测定醋（稀醋酸）的Ka值

Experiment C31: Neutralization of Vinegar with Drain
Cleaner
(pH Sensor) (pH 传感器)
Concept: acids, bases & salts概念：酸、碱和盐
Time: 30 m
SW Interface: 300, 500 & 700
Macintosh_ file: C31 Vinegar
Windows_ file: C31_VINE.SWS
Adapted by Tom Russo from MicroChemistry, distributed by Theta Technologies, 203 Bluegrass Ave., Suite 179H, South Gate, KY 41071, (606) 441-4768.
EQUIPMENT NEEDED
•Science Workshop_ Interface
•pH sensor传感器
•balance, accurate to 0.01 g天平（准确到0.01g）
•base and support rod铁架台
•beaker, 烧杯，250 mL (2)
•buret clamp滴定管夹
•graduated cylinder, 量筒，100 mL
•magnetic stirrer & spin bar磁力搅拌器和搅拌棒
•printer打印机
•straight-edge直尺
•apron and safety goggles围裙和防护风镜
CHEMICALS AND CONSUMABLES化学品和消耗品
•buffer solutions, pH 7 and pH 4缓冲溶液，pH 7 和 pH 4
•distilled water蒸馏水
•drain cleaner, solid (without aluminum)清洁剂（不含铝）
•vinegar, 5% acetic acid醋，5%醋酸
PURPOSE目的
The purpose of this laboratory activity is to determine the Ka of vinegar (mild acetic acid). You will neutralize the acid with a base that is a common household chemical. You will also quantitatively determine the Ka (acid dissociation constant) of the weak acid, the equivalence point of the neutralization, and the Ksp (solubility product dissociation constant) for the salt of the acid, sodium acetate. (Ksp is the degree to which the salt breaks apart.) 本实验的目的是测定醋（稀醋酸）的Ka值。

你将用一种常见的家用化学品作为碱来中和这种稀醋酸。

你也可以定量的测定这种弱酸的Ka值（酸的电离常数）、这一中和反应的平衡点和醋酸钠这种弱酸盐的Ksp值（可溶性物质的电离常数，指盐电离的程度）。

THEORY原理
Vinegar is a solution of acetic acid in water at a concentration of about 5% or 0.83 Molar. Granulated drain cleaner is a solid consisting mostly of lye or sodium hydroxide. Vinegar is an acid and drain cleaner is a base. An acid reacts with base to form a salt and water in a reaction called neutralization.醋是浓度为5%或0.83M的醋酸水溶液。

颗粒状的清洁剂是一种固体，主要由碱或氢氧化钠组成。

醋是一种酸而清洁剂是一种碱。

酸与碱反应形成盐和水，这种反应称为中和反应。

All acids ionize in water to form hydronium ions (H3O+) and an anion. The generalized equation for this ionization is:所有的酸都可以在水中电离形成氢离子(H3O+)和酸根。

这种电离反应的一般方程式是：
HQ + H2O =====> H3O++ Q-
For strong acids such as hydrochloric acid (HCl), nitric (HNO3), and sulfuric acid (H2SO4), ionization is complete. No acid remains in the molecular form.对于强酸如盐酸(HCl)、硝酸(HNO3)和硫酸(H2SO4)，电离是完全的，溶液中没有剩余的酸分子。

HCl + H2O =====> H3O++ Cl-
For weak acids however, the ionization reaction reaches an equilibrium. When a weak acid ionizes it reaches a point where the ions recombine to form a molecule of acid at the same rate that acid molecules ionize. 然而对于弱酸，电离反应只能达到平衡状态。

当弱酸电离时，它可以达到一个点，在这个点处，离子重新结合形成酸分子的速度与酸分子电离的速度相等。

HX + H2O <=====> H3O++ X-
Acetic acid is a weak acid. When it dissolves it ionizes to form hydronium ions and acetate ions. However, at a particular concentration the ions recombine to form acetic acid at the same rate that the molecules of acid ionize.醋酸是一种弱酸。

