EXPERIMENT 1

Transcription

1 EXPERIMENT 1 Calibration of Volumetric Glassware Introduction Table 1.1 shows some of the tolerance values established for volumetric glassware by the National Bureau of Standards. It may be noted that glassware meeting these specifications is adequate for all but the most exacting work of the analytical laboratory. Such glassware, stated by the manufacturer to conform to NBS standards, may be purchased. For an extra fee, it is also possible to obtain glassware that has actually been tested by the NBS. Because of the expense, however, the beginning analytical laboratory will rarely be equipped with glassware guaranteed to meet NBS tolerance specifications. Less expensive glassware stated by the manufacturer to meet tolerances about double those of the NBS is available, but even this glassware may not be furnished the student. For this reason, and also because the history of a particular item involving possible damage may be obscure, it is often advisable for students to calibrate their volumetric glassware. Of course, the individual instructor will make the decision in any case. Since most analytical work involves dilute, aqueous solutions, water is generally used as the reference material in the calibration of volumetric glassware. The general principle in calibration is to determine the weight of water contained in or delivered by a particular piece of glassware. Then, with the density of water known, the correct volume is found. The units of volume commonly employed in analytical chemistry are the liter and the milliliter. The liter was formerly defined as the volume occupied by 1 kg of water at the temperature of its maximum density (about 4 ) under a pressure of 1 atm. In 1964 the Twelfth General Conference on Weights and Measurements, meeting in Paris, France, abolished this definition and instead made the liter a special name for the cubic decimeter. This new definition eliminates the previous discrepancy of 28 parts per million between the milliliter and cubic centimeter (1 ml was cc), and these two units are now equivalent. 1

2 TABLE 1.1 TOLERANCES FOR VOLUMETRIC GLASSWARE (ml) Capacity, Less Than and Including: Volumetric Flasks Transfer Pipets Burets The National Bureau of Standards has specified 20 as the temperature at which glassware is calibrated. Since the laboratory temperature will usually not be exactly 20, glassware must, strictly speaking, be corrected when used at other temperatures because of errors due to expansion (or contraction) of both the glass vessel itself and the solution contained therein. The coefficient of expansion of glass is sufficiently small that the correction required for this factor is negligible for most work (it amounts to the order of 1 part per 10,000 for a change of 5 ). The change in the volume of the solution itself, on the other hand, is more important, but it can still be ignored in many cases if the working temperature is not far removed from 20 (the volume change is of the order of 1 part per thousand over a 5 range). As noted above, calibration data are secured by converting weight of water into volume via the density. Tables showing the density of water at various temperatures are available in handbooks. However, the data in such tables are usually given on the bassis of weights in vacuum, while the actual weighings are made in air. Since the water being weighed generally displaces more air than do the weights, it is necessary to correct the weighings for the buoyancy effect of the air if such handbook tables are to be used. On the other hand, we may change in vacuum densities into densities that would be obtained in air with steel weights, and use these directly with our weighings obtained in air. The values in Table 1.2, which are the reciprocals of such adjusted densities, may be used directly by the student without taking the buoyancy effect into consideration. Methods of Calibration Commonly Employed There are three general approaches to the calibration of volumetric glassware that 2

3 are in wide use and with which the student should be familiar. Table1.2 VOLUME OF 1G OF WATER WEIGHED IN AIR WITH STEEL WEIGHTS AT VARIOUS TEMPERATURES ml ml The first method, which we may designate as a direct, absolute calibration, is based on the principles outlined above. The volume of water delivered by a buret or pipet, or contained in a volumetric flask, is obtained directly from the weight of the water and its density. Directions are given below for the calibration of a buret, a pipet, and a volumetric flask using this method. 2. Volumetric glassware is sometimes calibrated by comparison with another vessel previously calibrated directly. We may refer to this as an indirect, absolute calibration, or calibration by comparison. This method is especially convenient if many pieces of glassware are to be calibrated, and it is sufficiently accurate for all ordinary usages provided that the reference vessel itself has been accurately calibrated. Calibrating bulbs are not available in many student laboratories, and specific directions for their use are not included in this chapter. They are not difficult to use if proper care is taken. Students may obtain directions from their instructors if they are to use such equipment. 3. Sometimes it is necessary to know only the relationship between two items of glassware without knowing the absolute volume of either one. This situation arises, for example, in taking an aliquot portion of a solution. Suppose that is desired to titrate one-fifth of an unknown sample. The unknown might be dissolved, appropriately treated preparatory to the titration, and diluted to volume in a 250-mL volumetrc flask. A 50-mL pipet would then be used to withdraw an aliquot for titration. For the calculations in this analysis, it would not be necessary to know the exact volume of the flask or the pipet, but 3

4 it would be required that the pipet hold exactly one-fifth as much solution as the flask. The method used for a relative calibration of this sort simply involves discharging the pipet five times into the flask and marking the level of the meniscus on the flask. Calibration of Buret The buret should be thoroughly cleaned so that it drains well, and the stopcock should be properly lubricated. Fill the buret with water and test for leakage, making a reading to the nearest 0.01 ml and repeating the reading after waiting at least 5 min. No noticeable change should have occurred. During the waiting period, weigh a stoppered 125-mL Erlenmeyer flask (or other suitable container) to the nearest milligram. Record this weight. Fill the buret with distilled water that is at the temperature of the laboratory. This temperature must be measured and recored. Then sweep any air bubbles from the tip of the buret by opening the stopcock to permit rapid outflow. Now withdraw water more slowly until the meniscus is at, or slightly below, the zero mark on the buret. After drainage is complete (at least 30 s), read the buret to the nearest 0.01 ml. Record this "initial" reading. Remove any hanging drop of water from the tip of the buret by touching it lightly to the side of a vessel such as a beaker. Now run about 10 ml of water from the buret into the weighed flask. The tip of the buret should extend somewhat into the mouth of the flask, and care should be taken against spattering. The neck of the flask where contact with the stopper is made should not be wet. Quickly stopper the flask. Read the buret after allowing time for drainage and record this "final" reading. Then weigh the stoppered flask to the nearest milligram. Record this weight. Tabulation of the data as suggested in Table 1.3 is convenient. Now refill the buret and obtain another "initial" reading. Run about 20 ml of water into the flask, obtain the "final" buret reading, and reweigh the flask. Note that we are calibrating the buret in 10-mL intervals, but starting each time from an initial reading near zero since in titrations we generally start near this point. This process is repeated for the 30-, 40-, and 50-mL volumes. The flask, of course, should never be allowed to exceed the capacity of the balance. It should first be emptied, the neck dried with a clean towel, and reweighed. For each calibration interval, multiply the weight of water by the appropriate value from Table 1.2 to obtain the actual volume of the water. 4

