{"id":4316,"date":"2023-10-24T13:55:17","date_gmt":"2023-10-24T11:55:17","guid":{"rendered":"https:\/\/www.em-grade.com\/general-fr\/definition-of-ph-ka-and-pka\/"},"modified":"2025-05-16T15:14:05","modified_gmt":"2025-05-16T13:14:05","slug":"definition-of-ph-ka-and-pka","status":"publish","type":"post","link":"https:\/\/www.em-grade.com\/fr\/definitions-de-chimie\/definition-of-ph-ka-and-pka\/","title":{"rendered":"D\u00e9finition du pH, du Ka et du Pka"},"content":{"rendered":"<h4><strong>D\u00e9finition du pKa<\/strong><\/h4>\n<p>\u00c0 partir de la constante d\u2019acidit\u00e9 <strong>Ka<\/strong>, on d\u00e9finit le <strong>pKa<\/strong> de la mani\u00e8re suivante :<\/p>\n<p style=\"text-align: center;\"><strong><span class=\"katex-display\"><span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mord\"><span class=\"vlist-t vlist-t2\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"boxpad\"><span class=\"mord text\">pKa<\/span><span class=\"mrel\">=<\/span>\u2212<span class=\"mop\">log<\/span><span class=\"mopen\">(<\/span><span class=\"mord text\">Ka<\/span><span class=\"mclose\">)<\/span><\/span><\/span><span class=\"vlist-s\">\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/strong><\/p>\n<p>On utilise g\u00e9n\u00e9ralement le pKa plut\u00f4t que le Ka pour qualifier la force d\u2019un acide.<\/p>\n<ul>\n<li><strong>Plus l\u2019acide est fort, plus le pKa est faible.<\/strong>\n<ul>\n<li>Cela s\u2019explique car un acide fort est tr\u00e8s dissoci\u00e9\u202f: la concentration de l\u2019acide non dissoci\u00e9 <span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mopen\">[<\/span><span class=\"mord mathnormal\">A<\/span><span class=\"mord mathnormal\">H<\/span><span class=\"mclose\">]<\/span><\/span><\/span><\/span> devient tr\u00e8s faible.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<hr \/>\n<h4><strong>Lien entre pKa, pH et forces acides<\/strong><\/h4>\n<p>Partons de la formule du Ka pour un couple acide-base :<\/p>\n<p style=\"text-align: center;\"><strong>Ka = [H<sup>+<\/sup>][A<sup>\u2212<\/sup>] \/ [AH]<\/strong><\/p>\n<p>En prenant le logarithme d\u00e9cimal des deux c\u00f4t\u00e9s, cela devient :<\/p>\n<p style=\"text-align: center;\"><strong><span class=\"katex-display\"><span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mord\">\u2212<\/span><span class=\"mop\">log<\/span><span class=\"mopen\">(<\/span><span class=\"mord text\"><span class=\"mord\">Ka<\/span><\/span><span class=\"mclose\">)<\/span><span class=\"mrel\">=<\/span><\/span><span class=\"base\"><span class=\"mord\">\u2212<\/span><span class=\"mop\">log<\/span><span class=\"mopen\">([<\/span><span class=\"mord\"><span class=\"mord mathrm\">H<\/span><span class=\"msupsub\"><span class=\"vlist-t\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"sizing reset-size6 size3 mtight\"><span class=\"mbin mtight\">+<\/span><\/span><\/span><\/span><\/span><\/span><\/span><span class=\"mclose\">])<\/span><span class=\"mbin\">\u2212<\/span><\/span><span class=\"base\"><span class=\"mop\">log<\/span><span class=\"minner\"><span class=\"mopen delimcenter\"><span class=\"delimsizing size3\">(<\/span><\/span><span class=\"mord\"><span class=\"mfrac\"><span class=\"vlist-t vlist-t2\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">AH<\/span><span class=\"mclose\">]<\/span><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">A<\/span><span class=\"msupsub\"><span class=\"vlist-t\"><span class=\"sizing reset-size6 size3 mtight\"><span class=\"mbin mtight\">\u2212<\/span><\/span><\/span><\/span><span class=\"mclose\">]<\/span><\/span><span class=\"vlist-s\">\u200b<\/span><\/span><\/span><\/span><\/span><span class=\"mclose delimcenter\"><span class=\"delimsizing size3\">)<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/strong><\/p>\n<p>On reconna\u00eet alors les termes :<\/p>\n<ul>\n<li><strong>pKa = -\\log(Ka)<\/strong><\/li>\n<li><strong>pH = -\\log([\\mathrm{H}^+])<\/strong><\/li>\n<\/ul>\n<p>Ce qui donne la relation :<\/p>\n<p style=\"text-align: center;\"><strong><span class=\"katex-display\"><span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mord\"><span class=\"vlist-t vlist-t2\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"boxpad\"><span class=\"mord text\">pKa<\/span><span class=\"mrel\">=<\/span><span class=\"mord text\">pH<\/span><span class=\"mbin\">\u2212<\/span><span class=\"mop\">log<\/span><span class=\"minner\"><span class=\"mopen delimcenter\"><span class=\"delimsizing size3\">(<\/span><\/span><span class=\"mfrac\"><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">AH<\/span><span class=\"mclose\">]<\/span><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">A<\/span><span class=\"msupsub\"><span class=\"vlist-t\"><span class=\"sizing reset-size6 size3 mtight\"><span class=\"mbin mtight\">\u2212<\/span><\/span><\/span><\/span><span class=\"mclose\">]<\/span><span class=\"vlist-s\">\u200b<\/span><\/span><span class=\"mclose delimcenter\"><span class=\"delimsizing size3\">)<\/span><\/span><\/span><\/span><\/span><span