9E+08 l.0E+30 28 39 Pflll PPI c c Print cri.t: Pari t1:1: drHt 00 2400 s none @Metrohm No. 133/2 e Application Bulletin Page 7 Determination of ammonium with the ion-selective electrode Standard addition In principle, there are three types of standard addition: - manual -auto dos -auto Please consult the Instructions for Use for the characteristics of the individual types. The quickest method is the automatic standard addition {«auto»). Note, however, that in order to get exact results the difference in potential AU chosen must be at least 12 mV per standard addition and that at least 3 standard additions must be performed {total AU at least 30 mV). If exactly defined volumes of the standard additions are required but maximum ease of operation is also desirable, choose cauto dos» and enter the individual standard addition volumes (see Instructions for Use). Example for a concentration determination by standard addition: Sample size: ISA solution: Standard: No. of additions: datt. 93-11-15 20 ml 1 ml 1OOO mg/l in 1O ml Exchange Unit 5 ti• 17:88:56 Par•wr -.-: sld.ldd )Masuri111 l'il'•lfrs 1114<+!) ion Sift! cooc:.111itl lltH. il'PU.: tiectr.id: 1113 4rilt t.5 lll!lsin i-rawre 25.8 c iM<•) •Chad id stirrer= tll lc.ilal'-'ian 21l.t •l • l Slllt U toW 21.1 Ill ,.,. ,.,..wrs fil:lar SIP! SUit lllill 1.e Ii 93-11-15 time mee::s& additiavsi.b1r;,clioo 11111110d sld.ilid INS.iwt: llll tlte~r.id 14 dlW lfU.irAJU 25.8 i.eraturt Al. cooc:.sld c 1aa .... u io~l 21.1 ai -1ti4.l 11J initial wl :.age 28.811 •1 i'IIt 93-11-15 11:• claw "'111 sld.1ncr. std.incr. std.incr. sld.incr. 1 2 3 4 ;Id. (Iler. 5 IYalJ ~ 8.572 -169.4 -5.! e.712 -m.~ -5.1 Ull -179.~ 1.221 -191.4 1.668 -18'.2 -5.8 -4.~ -4.8 >sundard idlii lion liffl ;:mc,s;;i. nl'Cirll aadit.illlll dthaU add 1&1!'1'11 iull auio 5.., dos.raw: no.of addl:i.n llldi.IJI IJ EJCdlal1lt tilii: 18 ' UIMU -1''r -m~ -·1-mr t\ -191t \\ -1951 2 I -181~ i I -185t I ••• c variru sl.w E<8> lt!C(tl) a 8.81!2 -S.1.., -4i' .s "' 111- No. 133/2 e Application Bulletin Page8 Determination of ammonium with the ion-selective electrode Practical examples 1. Ammonium in drinking water using automatic standard addition Sample preparation: none ISAITISAB: not required Analysis: 20 ml sample is pipetted into the measuring vessel. Shortly before beginning the measurement, 1 ml of NaOH solution is added. The measuring vessel should be closed after the addition. 93-11-18 tile 13:48:24 Paraeier aeas. ~= std.add :wiSIJriM Parameters icn l\if'i: 1'114<+t> date cone.unit: ltiS.iOPUU elecir. id: · drift taffrasure •ttm id stirrer: ?Fii 1 lfl3 8.5 .U11in 25.8 c lif4(+) ~ >calcvlatiOfl Piralll!WS SIPl Si.zt . tff cl u total 21.0 11 factor 1.e SIP I size mi t: 1l >standird add.itioo tlft: add ccnc.std. report: addition: delta u dos.rate: oo.ot additions I.I ExctlanM lkli t: 1eee.e ,,. Ml auto lhlJ lediUI 3 10 daie 93-11-!ll ti• 13:57:41 additioo/su'.ltracdon 1ttllod 1eas. tlffl sw. add electr. id iti3 l 25.0 c 1000.e 1'1'61 21.0 •l -35.6 llU 93-11-18 13:57 waaPerature .an. cor.c.std I.I total initial uol ta9f da~ <.lVal 8.844 0.829 std.lr11:r. 3 8.942 UlillU illl.O -6S? -32.6 ·- sld.incr. 1 std. incr. 2 -78.2 -87.