5 TABLE 1.3 SAMPLE DATA FOR CALIBRATION OF BURET Initial buret reading Final buret reading Apparent volume. ml Initial weight of flask Final weight of flask Weight of water, g Temperature, Actual volume, ml Correction, ml The difference between this actual volume and the apparent volume (from the buret readings) is, of course, the correction. If the actual volume is larger than the apparent volume, the correction obtained by subtracting in this way will be positive, which means that it is to be added to the buret reading in future work with the buret. The calibration should be repeated as a check on the work. Duplicate results should agree within 0.04 ml. With the weighings carried out to the nearest milligram, as directed above, errors in weighing should not affect the results because the weight is actually needed only to the nearest 0.01 g. Hence failure of duplicate results to agree is generally due to (1) leakage around the buret stopcock, (2) failure to wait for drainage before reading the buret, or (3) careless technique in collecting or handing the container and stopper. Since our calibration is performed over 10-mL intervals, students often ask about the correction to be applied to a buret reading of an intermediate value such as 46ml. Ordinarily, applying the correction of the nearest interval, in this case 50mL, will be sufficient. It is perhaps somewhat better to interpolate between the known corrections. This is done most conveniently by a graphical method. The student may plot corrections against intervals 10, 20 ml,..., connecting the points with straight lines so as to obtain linear interpolations by simple inspection of the graph. Such an interpolation obviously is not likely to reflect the true situation within the buret, but it will usually be more than sufficiently accurate for the need at hand. The student must use good judgment in consideration of the precision desired in the buret reading. For example, an error of 0.04 ml in a 40-mL buret reading represents only 1 ppt. Calibration of Pipet The method is essentially the same as that described above for a buret. The pipet should naturally be carefully cleaned and rinsed before calibration. Weigh to the neareat 5

6 milligram a clean, stoppered Erlenmeyer flask of adequate capacity. Now fill the pipet to a level above the etched line, using a beaker of distilled water that is at the temperature of the laboratory. This temperature must be measured, of course. Dry the outside of the pipet with a clean towel, and then release the finger pressure to allow the liquid level to fall to the etched line. The pipet should be held in a vertical position with the etched line at eye-level for this operation. Hold the liquid level so that the bottom of the meniscus coincides with the etched line, and touch the tip of the pipet to the side of the beaker to remove any hanging drop. Then discharge the contents of the pipet into the weighed container. It is best to insert the tip of the pipet well into the container and to keep the tip touching the inner wall of the container during the delivery. With the tip still touching the container, allow the pipet to drain for 20 to 30 s after the flow ceases. Do not disturb the last portion of water remaining in the tip. Then stopper the container and reweigh it. Calculate the volume of water delivered by the pipet from the weight and the appropriate value taken from Table 1.2. The calibration should be repeated, and the duplicate results should not differ by more than 1 ppt. Common errors are (1) warming the contents of the pipet by holding the enlarged portion in the palm of the hand, (2) failure to allow sufficient drainage time, (3) disturbing the residue of water that should remain in the tip of the pipet, and (4) general carelessness in handling the weighed container and stopper. 6

7 實驗一容器體積的校正 一 目的 1. 學習電子天平的正確操作及秤重方法 2. 了解玻璃器皿容積的校正方法與操作技巧 3. 學習數據處理及分析技巧 二 原理實驗室中常使用之器皿多為玻璃材質, 由於玻璃材質稍高溫加熱易軟化變形, 使玻璃器皿上的刻度改變, 故有精確刻度之玻璃器皿不可放入烘箱加熱乾燥, 如玻璃器材之吸量管 滴定管 定量瓶及量筒等 在大部分的分析實驗中, 常選擇水作為溶劑, 由於水的密度隨溫度而異 ( 如 Table 1-2 所示 ), 以及玻璃本身熱脹冷縮等因素而導致玻璃器皿視容積發生偏差 故為了得到準確的體積測量, 玻璃器皿在使用前須先校正 校正過程包括用體積容器來測定已知密度液體的重量, 其中體積受溫度影響最大, 其影響可分為兩方面 : 第一, 液體的體積隨溫度改變而有所不同 第二, 玻璃器皿的體積會隨溫度而改變 玻璃的溫度係數很小, 故校正體積時, 除非須做更精密之量測, 否則可將玻璃器皿的體積變化因素忽略 三 實驗器材與藥品 器材 :50mL 三角錐形瓶 13 個 溫度計 50mL 滴定管 25mL 吸量管 天平 藥品 :R.O 水四 實驗步驟 A. 滴定管的校正 ( 了解滴定管的正確使用方式 ) ( 每組領取十三個 50 ml 三角錐形瓶 ) 1. 徹底清洗滴定管, 並使活塞可順暢轉動 2. 取約 500mL 水, 靜置到室溫, 並測量溫度 3. 將滴管固定在滴定管架上 4. 將滴定管裝滿水, 打開活栓讓水快速流出以除去尖端的氣泡 5. 將液面 ( 新月型凹處 ) 流至刻度 0.00mL 處, 並去除滴定管尖端的水滴 6. ( 了解微量天平的正確使用方式 ) 秤重一 50mL 三角錐形瓶讀取至 0.1mg 7. 從滴定管中取 10mL 水, 盡量精確取 10.00mL, 待水完全沉靜下來後 ( 至少 30 秒 ) 再精確讀值到 0.01ml 8. 稱量三角錐形瓶加水的重量 ( 使用同一天平 ) 9. 計算水的重量 7