class=\"vlist-s\">\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/strong><\/p>\n<hr \/>\n<h4><strong>D\u00e9finition pratique du pH en fonction du pKa<\/strong><\/h4>\n<p>En r\u00e9arrangeant la formule pr\u00e9c\u00e9dente, on obtient l\u2019expression bien connue\u202f:<\/p>\n<p style=\"text-align: center;\"><strong><span class=\"katex-display\"><span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mord\"><span class=\"vlist-t vlist-t2\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"boxpad\"><span class=\"mord text\">pH<\/span><span class=\"mrel\">=<\/span><span class=\"mord text\">pKa<\/span><span class=\"mbin\">+<\/span><span class=\"mop\">log<\/span><span class=\"minner\"><span class=\"mopen delimcenter\"><span class=\"delimsizing size3\">(<\/span><\/span><span class=\"mfrac\"><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">AH<\/span><span class=\"mclose\">]<\/span><span class=\"mopen\">[<\/span><span class=\"mord mathrm\">A<\/span><span class=\"msupsub\"><span class=\"vlist-t\"><span class=\"sizing reset-size6 size3 mtight\"><span class=\"mbin mtight\">\u2212<\/span><\/span><\/span><\/span><span class=\"mclose\">]<\/span><span class=\"vlist-s\">\u200b<\/span><\/span><span class=\"mclose delimcenter\"><span class=\"delimsizing size3\">)<\/span><\/span><\/span><\/span><\/span><span class=\"vlist-s\">\u200b<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/strong><\/p>\n<ul>\n<li>O\u00f9 <span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mopen\">[<\/span><span class=\"mord\"><span class=\"mord mathrm\">A<\/span><span class=\"msupsub\"><span class=\"vlist-t\"><span class=\"vlist-r\"><span class=\"vlist\"><span class=\"sizing reset-size6 size3 mtight\"><span class=\"mbin mtight\">\u2212<\/span><\/span><\/span><\/span><\/span><\/span><\/span><span class=\"mclose\">]<\/span><\/span><\/span><\/span> \u202f: concentration de la base conjugu\u00e9e\u202f;<\/li>\n<li><span class=\"katex\"><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"base\"><span class=\"mopen\">[<\/span><span class=\"mord\"><span class=\"mord mathrm\">AH<\/span><\/span><span class=\"mclose\">]<\/span><\/span><\/span><\/span> \u202f: concentration de la forme acide.<\/li>\n<\/ul>\n<hr \/>\n<p>\ud83d\udca1 <em>Cette \u00e9quation, appel\u00e9e aussi <strong>\u00e9quation de Henderson-Hasselbalch<\/strong>, est fondamentale pour comprendre les tampons et la force acide\/base en chimie et en biologie.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>D\u00e9finition du pKa \u00c0 partir de la constante d\u2019acidit\u00e9 Ka, on d\u00e9finit le pKa de la mani\u00e8re suivante : pKa=\u2212log(Ka)\u200b On utilise g\u00e9n\u00e9ralement le pKa plut\u00f4t que le Ka pour&#8230;<\/p>\n","protected":false},"author":8,"featured_media":3511,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_seopress_titles_title":"","_seopress_titles_desc":"","_seopress_robots_index":"","_seopress_robots_follow":"","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"","_seopress_robots_breadcrumbs":"","_seopress_robots_freeze_modified_date":"","_seopress_robots_custom_modified_date":"","_seopress_robots_canonical":"","_seopress_social_fb_title":"","_seopress_social_fb_desc":"","_seopress_social_fb_img":"","_seopress_social_fb_img_attachment_id":0,"_seopress_social_fb_img_width":0,"_seopress_social_fb_img_height":0,"_seopress_social_twitter_title":"","_seopress_social_twitter_desc":"","_seopress_social_twitter_img":"","_seopress_social_twitter_img_attachment_id":0,"_seopress_social_twitter_img_width":0,"_seopress_social_twitter_img_height":0,"_seopress_redirections_value":"","_seopress_redirections_enabled":"","_seopress_redirections_enabled_regex":"","_seopress_redirections_logged_status":"","_seopress_redirections_param":"","_seopress_redirections_type":0,"_seopress_analysis_target_kw":"","_seopress_news_disabled":"","_seopress_video_disabled":"","_seopress_video":[],"_seopress_pro_schemas_manual":[],"_seopress_pro_rich_snippets_disable_all":"","_seopress_pro_rich_snippets_disable":[],"_seopress_pro_schemas":[],"itsec_x_frame_options":"","footnotes":""},"categories":[288],"tags":[],"class_list":["post-4316","post","type-post","status-publish","format-standard","has-post-thumbnail","category-definitions-de-chimie"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/posts\/4316","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/comments?post=4316"}],"version-history":[{"count":5,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/posts\/4316\/revisions"}],"predecessor-version":[{"id":11109,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/posts\/4316\/revisions\/11109"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/media\/3511"}],"wp:attachment":[{"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/media?parent=4316"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/categories?post=4316"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.em-grade.com\/fr\/wp-json\/wp\/v2\/tags?post=4316"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}