它溶解时将电离形成氢离子和醋酸根离子。

然而，在一个特定的浓度时，这些离子又会以一个与醋酸分子电离相等的速度重新结合形成醋酸。

HC2H3O2+ H2O <=====> H3O++ C2H3O2-
When the rate of the forward and reverse reactions are equal, the acid has reached an equilibrium between ionized and unionized molecules. This equilibrium point has a special name. It is called the K a of the acid. Mathematically, the K a of an acid is:当正反应的速度与逆反应的速度相等时，醋酸就到到了电离分子和未电离分子之间的平衡状态。

这个平衡点有一个特殊的名字——酸的Ka值。

从数学意义上讲，酸的Ka值：
K =
H O + [ ] ⨯ X [ ]
HX
[ ]
and for acetic acid:而对于醋酸：
K =
H O +
[ ] ⨯ C H O -
[ ] HC H O -
[ ]
When sodium hydroxide reacts with acetic acid, sodium acetate (a salt) and water are formed.当氢氧化钠与醋酸反应时，形成醋酸钠（一种盐）和水：
NaOH + HC2H3O2========> NaC2H3O2 + H2O
SAFETY PROCEDURES安全规程
1. Do not let the drain cleaner come in contact with your skin or clothing. Wear an apron and
safety goggles while handling or working near chemicals.不要让清洁剂接触到你的皮肤或衣服上。

操作化学品或靠近化学品工作时，要穿戴防护风镜。

2. Follow your teacher’s directions for the handling and disposal of all chemicals and
solutions.遵循老师的指导来操作和处理所有的化学品和溶液。

PROCEDURE步骤
For this activity, the pH sensor measures the change of pH in a mixture of acetic acid and a bas e. The Science Workshop program records and displays the data. 在这个实验中，pH传感器将测量醋酸和碱的混合物中的pH值的变化。

Science Workshop程序将记录幷显示这些资料。

PART I: Computer Setup计算机设置
1. 将ScienceWorkshop接口连接到计算机上，打开接口，然后打开计算机。

2. 将pH传感器的电子盒的DIN插头连接到接口上的模拟信道A。

3. 将pH电极的BNC插头连接到pH传感器的电子盒的接头上。

还不要把pH电极从装有缓冲
溶液的瓶中取出。

4. 打开 ScienceWorkshop 如下命名的文件：
1. Connect the Science Workshop interface to the computer, turn on the interface, and turn on
the computer.
2. Connect the DIN plug of the pH sensor electronics box to Analog Channel A on the
interface.
3. Connect the BNC plug of the pH electrode to the connector on the pH sensor electronics
box. Do not remove the pH electrode from the bottle of buffer solution yet.
4. Open the Science Workshop file titled as shown;
Macintosh: C31 Vinegar
Windows: C31_VINE.SWS
•The document has a Graph display of pH versus Time (sec).文件有一个pH——时间（秒）的图表显示。

•Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click on its window or select the name of the display from the list at the end of the Display menu. Change the Experiment Setup window by clic king on the “Zoom” box or the Restore button in the upper right hand corner of that window. 注意：如需快速参考，请参见实验注释窗口。

欲将一个显示图放至前台，单击其所在窗口或从显示器底部的菜单栏中选择该显示图的名称。

单击窗口右上角的扩大框或恢复按钮来改变实验设置窗口的大小。

4. The "Sampling Options_" for this experiment are as follows: Periodic Samples = Fast at 10
Hz.
5. The vertical axis (pH) of the Graph is scaled from 0 to 12 pH and the horizontal axis (Time) is scaled from 0 to 80 seconds.图表竖轴（pH）的刻度是从0-12pH，横轴（时间）的PART II: Sensor Calibration and Equipment Setup传感器校准和仪器设置
•It is important to calibrate the pH sensor. For calibration you will need distilled water and buffer solutions of pH 7 and pH 4. pH传感器的校准是非常重要的。

要进行校准，需要蒸馏水及pH7和pH4的缓冲溶液。

1. Remove the pH electrode from its bottle of buffer solution. Soak the pH sensor in distilled
water for 10 minutes and then dry the electrode with a tissue. 将pH电极从它的缓冲溶液瓶上取下。