8 10. 將水的重量依表 (1-2) 轉換成體積 ( W * Factor = ml) 11. 重覆 3-7 步驟, 兩次誤差應在 0.1mL 之內 12. 求取平均值 13. 重覆 3-9 步驟, 除步驟 6 改取 20mL 水 14. 重覆 3-9 步驟, 除步驟 6 改取 30mL 水 15. 重覆 3-9 步驟, 除步驟 6 改取 40mL 水 16. 重覆 3-9 步驟, 除步驟 6 改取 45mL 水 17. 以修正值對刻度值作圖, 畫出修正曲線 ( 修正值 = 實驗值 - 刻度讀值 ) B. 吸量管的校正 ( 了解吸量管及吸球正確使用方式 ) 1. 稱重一 50mL 三角錐形瓶 ( 至 0.1mg) 2. 從 25mL 吸量管中取 25mL 水到三角錐形瓶中 3. 稱量三角錐形瓶加水的重量 ( 使用同一天平 ) 4. 計算水的重量 5. 將水的重量依表 (1-2) 轉換成體積 6. 重覆 1-5 步驟二次 ( 共三次 ), 兩次誤差應在 0.06mL 之內 7. 求取平均值 8. 求出標準差, 平均偏差及相對平均偏差 五 注意事項 ( 將三角錐形瓶交回放在指定的位置 ) 1. 架設滴定管時, 需至正面及側面觀看, 確保滴定管直立 2. 滴定管控制閥若不易轉動, 或是會漏水請旋轉旁邊旋鈕到鬆緊適當的位置 3. 移液管插入安全吸須球須小心, 誤插太深使吸球損壞, 也不要太用力使移液 管折斷, 若不易插入可沾取少許水分使其潤滑 4. 讀值時兩眼須平視刻度, 讀取位數為最小刻度再加一位估計值 ( 例 :10.42) 5. 注意有效數字的讀取與運算 參考公式 : 標準偏差 : 平均偏差 : n 2 xi x 2 ( ) 1 x x i i i s n 1 n 1 n xi x i d 1 n 2 xi x / n d i 1 相對平均偏差 : 100% 100% x x n d ppt 1000 x / n 8

9 x x i t 相對誤差 : % 100% x t xi xt ; ppt 1000 x t xi: 每次的測量值 x : 平均值 x t : 真實值在此定為 x truevalue = x 9

10 八 實驗數據與結果 A: 滴定管的校正 溫度 : 測量體積 10 ml 20 ml 30 ml 40 ml 45 ml 次數 初刻度 (V i ) 末刻度 (V f ) 刻度體積,ml (V a = V f -V i ) 刻度體積平均,ml V a = (V a1 +V a2 )/2 空瓶重 (W 0 ) 瓶加水重 (W 1 ) 水重,g (ΔW = W 1 -W 0 ) 實際體積,ml (V b =ΔW F) 實際體積誤差,ml (V b1 -V b2 ) 實際體積平均 V b =(V b1 +V b2 )/2 修正值,ml (C = V b -V a ) 修正曲線 : 請用電腦作圖 (XY 散佈圖中的有點折線圖 ) 10

11 B: 吸量管的校正 空瓶重 (g) 空瓶加水重 (g) 水重 (g) 水的 體積 (ml) 平均偏差 平均值 1 25 ml 2 3 標準偏差 : 平均偏差 : 相對平均偏差 : 九 問題與討論問題 : 1. 實驗室中, 準確的體積測量常使用那些工具, 應用範圍為何? 2. 測量體積之容器為何常需要校正? 3. 在 buret 的校正實驗時 : 兩次重覆實驗如有較大的誤差 ( 大於 0.1mL), 可能是何原因造成? 討論 : 請針對本次實驗的結果與收穫加以探討 11

12 EXPERIMENT 2 Neutralization Titrations Introduction Neutralization titrations are performed with standard solutions of strong acids or bases. While a single solution (of either acid or base) is sufficient for the titration of a given type of analyte, it is convenient to have standard solutions of both acid and base available in the event a back-titration is needed to locate end points more exactly. The concentration of one solution is established by titration against a primary standard; the concentration of the other is then determined from the acid/base ratio (that is, the volume of acid needed to neutralize ml of the base). The Effect of Atmospheric Carbon Dioxide on Neutralization Titrations Water in equilibrium with the atmosphere is about 1*10-5 M in carbonic acid as a consequence of the equilibrium CO 2 (g) + H 2 O H 2 CO 3 (aq) (2-1) At this concentration level, the amount of 0.1 M base consumed by the carbonic acid in a typical titration is negligible. With more dilute reagents (<0.05 M), however, the water used as a solvent for the analyze and in the preparation of reagents must be freed of carbonic acid by boiling for a brief period. Water that has been purified by distillation rather than by deionization is often supersaturated with carbon dioxide and may thus contain sufficient acid to affect the results of an analysis. The instructions that follow are based upon the assumption that the amount of carbon dioxide in the water supply can be neglected without causing serious error. Procedure Preparation of Dilute Hydrochloric Acid Solutions For a 0.1 M solution, add about 4 ml of concentrated HCl to about 500 ml of distilled or deionized water (Note). Mix thoroughly, and store in a glass-stoppered bottle. Note: It is advisable to eliminate CO2 from the water by a preliminary boiling if very dilute solutions (<0.05 M) are being prepared. 12

13 Preparation of Carbonate-Free Sodium Hydroxide If so directed by the instructor, prepare a bottle for protected storage (Note 1). Transfer 250mL of distilled or deionized water to the storage bottle. Decant 1.5 to 2 ml of 50% NaOH into a small container (Note 2), add it to the water, and mix thoroughly. USE EXTREME CARE IN HANDLING 50% NaOH, which is highly corrosive. If the reagent comes into contact with skin, IMMEDIATELY flush the area with copious amount of water. Protect the solution from unnecessary contact with the atmosphere. Notes: 1. A solution of base that will use up within two weeks can be store in a tightl capped polyethylene bottle. After each removal of base, squeeze the bottle while tightening the cap to minimize the air space above the reagent. The bottle will become embrittled after extensive use as a container for bases. 2. Be certain that any solid Na 2 CO 3 in the 50% NaOH has settle to the bottom of the container and that the decanted liquid is absolutely clear. If necessary, filter the base through a glass mat in a Gooch crucible; collect the clear filtrate in a test tube inserted in the filter flask. The Determination of the Acid/Base Ratio When both acid and base solutions have been prepared, it is useful to determine their volumetric combining ratio so that the two solutions can be used to establish end points more accurately in any of methods given later in this section, knowledge of this ratio and the concentration of one solution permits calculation of the molarity of the other. Place a buret into service. Place a test tube or small beaker over the top of the buret that holds the NaOH solution to minimize contact between the solution and the atmosphere. Record the initial volumes of acid and base in the buret to the nearest 0.01 ml. Deliver 30 ml of the acid into a 250 ml conical flask. Touch the tip of the buret to the inside wall of the flask, and rinse down with a little distilled water. Add two drops of phenolphthalein (Note 1) and then sufficient base to render the solution a definite pink. Introduce acid dropwise to discharge the color, and again rinse down the walls of the flask. Carefully add base until the solution acquires a faint pink hue that persists for at least 30 sec (Note 2,3). Record the final buret volumes (again, to nearest 0.01 ml). Repeat the titration. Calculate the acid/base volume ratio. The ratio for duplicate titration should agree to within 1 to 2 ppt. Perform additional tirtations, if necessary, to achieve this order of precision. 13