~ -!i:l.0 -9.? U/..V -3flr j I -<WLI I t I -58~ I t I -68t l ! \\ \ \\ ' \ \ -78t • -88~ \ \ f -981 -1 vari..-.ce slOPe E<il) lil4(+1) 8 log c I 0.000 -59.3 llU -48.7 0.822 ~ Pftl 1 @Metroln No. 133/2 e Application Bulletin Page9 Determination of ammonium with the ion-selective electrode 2. Ammonium in a waste water using automatic standard addition Sample preparation: none ISA/TISAB: not required Analysis: 20 ml sample is pipetted into the measuring vessel. Shortly before beginning the measurement, 1 ml of NaOH solution is added. The measuring vessel should be closed after the addition. Inlet of waste water treatment plant da1e 9:5-11-15 lilll! 15:16:24 addi tiOIVSlb"ac:tion llEtl'lod llNS.iwie: std.acld lllKtr.id . Ii() 11eas. inPUi: li.8 c w.erawrt 11a11. 10119.8 PPll an::.sld IJ wial· 21.3 rd -194.8 rJJ ini Ual uol me 28.8 •l Slll'l size 93-11-15 15:16 dase date 93-11-15 tile. 15:36:12 iddili.rilsdltraciuin 11Ulod meas. Wflt: 9 s1d.idd 1113 electr.id 11eas.inl'u1: teRrawre w.. conc.sed IJ iotal inHial uol ta9e 1 25.8 C 11Bl.8 ..,. 21.8 Ill -52.5 ~ 29.9 •l 93-11-15 15: 36 !IJl.U e.m -119.8 -14.2 . 9.117 -123.9 -4.9 -4.'3 8.186 -128.8 -133. 7 -4.9 8.1l8 8.159 -138.6 -4.9 ;j)/11 st.d. incr. 1 sid.incr. 2 std.incr. 3 sid.incr. 4 std.incr. 5 8 Outlet of waste water treatment plant Ui.U d.1/11 std.inc:r. s&ci. inc:r. std.inc:r. std. inc:r. Utlil dUllO 1 2 3 8.1141 3.815 0.818 -75.2 -21..7 -98. Z -s. e -85.8 -4.8 4 8.022 -8'9.S -4.S std.mcr. 5 8.927 -94.7 -4.9 U/..U -111 -51 \ -6 -71 \ -88 •• -141 l.lif'ianta slOPt EC8> 1114<+1> log c 1 -1 -181 2 8.888 -57.9 rJJ -45.5 .u 11.1 PPll • 11arin:e \ log c 1 ' 8.882 SlOPt -57.2 .u E(8) -46.8 !114(+1) - .u 1.36 PPI @Metrohm Application Bulletin Determination of ammonium with the ion-selective electrode Parameter report for waste water: date 93-11-15 iillle 16:42:56 parameter std.add ins Parameters tti4(+D ion i.Yf.e: PPI conc.ooii.: 1 aeas. il'lPUt: tfi3 electr.id: e.5 llJ/flin drift ~r 25.0 ~rawre 1ethod id stirrer: .~ I,, NH-1(+) ~ >calculation Parameters 51Fl size u totil factor S11Pl size unit: >standard addi tioo tl.:IPe: conc.std. fiP'Ort: addition: delta u dos.raw: oo.of additions IJ Exchan9e i))i i: 28.8 11 21.0 Ii 1.0 ml <)jd 1000.0 F'Pfl full auto S IV lleciiUI 5 10 No. 133/2 e Page 10 @Metrohm No. 133/2 e Application Bulletin Page 11 Determination of ammonium with the ion-selective electrode 3. Uquid fertilizer for ornamental flowers Sample preparation: The sample is diluted by 1 : 100 (= sample solution, factor 100) = ISAITISAB: not required Analysis: 20 ml sample solution and 1 ml 10% disodium tartrate solution or 1 ml 10% tartaric acid are pipetted into the measuring vessel. The NaOH solution is added just before the start of the determination. date 93-11-1'5 t.i• 16:26:38 Pa;•wr •as.~= s":l.add >measur iM Para1eiers ion "'": conc.ooit: •as.inPVU lfM<+D Pf'll l electr.idl ~ drift 0.5 I0/1ln 25.8 c w..raiure lfi4(+) •&iwld id ~ stimr: >cilcului.on Para1Hers 2'1.8 11 • l size 22.e 11 v iotal i~.e factor 511'1 Sitt llli t: 11 >siaidar'd addltior. ~ conc.std. l'li!POrt: addition: delta u add 1000.e PPis . IJll\I dUlllV d!Jlli -4.9 -175.4 ,;td. incr. 1 0.765 std. inc:r. 2 1.915 -188.7 -5.3 1.323 -18'5.6 -4.9 sid. iocr. 3 std. ir.:r. 4 1.812 -190.6 -5.t1 sui. i11er. 5 2.574 -1''5.5 -4.9 full auw 5 Ill no. of additions aediUI 5 u Exd'lwt ij)i;: 19 dos.raie~ daie 93-11-15 ti• 13:42: 13 addi t.iwsi.btraction ll!Ukxi INS. t..,.; std.ai! till3 electr.1d .as. iriPUtl .! 