将pH传感器浸在蒸馏水中10分钟，然后用薄棉纸将它擦干。

2. In the Experiment Setup window, double-click on the pH sensor icon.在Experiment Setup
窗口中双击pH传感器图标。

·The Analog Sensor Setup dialog box will open.模拟传感器对话框将打开。

3. Immerse the end of the pH electrode in the high pH buffer solution. When the voltage
displayed as the Cur Value: (current value) stabilizes, click on the Read button for the
High Value: and enter the pH value of the buffer solution.将pH电极的尾端浸没在高pH 值缓冲溶液中。

当电压显示为Cur Value:（电流值）稳定时，双击Read按钮得到High Value:然后键入缓冲溶液的pH值。

4. Thoroughly rinse the pH electrode in distilled water, dry it with a tissue, and immerse the
end of the electrode in the low pH buffer solution. 在蒸馏水中彻底清洗pH电极，用薄棉纸擦干，然后将电极的尾端浸没在低pH值缓冲溶液中。

5. When the voltage displayed as the Cur Value: stabilizes, click on the Read button for the
Low Value: and enter the pH value of the buffer solution. Click OK to return to the
Experiment Setup window. 当电压显示为Cur Value:稳定时，双击Read按钮得到Low Value:然后键入缓冲溶液的pH值。

单击OK回到Experiment Setup窗口。

6. Thoroughly rinse the pH electrode with distilled water and dry gently. 用蒸馏水彻底清洗
pH电极幷轻轻擦干。

Equipment Setup仪器设置
1. Dissolve 3.3 grams of drain cleaner in 100 mL of distilled water.将3.3g清洁剂溶解在
100mL蒸馏水中。

2. Place 100 mL of vinegar in a 250 mL beaker. Put a spin bar in the beaker. 将100mL粗放在
一个250mL烧杯中，然后放入一根搅拌棒。

3. Place the beaker with vinegar and spin bar on the magnetic stirrer. 将装有醋和搅拌棒的
烧杯放到磁力搅拌器上。

4. Use the buret clamp and base and support rod to position the pH electrode so the end of the
electrode is in the vinegar, but will not interfere with the spin bar.用夹子和铁架台固定好pH电极，使电极的末端浸入醋中，但不要碰到搅拌棒。

Neutralization of Vinegar with Drain Cleaner
5. Turn on the magnetic stirrer.打开磁力搅拌器。

PART III: Data Recording 资料记录
1. Click the “REC” button to begin recording data.单击“记录”按钮开始资料记录。

2. After 5 seconds, begin adding the drain cleaner solution to the vinegar at a slow, constant
rate. 5秒钟后，开始缓慢、连续的往醋中加入清洁剂溶液。

•Try to make the 100 mL of drain cleaner solution last for about 45 seconds (a rate of about
3 mL per second).尽量用大约45秒钟的时间加入100mL清洁剂溶液（速度大约为3mL/
分钟）。

3. When all of the drain cleaner solution is added to the vinegar, click the “STOP” button to
end data recording. 当所有的清洁剂溶液都已加入到醋中时，单击“停止”按钮结束资料记录。

•Run #1 will appear in the Data list in the Experiment Setup window. Run #1将出现在实验设置窗口的资料列表中。

4. Turn off the magnetic stirrer.关掉磁力搅拌器。

5. Carefully remove the pH electrode, rinse it thoroughly in distilled water, and put it in its
bottle of buffer solution. 小心地取下pH传感器，用蒸馏水彻底冲洗，然后把它放回原来的缓冲溶液瓶中。

6. Dispose of the solution in the beaker as instructed.遵照指导处理烧杯中的溶液。

ANALYZING THE DATA分析资料
1. Select “Save As_” from the File menu to save your data. Click on the Graph to make it
active. Select “Print Active Display” from the File menu to print your graph.从文件菜单中选择“保存为_”来保存你的资料。

单击图表使之活动。

从文件菜单中选择“打印活动显示”来打印你的图表。

2. In the Graph display, click the “Autoscale” button to resize the Graph to fit the data. 在图
表显示中，单击“自动分度按钮”来重新分度图表以适应数据。

3. Click n the “Smart Cursor” button. The cursor will change to a cross-hair when you move it
into the data area of the Graph display. The Y-coordinate of the cursor/cross-hair will
appear in the area next the label of the vertical axis. The X-coordinate of the
cursor/cross-hair will appear in the area above the label of the horizontal axis.单击“Smart Cursor”按钮。