14 Notes: 1. The volume ratio can also be determined with an indicator that has an acidic transition range, such as bromocresol green. If the NaOH is contaminated with carbonate, the ratio obtained with this indicator will differ significantly from the value obtained with phenolphthalein. In general, the acid/base ratio should be evaluated with the indicator that be used in subsequent titrations. 2. Fractional drops can be formed on the buret tip, touched to the wall of the flask, and then rinsed down with a small amount of water. 3. The phenolphthalein end point fades as CO 2 is absorbed from the atmosphere. Standardization of Sodium Hydroxide Against Potassium Hydrogen Phthalate Dry a quantity of primary-standard potassium hydrogen phthalate (KHP) for about 2 h at 100 and cool in a desiccator. Weigh individual 0.7-to 0.8-g samples (to the nearest 0.1 mg) into 250-mL conical flasks, and dissolve each in 50 to 75 ml of distilled or deionized water. Add 2 drops of phenolphthalein; titrate with base until the pink color of the indicator persists for 30 s (Note). Calculate the concentration of the NaOH solution. Note: It is permissible to back-titrate with acid to establish the end point more precisely. The volume of acid must, of course, be recorded so that a correction to the volume of base used in the titration can be computed from the acid/base ratio. The Determination of Sodium Carbonate in an Impure Sample Dry the unknown at 110 for 2 h, and then cool in desiccator. Consult with the instructor on an appropriate sample size. Weigh (to the nearest 0.1mg) individual samples into 250-mL conical flasks. Dissolve each in 50 to 75 ml of distilled water, add 2 drops of bromocresol green, and titrate with standard acid until the indicator just begins to turn green. Boil the solution for 2 to 3 min, cool to room temperature, and complete the titration. If additional acid is not required after boiling, the solution already contains excess. Either back-titrate this excess with standard base, or discard the sample if a base solution is not available. Report the percentage of Na 2 CO 3 in the sample. (fw = ). 14

15 實驗二 中和滴定 一 目的 1. 學習酸鹼反應在分析上的應用 2. 學習指示劑的原理及使用, 以及反滴定的應用 二 原理酸鹼中和滴定是以一已知濃度之標準鹼或酸滴定未知之酸或鹼, 當雙方當量數相等時, 可以酸鹼濃度計或指示劑的顏色變化而得知當量點, 進而算出未知酸或鹼的濃度 其方程式如下 : N acid V acid = N base V base (2-1) N base N acid : 為酸 - 鹼的當量濃度 V base V acid : 為酸 - 鹼的體積如酸 鹼物質之當量濃度等於容積莫耳濃度時, 則在當量點 : M acid V acid = M base V base (2-2) M acid M base : 為酸 - 鹼的容積莫耳濃度 V acid V base : 為酸 - 鹼的體積 三 實驗器材與藥品 使用器材 : 吸量管 1000mL 燒杯 250mL 錐形瓶 500mL 燒杯 50mL 滴定管 藥品 :0.1N 氫氧化鈉 NaOH (aq) 60mL 濃鹽酸 Conc. HCl 2mL Phenolphthalein 指示 劑 0.5M NaCl (aq) 食鹽水 10mL Bromocresol green 指示劑 15

16 四 實驗步驟 A. 製備 0.1 N 的稀鹽酸 250 ml: 1. 取約 250ml 的水於 500ml 的燒杯中 2. 取 2mL 濃鹽酸加入步驟 1 中, 以玻棒攪拌徹底混合均勻 3. 放入滴定管中備用, 需標示 B. 酸實際濃度的測定 : 1. 從酸的滴定管中取 30ml 的酸到 250ml 錐形瓶中 2. 加兩滴 phenolphthalein 溶液 3. 取另一滴定管加入 0.1N NaOH (aq), 後將鹼滴入上述溶液使之呈永久的粉紅色 4. 從酸滴定管中加數滴的酸使溶液退色 5. 從鹼滴定管中小心加入鹼使溶液呈淡粉紅色至少三十秒 6. 記錄最後所使用酸及鹼的總體積 ( Va = step 1 + step 4 ; Vb = step 3 + step 5) 7. 重覆 1-6 步的步驟共兩次 ( 可直接加入約略少於步驟 3 三之體積的鹼, 再慢慢滴定到終點 ) 8. 計算酸的濃度 9. 兩次滴定所得的值應在 5% 內 ( 相對誤差 ) 10. 如有相對誤差未在要求之內, 重覆上述的滴定步驟, 以符合要求的精確度為止 ( 最多三次 ) C. 碳酸鈉純度的測定 ( 先操作空白實驗 ): 1. 取 0.5M 氯化鈉溶液自行稀釋為 0.05M 氯化鈉溶液 100ml, 取 50ml 作為一次的空白試劑 2. 加兩滴 bromocresol green 指示劑 3. 以酸滴定到溶液開始變綠色 ( 即藍色消失 ) 4. 煮沸溶液三分鐘, 冷卻到室溫並完成滴定 ( 如指示劑煮沸後顏色消失, 則再加三滴指示劑 ) 如果加溫後, 顏色無法變回藍色, 表示滴定過量, 須以鹼反滴定到 16