25.0 c \ell'trature aan. 1000. 3 Pf'tl cone.SW 22.8 11 i) total -178.5 If.I initial voi ta9t 100.0 factor 29.0 •l s.l s.iu 93-11-15 13:42 date U/MU -165r I I -171~ \ !\ -115t \ -188tI \\ -185~ t I -19~ I -ust \ \< \ \ ' -28811...._ _ _ _ _ ___.log c 2 3 Vifi~ 8.007 !ilOPe -~.l i;J Em -45.e td.J lt14(tl) 1. 46E+94 Pf'SI @Metrohm No. 133/2 e Application Bulletin Page 12 Determination of ammonium with the ion-selective electrode Sample preparations for further application examples 1. Ammonium in soil samples After weighing, the soil sample is dried and then boiled for 1 hour in c(HCI) = 2 moVL. The solution is filtered and the volume to be pipetted is adjusted to pH= 12. 2. Ammonium in an aquarium Same as for drinking water- see above, «Practical Examples». 3. Ammonium in urine = c(HCI) 0.1 mol/l is added to the sample, the resulting mixture stirred and then filtered off. The volume to be pipetted is adjusted to pH 12. = 4. Ammonium in boiler feed water Same as for drinking water. 5. Total nitrogen in waste water For measurements of total nitrogen content, a standard sulphuric acid digestion according to Kjeldahl is carried out on a 1O ml sample. The resulting filtrate is diluted with distilled water, topped off to 50 ml and adjusted to pH 12. = Example: Ammonium concentration determined by automatic standard addition: 38.6 mg/l Calculation of total nitrogen content: total nitrogen content (mg I L) = ammonium content (mg I L) x molar mass N (g I mol) molar massNH 4 + (g I mol) 38.6mg/Lx 14g/mol = 30 m /L 18g/mol g = @Metrohm Application Bulletin Determination of ammonium with the ion-selective electrode No. 133/2 e Page 13 Literature Application Bulletin No. 79, Bibliography - Ion-selective electrodes EPA 350.3 (1983), Nitrogen, ammonia (Potentiometric, ion selective electrode). US Environmental Protection Agency Hoge, J.H.C., Hazenberg, H.J.A., Gips, C.H., Determination of ammonia in urine with an ammonia electrode and with a direct method. Clin. Chim. Acta, (1974), 55 (3), 273-279 Ref.: Electroanal. Abstr. 13 (1975) 410 ISO 6778: (1984), Water quality - Determination of ammonium. Potentiometric method. Ito, K., Sunahara, H., Simple determination of ammonium and nitrate ions in activated sludge process waters with plastic-membrane ion selective electrodes Bunseki Kagaku, (1987), 36, 123-125 (Japanese) Ref.: Fresenius, Z. Anal. Chem. 328 (1987) 613 Love, M.D., Pardue, H.L., Pagan, G., Evaluation of transient response of ammonia-selective potentiometric electrodes for quantitative applications. Anal. Chem., (1992), 64 (11), 1269-1276 Navarro, R.A., Rojas, P., Determination of ammoniacal and total nitrogen in fertilizers by ammonia-selective electrode J. Assoc. Off. Anal. Chem., (1984), 67, 890-892 Rice, T.D., Sweeney, V., Semitekolos, R., Rhyder, G.J., Standard-addition determination of nitrogen in coal with an ammonia-sensitive electrode Talanta, (1984), 31, 607-610