{"id":4232,"date":"2017-05-24T00:13:48","date_gmt":"2017-05-23T22:13:48","guid":{"rendered":"http:\/\/agora.xtec.cat\/ies-merce-rodoreda\/?p=4232"},"modified":"2017-05-24T00:13:48","modified_gmt":"2017-05-23T22:13:48","slug":"potenciometria-amb-electrode-selectiu","status":"publish","type":"post","link":"https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/modul-a\/potenciometria-amb-electrode-selectiu\/","title":{"rendered":"Potenciometria amb electr\u00f2de selectiu"},"content":{"rendered":"<p style=\"text-align: center;\"><a href=\"http:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4643 aligncenter\" src=\"http:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1-1024x768.png\" alt=\"practica nitratos ESI\" width=\"610\" height=\"457\" srcset=\"https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1-1024x768.png 1024w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1-300x225.png 300w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1-768x576.png 768w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1-200x150.png 200w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/practica-nitratos-ESI-1.png 1600w\" sizes=\"auto, (max-width: 610px) 100vw, 610px\" \/><\/a>\u00a0\u00a0\u00a0\u00a0Imatge 1 \u00a0El\u00e9ctrode de refer\u00e8ncia i l&#8217;ESI<\/p>\n<p style=\"text-align: justify;\"><span id=\"result_box\" lang=\"ca\">La potenciometria \u00e9s una t\u00e8cnica instrumental quantitativa amb la qual es pot determinar la concentraci\u00f3 d&#8217;una esp\u00e8cie qu\u00edmica en una mostra l\u00edquida mitjan\u00e7ant un el\u00e8ctrode de refer\u00e8ncia, un el\u00e8ctrode d&#8217;indicador i un volt\u00edmetre. Aquest m\u00e8tode \u00e9s molt utilitzat, ja que, en contraposici\u00f3 d&#8217;altres t\u00e8cniques instrumentals, no influeix ni el color ni la terbolesa de la mostra, i el rang de treball \u00e9s m\u00e9s ampli que moltes altres t\u00e8cniques. Al Cicle Superior de Qu\u00edmica Ambiental del Merc\u00e8 Rodoreda, apliquem aquesta t\u00e8cnica, per exemple, per determinar quantitativament la concentraci\u00f3 de nitrats d&#8217;una mostra de patates. (Imatge 2)<\/span><!--more--><\/p>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div><span id=\"result_box\" lang=\"ca\">L&#8217;el\u00e8ctrode de refer\u00e8ncia \u00e9s un el\u00e8ctrode que t\u00e9 un potencial estable i conegut; mitjan\u00e7ant una reacci\u00f3 redox s&#8217;aconsegueix un potencial constant. El m\u00e9s utilitzat \u00e9s l&#8217;el\u00e8ctrode plata \/ clorur de plata (Ag \/ AgCl); format per un fil de Ag sobre el qual es diposita AgCl, generalment per via electroqu\u00edmica, en una soluci\u00f3 de NaCl o KCl, en la qual el fil de Ag actua com a \u00e0node. Tamb\u00e9 hi ha altres el\u00e8ctrodes de refer\u00e8ncia, com l&#8217;el\u00e8ctrode de calomelans, per\u00f2 est\u00e0 en clar des\u00fas, a causa de la toxicitat del propi mercuri.<\/span><\/div>\n<div><\/div>\n<p><span id=\"result_box\" lang=\"ca\">L&#8217;el\u00e8ctrode indicador, s&#8217;usa juntament amb l&#8217;el\u00e8ctrode de refer\u00e8ncia. El senyal que produeix l&#8217;el\u00e8ctrode indicador dep\u00e8n de l&#8217;analit; \u00e9s sensible a l&#8217;analit de qu\u00e8 es vol saber la seva concentraci\u00f3. Existeixen de dos tipus fonamentalment: met\u00e0l\u00b7lics, o selectius d&#8217;ions (ESI). En aquest tipus de potenciometr\u00eda s&#8217;utilitza l&#8217;ESI com el\u00e8ctrode indicador.<br \/>\n<\/span><\/p>\n<div><span id=\"result_box\" lang=\"ca\">En resum, l&#8217;el\u00e8ctrode de refer\u00e8ncia s&#8217;utilitza per mesurar el potencial de l&#8217;el\u00e8ctrode indicador, a trav\u00e9s la difer\u00e8ncia de potencial entre els dos. Com el potencial de l&#8217;el\u00e8ctrode de refer\u00e8ncia \u00e9s conegut, es podr\u00e0 saber el potencial de l&#8217;el\u00e8ctrode indicador.