17 變回藍色, 再以酸滴定到藍色消失 ( 每張實驗桌僅使用 3 台加熱攪拌器 ) 5. 取步驟 1 的另 50ml 空白試劑 重覆步驟 2-4, 記錄所使用酸的體積 做兩次空白試驗並取平均值 6. 由助教處取得碳酸鈉分析樣品, 依照組別領取 7. 將樣品於 110 中烘兩小時, 並於乾燥器中冷卻 助教已完成此步驟 8. 秤取 g 的樣品於 250ml 錐瓶中 9. 加 50ml 的水以溶解樣品 10. 重覆 2-4 步驟 11. 計算樣品中碳酸鈉的純度 (Fw=105.99) [(V acid M 2 - B mol ) /2] / weight of sample 100%= Purity (%) (2-4) 12. 計算平均值 補充 -- 滴定之操作方法 : 1. 將待滴定之溶液置於錐形瓶內, 加入指示劑 2. 錐形瓶下置一白紙或白瓷板 3. 滴定管洗淨後再用標準溶液清洗兩次 4. 滴定管內加滿標準溶液, 左手拇指在滴定管前, 食指與中指在後操作旋塞, 如圖 2-1( 左 ) 所示 5. 右手輕搖錐形瓶以攪拌內盛溶液, 圖 2-1( 右 ) 6. 觀察溶液顏色變化, 接近滴定終點時需格外小心, 勿滴定過量 圖 2-1 滴定操作示範圖 17

18 五 注意事項 1. 同組同學要互相合作, 一人負責酸的滴定, 一人負責鹼的滴定 2. 先進行空白實驗, 以利顏色變化的判斷,0.05M 氯化鈉溶液需自行稀釋配製 3. 滴定管裝填時漏斗不可一直插在滴定管上, 使用完要洗淨, 管尖朝上活栓打開 4. 未知樣品依照組別領取, 用畢瓶子洗淨依照組別交回 18

19 八 實驗數據與結果 NaOH 濃度 M 1 : B. 酸濃度的測定 : 酸的總體積鹼的總體積酸的濃度 HCl 濃度平 均值 M 2 相對誤差 1 2 C-1. 碳酸鈉純度的測定之空白實驗 B mol =V a M 2 -V b M 1 HCl 滴定前刻度 HCl 滴定後刻度 第一次用的酸體積 V a1 NaOH 反滴定的體積 V b 第二次用的酸體積 V a2 空白實驗使用酸的體積 V a 空白實驗使用酸的莫耳數 B mol 空白實驗使用酸的莫耳數平均值 1 2 C-2. 碳酸鈉純度的測定, 未知物編號 樣品重 滴定前刻度 滴定後刻度 酸的體積 反滴定的體積 實際使用酸的體積 碳酸鈉純度 平均值 相對誤差

20 九 問題與討論問題 : 1. 溶解在水中的二氧化碳對滴定結果將造成正偏差或負偏差? 如何消除此偏差? 2. 一般氫氧化鈉試劑常含有碳酸鈉, 如何將之分離以配出純的氫氧化鈉溶液? 3. 以鹼滴定酸, 達到 Phenolphthalein 當量點呈粉紅色時, 置在桌上一段時間後, 又會變回無色, 是何原因? 是否須要再滴定到粉紅色, 以求得正確的滴定結果? 討論 : 請針對本次實驗的結果與收穫加以探討 20

21 EXPEREMENT 3 Complex-Formation Titrations with EDTA Procedure Preparation of Standard 0.01 M EDTA Solution Dry about 4 g of the purified dihydrate Na 2 H 2 Y.2H 2 O (Note 1) at 80 to remove superficial moisture. Cool to room temperature in a desiccator. Weigh (to the nearest milligram) about 1.0 g into a 250mL volumetric flask (Note 2). Use a powder funnel to ensure quantitative transfer; rinse the funnel well with water before removing it from the flask. Add 150 to 200 ml of water (Note 3) and swirl periodically. Dissolution may take 15 min or longer. When the entire solid has dissolved, dilute to the mark with water and mix well (Note 4). In calculating the molarity of the solution, correct the weigh of the salt for 0.3% moisture it ordinarily contains after drying at 80 Notes: 1. Directions for the purification of the disodium salt are described by W. J. Blaedel and H. T. Knight, Anal. Chem., 1954, 26(4), The solution can be prepared from the anhydrous disodium salt, if desired. The weight taken should be about 0.9 g. 3. Water used in the preparation of standard EDTA solutions must be totally free of polyvalent cations. If any doubt exists concerning its quality, the water should be passed through a cation-exchange resin before use. 4. As an alternative, an EDTA solution that is approximately 0.01 M can be prepared and standardized by direct titration against a Mg 2+ solution of known concentration or against primary-standard CaCO 3 by displacement titration. The Determination of Magnesium by Direct Titration Weight gram sample containing magnesium sulfate to a clean 250-mL volumetric flask, dilute to the mark with water, and mix thoroughly. Transfer mL aliquots to 250-mL conical flasks; add 1 to 2 ml of ph10 buffer and 3 to 4 drops of Erio T or Calmagite indicator to each. Titrate with 0.01 M EDTA until the color changes from red to pure blue (Notes 1, 2). Express the results as percentage of Mg 2+ in the sample. Calculate the purity of magnesium sulfate in the sample. ( MgSO 4 :Fw = ) Notes: 21

22 1. The color change tends to be slow in the vicinity of the end point. Care must be taken to avoid overtitration. 2. Other alkaline earths, if present, are titrated along with the Mg 2+ ; removal of Ca 2+ and Ba2+ can be accomplished with (NH 4 ) 2 CO 3. Most polyvalent cations are also titrated. Precipitation as hydroxides or the use of a masking reagent may be needed to eliminate this source of interference. Preparation of the Magnesium/EDTA Complex, 0.1 M (Sufficient for 90 to 100 titrations.) To 3.72 g of Na 2 H 2 Y*2H 2 O in 50 ml of distilled water, add an equivalent quantity (2.46 g) of MgSO 4 *7H 2 O. Add a few drops of phenolphthalein, followed by sufficient 0.1 M NaOH to turn the solution faintly pink. Dilute to about 100 ml with water. The addition of a few drops of Erio T to a portion of this solution buffered to ph 10 should cause development of a dull violet color. Moreover, a single drop of 0.01 M Na 2 H 2 Y solution added to the violet solution should cause a color change to blue, and an equal quantity of 0.01 M Mg 2+ should cause a change to red. The composition of the original solution should be adjusted with additional Mg 2+ or H 2 Y 2- until these criteria are met. The Determination of Calcium by Replace Titration Weight gram sample containing calcium chloride to a clean 250mL volumetric flask, dilute to the mark with water, and mix thoroughly. Transfer 50.0mL aliquots to 250mL conical flasks, add the sample with a few drops of HCl, and boil gently for a few minutes to eliminate CO 2 Cool, add 3 to 4 drops of methyl red, and neutralize with 0.1 M NaOH. Introduce 2 ml of ph 10 buffer, 3 to 4 drops of Erio T or Calmagite, and titrate with standard 0.01 M Na 2 H 2 Y to a color change from red to pure blue (Note). Express the results as percentage of Ca 2+ in the sample. Calculate the purity of calcium chloride in the sample. ( CaCl 2 :Fw = ) Note: The color change is sluggish if Mg 2+ is absent. In this event, add 1 to 2 ml of 0.1 M MgY 2- before starting the titration. 22

