實驗室PH計電(dian)(dian)位(wei)(wei)(wei)(wei)分析(xi)法(fa)(fa)(fa)是利(li)用電(dian)(dian)極(ji)(ji)電(dian)(dian)位(wei)(wei)(wei)(wei)和(he)濃(nong)度(du)之間的(de)(de)(de)(de)關系來確定物質含量(liang)(liang)的(de)(de)(de)(de)分析(xi)方法(fa)(fa)(fa),表示電(dian)(dian)極(ji)(ji)電(dian)(dian)位(wei)(wei)(wei)(wei)的(de)(de)(de)(de)基本公式(shi)是能斯特方程式(shi)。由于(yu)單(dan)個(ge)電(dian)(dian)極(ji)(ji)電(dian)(dian)位(wei)(wei)(wei)(wei)的(de)(de)(de)(de)絕對(dui)(dui)值無法(fa)(fa)(fa)測量(liang)(liang),在大多(duo)(duo)數情(qing)況下,電(dian)(dian)位(wei)(wei)(wei)(wei)法(fa)(fa)(fa)是基于(yu)測量(liang)(liang)原電(dian)(dian)池(chi)的(de)(de)(de)(de)電(dian)(dian)動勢,構(gou)成(cheng)(cheng)電(dian)(dian)池(chi)的(de)(de)(de)(de)兩個(ge)電(dian)(dian)極(ji)(ji),一個(ge)電(dian)(dian)極(ji)(ji)的(de)(de)(de)(de)電(dian)(dian)位(wei)(wei)(wei)(wei)隨待(dai)測離子(zi)濃(nong)度(du)而變化(hua),能指(zhi)示待(dai)測離子(zi)濃(nong)度(du),稱(cheng)為(wei)指(zhi)示電(dian)(dian)極(ji)(ji);另一個(ge)電(dian)(dian)極(ji)(ji)的(de)(de)(de)(de)電(dian)(dian)位(wei)(wei)(wei)(wei)則不(bu)受試(shi)液組成(cheng)(cheng)變化(hua)的(de)(de)(de)(de)影響,具有較恒定的(de)(de)(de)(de)數值,稱(cheng)為(wei)參比電(dian)(dian)極(ji)(ji)。指(zhi)示電(dian)(dian)極(ji)(ji)和(he)參化(hua)電(dian)(dian)極(ji)(ji)共同(tong)浸入試(shi)液中(zhong),構(gou)成(cheng)(cheng)一個(ge)原電(dian)(dian)池(chi),通過測定原電(dian)(dian)池(chi)的(de)(de)(de)(de)電(dian)(dian)動勢,便可求得待(dai)測離子(zi)的(de)(de)(de)(de)濃(nong)度(du),這(zhe)一方法(fa)(fa)(fa)亦稱(cheng)為(wei)直(zhi)接(jie)電(dian)(dian)位(wei)(wei)(wei)(wei)法(fa)(fa)(fa),離子(zi)選擇(ze)電(dian)(dian)極(ji)(ji)分析(xi)法(fa)(fa)(fa)是電(dian)(dian)位(wei)(wei)(wei)(wei)分析(xi)法(fa)(fa)(fa)中(zhong)發展最(zui)為(wei)迅速、最(zui)活(huo)躍的(de)(de)(de)(de)分支。對(dui)(dui)某些離子(zi)測定的(de)(de)(de)(de)靈敏度(du)可達10-6數量(liang)(liang)級。在許多(duo)(duo)情(qing)況下不(bu)破壞試(shi)液或不(bu)用進(jin)行(xing)復雜的(de)(de)(de)(de)預處理(li),對(dui)(dui)有色(se)、渾濁溶液都可進(jin)行(xing)分析(xi)。離子(zi)選擇(ze)性電(dian)(dian)極(ji)(ji)分類(lei) 基于(yu)離子(zi)選擇(ze)電(dian)(dian)極(ji)(ji)絕大多(duo)(duo)數都是膜電(dian)(dian)極(ji)(ji)這(zhe)一事實(shi),依(yi)據膜電(dian)(dian)位(wei)(wei)(wei)(wei)響應機制、膜的(de)(de)(de)(de)組成(cheng)(cheng)和(he)結(jie)構(gou),硼砂標(biao)準緩沖(chong)液 精密(mi)(mi)稱(cheng)取硼砂[Na2B4O7·加水使溶解并稀釋至1000ml,置聚乙(yi)烯(xi)塑料瓶中(zhong),密(mi)(mi)塞,避免與空氣中(zhong)二氧化(hua)碳接(jie)觸,酸(suan)度計(ji)標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)的(de)(de)pH值(zhi)如下表。注意(yi)事(shi)項 (1)測(ce)定前校(xiao)準(zhun)儀(yi)器時(shi),應(ying)(ying)選(xuan)擇與供(gong)(gong)試液(ye)(ye)(ye)(ye)pH值(zhi)接(jie)近的(de)(de)標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)。(2)在測(ce)定時(shi)用(yong)標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)校(xiao)正(zheng)儀(yi)器后(hou)(hou),應(ying)(ying)再用(yong)另(ling)一種pH值(zhi)相差約3的(de)(de)標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)核對一次,誤(wu)差不應(ying)(ying)超過±0.1。(3)每次更換標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)或供(gong)(gong)試液(ye)(ye)(ye)(ye)前,應(ying)(ying)用(yong)水(shui)(shui)(shui)充分(fen)洗滌(di)電極(ji),然(ran)后(hou)(hou)將水(shui)(shui)(shui)吸盡,也可用(yong)所換的(de)(de)標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)或供(gong)(gong)試液(ye)(ye)(ye)(ye)洗滌(di)。 (4)在測(ce)定高pH值(zhi)的(de)(de)供(gong)(gong)試品(pin)時(shi),應(ying)(ying)注意(yi)堿誤(wu)差的(de)(de)問題,必要時(shi)選(xuan)用(yong)適(shi)用(yong)的(de)(de)玻璃電極(ji)測(ce)定。(5)對弱緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)(如水(shui)(shui)(shui))的(de)(de)pH值(zhi)測(ce)定,先用(yong)鄰(lin)苯二甲(jia)酸氫鉀標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)校(xiao)正(zheng)儀(yi)器后(hou)(hou)測(ce)定供(gong)(gong)試液(ye)(ye)(ye)(ye),并重取供(gong)(gong)試液(ye)(ye)(ye)(ye)再測(ce),直(zhi)至(zhi)pH值(zhi)的(de)(de)讀數(shu)在1分(fen)鐘內(nei)改變不超過±0.05為止;然(ran)后(hou)(hou)再用(yong)硼砂標(biao)(biao)(biao)準(zhun)緩(huan)沖(chong)(chong)(chong)液(ye)(ye)(ye)(ye)校(xiao)正(zheng)儀(yi)器,酸(suan)度計(ji)測(ce)定;二次pH值(zhi)的讀數相差(cha)應不超過0.1,取二次讀數的平均(jun)值(zhi)為其pH值(zhi)。配(pei)制標準(zhun)緩沖液與溶(rong)解供試品的水,應是新沸過的冷蒸餾水,其pH值(zhi)應為5.5~7.0。 (7)標準(zhun)緩沖液一般可保存2~3個月,但發現有渾濁、發霉(mei)或沉淀等現象時,不能繼續(xu)使用。
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