<\/span><\/div>\n<div><\/div>\n<div>La peculiaritat d&#8217;aquest tipus de potenciometria \u00e9s l&#8217;utilitzaci\u00f3 d&#8217;un electrode selectiu d&#8217;ions com a electrode indicador. Aquests s<span id=\"result_box\" lang=\"ca\">\u00f3n el\u00e8ctrodes que posseeixen una membrana sensible, selectiva a un i\u00f3 en particular. Aquesta membrana t\u00e9 una soluci\u00f3 amb l&#8217;analit del qual \u00e9s selectiva. Es mesurar\u00e0 la diferencia de potencial entre els dos costats de la membrana. El potencial d&#8217;un el\u00e8ctrode selectiu respon a l&#8217;activitat dels ions, no a la seva concentraci\u00f3.<br \/>\nPer a que l&#8217;activitat d&#8217;un i\u00f3 i la seva concentraci\u00f3 s&#8217;assemblin, s&#8217;afegeix tant als patrons com a les mostres un ajustador de for\u00e7a ionica. Aquesta \u00e9s una dissoluci\u00f3 de for\u00e7a i\u00f2nica elevada que no interfereix amb la mostra i que iguala la for\u00e7a i\u00f2nica de patrons i mostres. <\/span><\/div>\n<div><\/div>\n<div><span id=\"result_box\" lang=\"ca\">Com exemple, a la determinaci\u00f3 de fluorurs per potenciometria amb ESI es sol afegir a la mostra TISAB, una soluci\u00f3 que ajusta la for\u00e7a i\u00f3nica dels fluorurs.<\/span><\/div>\n<div>Els ESI desgraciadament no s\u00f3n 100% selectius d&#8217;un \u00fanic i\u00f3, encara que el seu nom ho indiqui. Hi poden haver interfer\u00e8ncies segons el tipus d&#8217;ESI utilitzat. Aquestes interfer\u00e8ncies solen tenir la mateixa c\u00e0rrega de l&#8217;analit. Llavors, abans de realitzar la potenciometria, cal enmascarar les possibles interfer\u00e8ncies.<\/div>\n<div><\/div>\n<div>Tot aix\u00f2 est\u00e0 molt b\u00e9, pero que passa amb els analits gasosos, com l&#8217;amon\u00edac?<\/div>\n<div>En aquest cas, l&#8217;electr\u00f2de indicador es posa dins d&#8217;un recipient, que est\u00e0 en contacte amb la mostra. Aquest recipient es tanca amb una membrana; aquesta membrana nom\u00e8s deixa passar el gas el qual volem saber la seva concentraci\u00f3.<\/div>\n<div><\/div>\n<div>Quan obtenim el potencial de la mostra, podem saber la concentraci\u00f3 de l&#8217;analit amb una expresi\u00f3 nerstiana.<\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><a href=\"http:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4644 aligncenter\" src=\"http:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4-1024x768.png\" alt=\"patatas C4\" width=\"522\" height=\"392\" srcset=\"https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4-1024x768.png 1024w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4-300x225.png 300w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4-768x576.png 768w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4-200x150.png 200w, https:\/\/agora.xtec.cat\/ies-merce-rodoreda\/wp-content\/uploads\/usu644\/2017\/05\/patatas-C4.png 1600w\" sizes=\"auto, (max-width: 522px) 100vw, 522px\" \/><\/a><\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div class=\"misc-pub-section misc-pub-post-status\" style=\"text-align: center;\">Mostres de patates\u00a0per determinar la concentraci\u00f3 de nitrats per<\/div>\n<div class=\"misc-pub-section misc-pub-post-status\" style=\"text-align: center;\">potenciometria amb electr\u00f2de selectiu de nitrats<\/div>\n<div class=\"misc-pub-section misc-pub-post-status\"><\/div>\n<div><\/div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u00a0\u00a0\u00a0\u00a0Imatge 1 \u00a0El\u00e9ctrode de refer\u00e8ncia i l&#8217;ESI La potenciometria \u00e9s una t\u00e8cnica instrumental quantitativa amb la qual es pot determinar [&hellip;]<\/p>\n","protected":false},"author":153,"featured_media":4643,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center 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