23 實驗三 錯化合物形成滴定 一 目的藉 EDTA 與陽離子形成穩定錯合物之特性, 以瞭解錯化合物形成的原理及在滴定分析法上的應用 二 原理 EDTA ( Ethylenediamine tetracetic acid ) 是乙二胺四醋酸的簡寫, 是一種四質子酸, 常以 H 4 Y 表示, 其結構如下 : HOOC CH 2 N H 2 C H 2 C N H 2 C COOH HOOC C H 2 H 2 C COOH 除了四個羧酸基外, 每一個氮原子具有未共用電子對 ( unshared pair of electrons ), 因此每個 EDTA 分子有六個可能位置可以與任何陽離子鍵結, 形成穩定錯合物, 是為六芽團的鉗合物 EDTA 與其所形成的離子通常寫成 H 4 Y, H 3 Y -,H 2 Y 2-,HY 3- 與 Y 4-, 以 EDTA 作為滴定劑時, 不論陽離子上的電荷有多少, 它與金屬離子都是按莫耳數 1:1 的比例結合, 且這些錯合物中大部分都很穩定, 此可作為定量分析的基準 指示劑變色原理如下 : 直接滴定法 MgIn - + HY 3- MgY 2- + HIn 2- (3-1) 紅色 藍色 置換法 +In Mn + + MgY 2-3- MY n-4 + MgIn - (3-2) 23

24 指示劑為 EBT( Eriochrom Black T ) 其結構如下 : OH OH HO 3 S N N O 2 N 三 使用器材及藥品 使用器材 : 50mL 滴定管 25ml 吸量管 250mL 量瓶 250mL 錐形瓶 玻棒 藥品 : 乙二胺四醋酸 (EDTA) Na 2 H 2 Y*2H 2 O M w1 = g 硫酸鎂 MgSO 4 *7H 2 O M w2 = 氯化鈣 CaCl 2 *2H 2 O M w3 = Methyl red 指示劑 EBT 指示劑 PH 10 buffer solution 0.1M 氫氧化鈉 NaOH 1N 鹽酸 HCl R.O. H 2 O 四 實驗步驟 A: 0.01M EDTA 標準溶液的製備 : 1. EDTA 於 80 中烘乾, 以除去表面的水, 並在室溫中於乾燥器內冷卻 助教已完成此步驟 2. 以秤量瓶精稱 1.0g EDTA, 倒入 250mL 燒杯中加 mL 的水, 攪拌使固體溶解 3. 將以完全溶解的液體倒入 250mL 量瓶中, 以水稀釋到標線 4. 計算溶液的濃度 乾燥過後, 該鹽含有約 0.3% 的水, 必須在計算濃度時, 修正扣除之 M(EDTA) = (1.0*0.997)/ * (1000/250) (3-3) 24

25 B: 以直接滴定法測定鎂離子的含量 1. 精秤硫酸鎂樣品 克並以 50 ml 水溶解後, 移入 250mL 量瓶中, 以水稀釋到標線並徹底混合 2. 以吸量管取 50mL 上述溶液到 250mL 錐形瓶中 3. 加 2mL P H 10 的緩衝溶液, 並加 3-4 滴的 EBT 指示劑 4. 以 0.01M EDTA 溶液滴定到顏色從紅色到藍色 5. 重覆 2-4 步, 並計算平均值 6. 計算鎂離子在樣品中的濃度 (w/w %) 及 7 個結晶水硫酸鎂的純度 (Mg Aw = ) % = = 100 % (3-4) 純度 = = C. 以置換法測定鈣離子的含量 1. 精秤氯化鈣樣品 克以 50 ml 水溶解後, 移入 250mL 量瓶中, 以水稀釋到標線並徹底混合 2. 取 50mL 上述溶液到 250mL 錐形瓶中, 以數滴 1N 的鹽酸酸化 3. 緩慢沸騰數分鐘, 以除去二氧化碳 4. 冷卻後, 加入 1 滴 methyl red, 並以 0.1M 氫氧化鈉溶液中和之 ( 從紅色變為黃色 ) 5. 加 2mL P H 10 的緩衝溶液及 1-2mL 0.1M Mg/EDTA 溶液及 3-4 滴 EBT 指示劑 6. 以 0.01M EDTA 溶液滴定到從紅色變成藍色 7. 重覆 2-6 步驟, 取平均值 8. 計算鈣離子在樣品中的濃度 (w/w %) 及氯化鈣的純度 (Ca Aw = ) 五 注意事項 M EDTA 配製需準確且小心使用, 配錯或用量不夠皆須重做實驗 2. 實驗 C 步驟較繁瑣, 請兩位同學同時進行, 一位進行實驗 A 和 B 時, 另一位可開始做實驗 C 的加熱除去二氧化碳 3. 實驗 C 中有兩種指示劑, 請注意加入的時間與名稱 25

26 八 實驗數據與結果 A. 溶液的製備與濃度 : Ethylenediamine teracetic acid EDTA 重量, g(w1) 濃度 (M1) 分子量 (Mw1) 硫酸鎂 MgSO 4 *7H 2 O 重量, g(w2) 濃度 (M2) 分子量 (Mw2) 氯化鈣 CaCl 2 重量, g(w3) 濃度 (M3) 分子量 (Mw3) B. 以直接滴定法決定鎂離子的含量 : 數據紀錄 硫酸鎂體積 (ml), V2 硫酸鎂樣品重 (mg), M2*V2*Mw2 滴定前刻度 滴定後刻度 EDTA 體積 1 2 濃度換算含公式 鎂含量 (%) 平均值相對誤差硫酸鎂純度平均值相對誤差

