厦门大学 仪器分析课件第章 伏安与极谱分析.ppt

* 三、特点与应用 1．? 方法迅速 2．? 灵敏度高（10-7mol/l） 3．? 分辨力强（0.1V） 4．? 前放电物质影响小 5．? 抗氧波能力强，不用除 O2 特别适用于络合吸附波和具有吸附性的催化波。 * §5—6 方波极谱法与脉冲极谱 一、方波极谱：在缓慢改变的直流电压上叠加一个低频小振幅（≤30mV），并在方波改变方向前一瞬间记录通过电解池交流成分的极谱方法。 1952年，Barker通过仪器过滤，消除减小了电容电流的影响，提高了测定的灵敏度，测定最低浓度可达5 ? 10-8 M * 二、脉冲极谱 方波极谱存在问题 1．毛细管噪音：汞滴滴落，毛细管汞线收缩，管口溶液渗入毛细管。在管壁与汞线间形成一层很薄的不规则液层，在方波电压作用形成充电电流与液层内离子电解电流。 2．电解池内阻：为有效提高灵敏度，应减少内阻R，但方波f大R内也大，使测量信号放大产生问题，为减R内引入大量电解质. 1960Barker 在每一滴汞滴内即在汞滴下落前，很短的时间内仅加入一个方波电压。?极谱方波中最灵敏 有常规与微分脉冲 * §5—7循环伏安法（Cyclic voltammetry, CV） 一．CV 原理 1．施加三角波电压 2 电压扫描速度10mV~1000mV/s（一般在 50~100mV/s） 3 固定电极 溶液中有氧化态物质O时，在电极上被还原生成还原态R O+ne-→R 回扫时 R被氧化成O R→ O + ne- * 二、可逆性判断 φpc=φ1/2-1.11RT/nF φpa=φ1/2+1.11RT/nF Δφ=2.22RT/nF =φpa—φpc=56.5/n(mv) 55-65mv 认为可逆，且ipa=ipc∝v 1/2 * * * * Cyclic Voltammetry The current response of a small stationary electrode in an unstirred solution is measured under the influence of a triangular potential wave form. The potential is first varied linearly from +0.8 V to –0.15 V. Then the scan direction is reversed and returned to its original value. * Cyclic Voltammogram {6.0 mM K3Fe(CN)6 and 1.0 M KNO3} A small anodic potential arises from the oxidation of water to give oxygen. No current between +0.7 and +0.4 V because no reducible or oxidizable species are present in this potential range. Cathodic current develops (point B) due to the reduction of hexacyanoferrate (III). This current increases rapidly due to trying to reach equilibrium (Nernst equation). At Point D the current is a sum of the diffusion-controlled current and the current surge that’s trying to adjust the concentration. * Cyclic Voltammogram {6.0 mM K3Fe(CN)6 and 1.0 M KNO3} Between points D and F, the current decays due to a larger and larger layer that has reached equilibrium. Current is reversed at point F but the current continues to be cathodic because the potentials are still negative enough to cause the reduction of Fe(CN)63-. Between H and I the potent