PASCO scientific 1 Chemistry Experiment Library: 06020Science Workshop C33 Half Titrationtr, sc _ 1996, PASCO scientific C33 - 1Experiment C33: Determination of pKa by Half Titration用部分滴定法测定 pKa值(pH Sensor) (pH 传感器)Concept: acids, bases & salts概念：酸、碱和盐Time: 30 mSW Interface: 300, 500 & 700Macintosh_ file: C33 Half TitrationWindows_ file: C33_HALF.SWSAdapted by Tom Russo from MicroChemistry, distributed by Theta Technologies, 203 Bluegrass Ave., Suite 179H, South Gate, KY 41071, (606) 441-4768.EQUIPMENT NEEDED所需仪器• Science Workshop_ Interface• pH sensorpH传感器• base and support rod铁架台• beaker, 烧杯，250 mL (2)• buret clamp夹子• graduated cylinder,量筒， 100 mL• magnetic stirrer & spin bar磁力搅拌器和搅拌棒• apron and safety goggles围裙和防护风镜CHEMICALS AND CONSUMABLES化学品和消耗品• acetic acid,醋酸， 0.1 Molar• buffer solutions,缓冲溶液， pH 7 和 pH 4• distilled water蒸馏水• sodium hydroxide, 氢氧化钠，0.05 MolarPURPOSE目的The purpose of this laboratory activity is to determine the pKa of weak acetic acid by half-titration. The weak acid will form a salt that produces a buffer solution. The half-titration of this system will determine the pH at the equivalence point. In this special case, the pH will be the pKa.本实验的目的是用部分滴定法测定弱醋酸的pKa值。
弱酸将形成一种可产生缓冲溶液的盐系统的部分滴定法将测定平衡点处的pH值在这种特殊情况下， pH值将是pKa值THEORY原理All acids ionize in water to form hydrogen (H+) or hydronium ions (H3O+). The degree of ionization is an indication of the strength of an acid. An acid which ionizes completely is said to be strong. The ionization is complete and no molecular acid remains in solution. The rate PASCO scientific 2 Chemistry Experiment Library: 06020Science Workshop C33 Half Titrationtr, sc _ 1996, PASCO scientific C33 - 2constant (kf) is a number which indicates the degree of dissociation of the acid. For strong acids like this example, kf (forward reaction rate) is large and there is no reverse reaction (kr = 0). 所有的酸都在水中电离而形成氢离子(H +)或水合氢离子(H 3O+)。
电离程度是酸的强度的一种表示能够完全电离的酸被称为强酸电离彻底，幷且溶液中没有剩余的酸分子速率常数(k f)是表示酸的离解程度的一个数对于如下例中的强酸，k f（向前反应的速率）很大，而没有逆反应（k r = 0）Strong Acid 强酸 kf HX ========> H+ + X-A weak acid ionizes but the ions can reform the original molecular form of the acid. Acids which ionize and then react again to form molecular acids reach equilibrium. Equilibrium is the condition reached when the rate of the ionization equals the rate of formation of the molecular species. 弱酸也可以电离，但是离子会重新形成这种酸的原始分子形态电离后又能反应形成酸分子的酸可以达到平衡状态平衡状态指的是当电离速率等于形成分子的速率时的那种状态。
Weak acid弱酸 kfHX H+ + X-krFor a weak acid the ratio of kf to kr is a number less than one. The ratio kf/kr is also known as Ka or the equilibrium constant. The Ka for the above reaction can be written:弱酸的k f 与 kr之比是一个小于1的数k f 与k r的比值也被称为K a或平衡常数上述反应的K a可以写成：K a H X - HX The negative log of Ka is known as the pKa of the acid. The smaller the equilibrium constant, the less ionized and weaker the acid. The smaller the Ka, the larger the pKa because the pKa is the negative log of Ka. If a base is added to an acid, the acid is neutralized to form a salt and water. Ka的-log值即酸的pKa值。
平衡常数越小，电离度越小，酸就越弱Ka值越小，pKa值就越大，因为 pKa值是 Ka的-log值如果把碱加入酸中，酸就被中和形成盐和水HX + NaOH =========> NaX + H2OThe salt of a weak acid in solution with the weak acid is said to form a buffer. A buffer is a solution which resists large changes in pH because it provides an acid source, in this case, the remaining HX and a base source, in this case, NaX. The sodium salt of the weak acid can react with hydrogen ions to reform an acid which does not ionize very well.溶液中的弱酸盐和弱酸一起被称为是形成了一个缓冲溶液缓冲溶液是一种阻止pH 值发生大的变化的一种溶液，因为它可以提供一个酸源（在这个例子中，指剩余的HX）和一个碱源（在这个例子中，指NaX）。
弱酸的钠盐可以与氢离子反应形成一种不会完全电离的酸PASCO scientific 3 Chemistry Experiment Library: 06020Science Workshop C33 Half Titrationtr, sc _ 1996, PASCO scientific C33 - 3X- + H+ ========> HXThe behavior of a buffer system is given by this buffer equation: 下面的缓冲方程式给出了一个缓冲系统的行为方式：pH = pKa + log [salt] / [acid]A buffer solution can be used to determine the pKa of an acid under special conditions. 缓冲溶液可用来测定一种酸在特殊状态下的pKa值For example, if 50 milliliters (mL) of a 0.1 Molar solution of a weak acid acid, HA, with a pKa of 4.0 is reacted with 50 mL of 0.1 Molar base (sodium hydroxide, or NaOH), the solution will reach the equivalence point and all of the acid will be neutralized. However, if only 25 mL of base are added, the acid will be 50 percent neutralized. The molar concentrations of the salt and the acid will be equal. This causes the log term in the buffer equation to cancel. The pH of the solution will equal the pKa of the weak acid:例如，如果50ml0.1M 的弱酸溶液（HA，pKa值为4.0）与50ml0.1M的碱（氢氧化钠，NaOH）反应，溶液将达到平衡点，幷且所有的酸都将被中和。
然而，如果只加入25ml碱，将只有50%的酸被中和盐和酸的摩尔浓度将相等这使缓冲方程式中的log项被取消溶液的pH值将等于弱酸的pKa值pH = pKa + log [salt] / [acid]= 4.0 + log [0.05] / [0.05]= 4.0 + log 1= 4.0 + 0 pH = 4.0 (the pKa of the acid这种酸的pKa值)This relationship can be used to ascertain the pH at the midpoint in the titration of a weak acid.这个关系式可用来确定弱酸的滴定中点的pH值SAFETY PROCEDURES安全规程1. Wear an a。