27 C. 以置換法決定鈣離子的含量 : 數據紀錄 氯化鈣體積 (ml), V3 氯化鈣樣品重 (mg), M3*V3*Mw3 滴定前刻度滴定後刻度 EDTA 體積 1 2 濃度換算含公式 鈣含量 (%) 平均值相對誤差氯化鈣純度平均值相對誤差 1 2 九 問題與討論 問題 : 一 鎂離子可由直接滴定法求得濃度, 何以鈣離子測量用置換法較準確? 二 在鈣離子測量中,Mg 2+ / EDTA 輔助指示劑加入量的多少對實驗有何影響? 三 為何在測定鈣離子含量時, 須加鹽酸酸化? 討論 : 請針對本次實驗的結果與收穫加以探討 27

28 實驗四 硝酸銀的沉澱滴定 一 目的 : 學習兩種以銀離子測定水溶液中氯離子含量的基本原理及分析方法 二 原理 : 銀離子和氯離子在水溶液中會形成微溶性的化合物, 故可以利用沉澱反應, 以滴定方法來測定樣品中氯離子的濃度 此類方法又可因滴定終點的識別原理不同而分為下列方法 : (1) 莫爾氏法 ( Mohr Method ) 由於鉻酸根和銀離子可以形成磚紅色的鉻酸銀沉澱, 所以可在水溶液中加入鉻酸根做為指示劑, 以磚紅色沈澱出現來判斷滴定終點的位置, 此反應式如下 : CrO 4 2- (aq) + 2 Ag + (aq) Ag 2 CrO 4(s) (4-1) 黃色磚紅色但此種方法必須考慮下列兩個因素 : 1. 滴定誤差最小時, 鉻酸根加入的濃度為 M, 而此濃度的鉻酸根黃色溶液將遮蔽了鉻酸銀沉澱的磚紅色, 故一般濃度皆降低到 M, 而此時需要做空白滴定來校正滴定誤差 2. 因為鉻酸銀在酸性下會形成重鉻酸銀, 而銀離子在強鹼性下會形成氧化銀沉澱, 所以反應須在中性或微鹼性溶液下進行 AgCl (s) Ag + + Cl - K sp = (4-2) Ag 2 CrO 4(s) 2Ag + + CrO 4 2- K sp = (4-3) 2CrO H + Cr 2 O H 2 O (4-4) 2Ag + + 2OH - 2AgOH (s) Ag 2 O (s) + H 2 O (4-5) [ 10 5 Ag ] K SP for AgCl 2 K [ CrO4 ] 10 [ Ag ] SP for indicator 2 28

29 (2) 斐恩氏法 ( Fajans Method ) 以具有吸附性的染料 -- 螢光黃染料 ( Fluorescein dye) 作指示劑, 由於它在水溶液中可以形成黃綠色帶負價的離子, 而在滴定終點時, 溶液會呈粉紅色, 此反應方程式如下 : HFlu (aq) + H 2 O H 3 O + (aq) + Flu - (aq) (4-6) 弱酸性 Flu - (aq) + (AgCl)Ag + (s) (AgCl)Ag + Flu - (s) (4-7) 黃色 AgCl 固體粉紅色 Cl O C OH Cl OH O O 2-7 -dichlorofluorescein HIn H + + In - (4-8) 當量點前到達當量點超過當量點 Cl - In - In - Ag + In - AgCl AgCl AgCl In- : Yellow In- : Yellow adsorption : pink 1. 使用器材及藥品使用器材 : 滴定管 1 支 滴管 1 支 250 ml 錐形瓶 2 個藥品 : 碳酸鈣 CaCO 3 硝酸銀 AgNO 3 二氯螢光黃 2',7'-dichlorofluorescein 碳酸氫鈉 NaHCO 3 糊精 Dextrin 5% 鉻酸鉀溶液 K 2 CrO 4 分析樣品 ( 依組別領取 )0.25g 29

30 四 實驗步驟 : (1) 莫爾法 : 1. 空白滴定 : 在 50mL 蒸餾水中, 加入少量碳酸鈣 ( 約 0.25g), 和加入少量 ( 約 0.1 克 ) 固體碳酸氫鈉至溶液中, 然後加入 1ml 的 5% 鉻酸鉀溶液, 並以 0.1M 硝酸銀 滴定到有磚紅色鉻酸銀沉澱產生為止, 作為空白滴定 2. 重複空白滴定共兩次, 取平均值 3. 待測樣品先在 110 下, 乾燥 1 小時並冷卻 助教已完成此步驟 4. 依組別領取分析樣品, 精稱約 0.25 克的分析樣品溶解於 50mL 蒸餾水中 5. 在上述溶液中, 加入少量 ( 約 0.1 克 ) 固體碳酸氫鈉至溶液中, 然後加入 1ml 的 5% 鉻酸鉀溶液, 並以 0.1M 硝酸銀滴定到有磚紅色鉻酸銀沉澱產生為止 注意 : 沉 澱顏色需與空白滴定顏色相同 6. 重複步驟 4~5 共兩次 7. 計算樣品中氯的含量 ( w /w %) 8. M Ag+ V Ag+ - M Ag+ V blank = mmol of Ag + = mmol of Cl - (4-9) mmol of - Cl % wt of sample(g) w % w = 樣品中氯的含量 (4-10) 9. 取平均值 註 : 氯化銀沉澱溶液回收至重金屬廢液桶中 (2) 斐恩氏法 : 1. 將樣品在 110 下乾燥 1 小時後並冷卻 助教已完成此步驟 2. 精秤約 0.25 克的分析樣品溶解於 50mL 蒸餾水中 3. 先在第一份樣品溶液中, 加入約 0.1 克的糊精和 5 滴二氯螢光黃指示劑, 以硝酸銀滴定到有粉紅色出現為止 4. 另取約 0.25 克樣品溶於 50mL 蒸餾水中, 配成另一份樣品溶液, 依步驟 3 得到的 30

31 結果, 先加入大部份的 0.1M 硝酸銀, 再加入糊精和二氯螢光黃指示劑, 再繼續滴定到滴定終點 5. 計算樣品中氯的含量 ( w/w % ) M Ag+ V Ag+ = mmol of Ag + = mmol of Cl - (4-11) mmol of - Cl % wt of sample(g) w % w = 樣品中氯的含量 (4-12) 6. 取平均值 五 注意事項 1. 未使用且無汙染的 0.1M 硝酸銀請回收到藥品桌上的回收瓶內 2. 本實驗的廢液須倒入重金屬廢液桶 31