当你把光标移到图表显示的数据区时，光标变成一个十字。

光标/十字的Y坐标出现在竖轴标签附近的区域中；X坐标出现在横轴标签上方的区域中。

4. Move the cursor to the beginning of the plot of pH versus time. Record the Y-coordinate as
the pH value of the starting vinegar solution in the Data Table.将光标移到pH—时间曲线的开头。

在资料表中记录这一点的Y坐标，作为开始时醋溶液的pH值。

5. Move the cursor to the end of the plot. Record the Y-coordinate as the pH value of the
solution of vinegar and drain cleaner.将光标移到pH—时间曲线的末尾。

记录这一点的Y 坐标，作为开始时醋和清洁剂的混合溶液的pH值。

6. On the printout of your Graph display, draw a straight line extending the acid line at the
beginning of the experiment. Draw a second line extending the acid line at the end of the
experiment. Draw a third line extending through the vertical part of the plot of pH.在打印出来的曲线图上，画第一条直线，延长实验开始处的酸的曲线；画第二直线，延长实验末尾处的酸的曲线。

延伸pH曲线的垂直部分，画第三条直线。

•The line through the vertical section of the plot should intersect the top and bottom extended lines.沿着pH曲线垂直部分的这条直线应该与上端和下端的那两条延长线相交。

7. Measure one-half the distance along the vertical extended line between the two points
where the vertical line intersects the other lines. 测量垂直延长线与另外两条直线的两个交点之间的垂直线段的1/2距离。

•This mid point is the pK sp of the salt of the acid/base combination (where pK sp is the negative antilog of K sp). It is also the equivalence point of the reaction between the acid and the base. 这个中点即酸/碱结合生成的盐的pKsp值（此处pKsp值为Ksp值的负对数），同时也是这个酸碱反应的平衡点。

8. Place a straight-edge on the printout of the graph so that it is parallel to the horizontal axis
and goes through the pK sp point on the plot. Examine where the straight-edge intersects the vertical (pH) axis. 将直尺放在打印出来的曲线图上，使至与图表的横轴平行幷穿过曲线的pKsp点。

看看直尺与竖轴（pH）在何处相交。

9. Estimate and record the pH value of the pK sp point on the plot (place where the
straight-edge intersects the vertical axis).估计曲线上pKsp点（直尺与竖轴相交的点）的pH值幷记录之。

10. Examine the beginning of the plot of pH. The horizontal part of the plot shows a gradual
increase. Measure one-half the distance along this line from the beginning of the addition of the drain cleaner to the point where the sharp increase in pH begins, and mark the point.
看看这条pH曲线的开头。

这条曲线的水平部分表示逐步增加。

测量这条直线在开始加入清洁剂到pH开始剧增的两个点之间的线段的1/2距离，幷标出这个中点。

•This mid-point is the pK a of the acid. The negative antilog of the pK a is the Ka of the acid.
这个中点时酸的pKa值。

PKa值的负反对数即酸的Ka值。

11. Place a straight-edge on the printout of the graph so that it is parallel to the horizontal axis
and goes through the pK a point on the plot. Examine where the straight-edge intersects the vertical (pH) axis. 将直尺放在打印出来的曲线图上，使至与图表的横轴平行幷穿过曲线的pKa点。

看看直尺与竖轴（pH）在何处相交。

12. Estimate and record the pH value of the pKa point on the plot (place where the
straight-edge intersects the vertical axis).估计曲线上pKa点（直尺与竖轴相交的点）的pH值幷记录之。

DATA TABLE资料表
QUESTION问题
1. What was the pH of the starting vinegar solution?开始时的醋溶液的pH值是多少？
2. What was the pH of the ending solution of vinegar and drain cleaner?结束时的醋与清洁
剂的混合溶液的pH值是多少？
3. The reaction of acetic acid and sodium hydroxide forms sodium acetate. What does the pH
of the pK sp of the salt (sodium acetate) tell you about the salt?醋酸与氢氧化钠反应形成醋酸钠。

这种盐（醋酸钠）的pKsp的pH值说明了这种盐的什么特性？
4. What is the pK a of the acid? What is the K a of the acid?这种酸的pKa值是多少？Ka值是
多少？。