32 八 實驗數據與結果 樣品編號 : 預測樣品 : 1:Mohr Method 滴定前刻度滴定後刻度空白滴定體積 (ml) 空白滴定平均值 (ml) 1 2 樣品重 滴定前刻度滴定後刻度硝酸銀體積 (ml) 氯含量 (%) 平均值相對誤差 1 2 2:Fajans Method 樣品重 滴定前刻度滴定後刻度硝酸銀體積 氯含量 (%) 平均值相對誤差 (ml) 1 2 九 問題與討論問題 : 一 何謂 空白滴定? 二 吸附指示劑用在沉澱實驗中, 有何特質? 三 試討論 P H 值的大小對本實驗不同方法的影響? 四 在本實驗斐恩氏法中為何要加入糊精? 討論 : 請針對本次實驗的結果與收穫加以探討 32

33 EXPERIMENT 5 Iodometric Methods of Analysis Principle Numerous methods are based upon the reducing properties of iodide ion: 2I I 2 + 2e - (5-1) Iodine, the reaction product, is ordinarily titrated with a standard sodium thiosulfate solution, with starch serving as the indicator: I 2 + 2S 2 O 3 2 2I + S 4 O 6 2 (5-2) Preparation of Solutions Preparation of 0.1N Sodium Thiosulfate Boil about 1 L of distilled water for 10 to 15 min. Allow the water to cool to room temperature; then add about 6.25 g of Na 2 S 2 O 3* 5H 2 O and 0.05 g of Na 2 CO 3. Stir until the solid has dissolved. Transfer the solution to a clean glass or plastic bottle, and store in a dark place. Potassium bromate, M Transfer about 1.5 g of reagent-grade potassium bromate to a weighing bottle, and dry at 110 for at least 1h. Cool in a desiccator. Weigh approximately 1.3 g (to the nearest 0.1 mg) into a 500 ml volumetric flask; use a powder funnel to ensure quantitative transfer of the solid. Rinse the funnel well, and dissolve the KBrO 3 in about 200 ml of distilled water. Dilute to the mark, and mix thoroughly. Solid potassium bromate can cause a fire if it comes into contact with damp organic material (such as paper toweling in a waste container). Consult with the instructor concerning the disposal of any excess. Standardization of Sodium Thiosulfate Against Potassium Bromate Introduction Iodine is generated by the reaction between a known volume of standard potassium bromate and an unmeasured excess of potassium iodide: BrO 3 + 6I + 6H + Br + 3I 2 +3H 2 O (5-3) The iodine produced is titrated with the sodium thiosulfate solution. 33

34 Procedure Pipet ml aliquots of the KBrO 3 solution into 250 ml conical flasks and rinse the interior wall with distilled water. Treat each sample individually beyond this point. Introduce 2 to 3 g of KI and about 5 ml of 3 M H 2 SO 4. Immediately titrate with 0.05 M Na 2 S 2 O 3 until the solution is pale yellow. Add 5 ml of starch indicator, and titrate to the disappearance of the blue color. Calculate the concentration of the thiosulfate solution. Iodometric Determination of Hydrogen Peroxide Procedure Pipet 25mL of the peroxide solution into a 250mL volumetric flask, dilute to the mark, and mix thoroughly. Transfer a 25mL aliquot of this solution to a 250mL Erlenmeyer flask and add 8mL of 1:6 sulfuric acid (about 6N), 3g of potassium iodide in 10mL of water, and 3 drops of 3% ammonium molybdate solution. Then add 5mL of starch solution and finish the titration to the disappeared. Then add 5mL of starch solution and finish the titration to the disappearance of the deep blue color. Titrate two other portions of the peroxide solution in the same manner. Calculate the weight of hydrogen peroxide in the original sample. Assuming that the weight of the sample was exactly 25g, calculate the percentage of hydrogen peroxide by weight. H 2 O 2 + 2H + + 2I I 2 + 2H 2 O (5-4) Determination of Bleaching Power by Iodometry Commercial bleaching products contain oxidizing agents such as hypochlorites or peroxides. The oxidizing power can be determined by the same procedure used in above. 4H + + 2ClO + 2I I 2 + Cl 2 + 2H 2 O (5-5) Procedure Place an accurately measured sample of the bleach in a 250mL Erlenmeryer flask. Liquid bleaches such Clorox and Purex contain sodium hypochlorite, and 2.00mL is a convenient sample if 0.1N thiosulfate is used as the titrant. Solid products such as Clorox II and Snowy Bleach contain peroxides. A sample of 0.7 to 0.8g is usually a convenient size for titration (Note 1). Add to the Erlenmeyer flask 75mL of distilled water, 3g of KI, 8ml of 1:6 sulfuric acid, and 3 drops of 3% ammonium molybdate solution (Note 2). Titrate the liberated iodine has almost disappeared. Then add 5mL of starch solution and finish the titration to the disappearance of the deep blue color. Titrate at least two portions of each bleaching product being compared. The oxidizing 34

35 ability of bleach is usually reported as percent chlorine. That is, the calculation assumes that chlorine is the oxidizing agent, although in fact it may not be. Report the percent by weight of chlorine in each product, assuming that the liquid bleaches have a density of g/ml. The equivalent weight of chlorine is the atomic weight. Notes: 1. The solids may not be homogeneous. If duplicate samples do give widely different results, a large sample can be taken, dissolved in a 500mL volumetric flask, and 50mL aliquots titrated. 2. Some of the bleaches react slowly with iodide ion. Molybdate ions catalyze the reaction. Alternatively, the solutions can be heated to about 60 to speed up the reachtion. 35

36 實驗五 碘滴定法 一 目的 1. 學習碘間接定量分析法的原理與應用 2. 測定雙氧水及漂白劑中氯的含量 二 原理碘 ( I 2 ) 本身是種相當弱的氧化劑, 氧化力不強, 故以碘為標準溶液的直接碘定量分析法, 應用有限 間接的碘定量分析法, 是用硫代硫酸鈉為標準溶液來滴定碘, 其用途比較廣泛 I 2 分子因為易揮發且較不穩定, 通常都是實驗時, 直接以氧化劑 ( 如 BrO - 3 ) 氧化生成, 如下列反應式 : BrO I H + Br I H 2 O (5-1) 利用此原理, 我們就可以利用待測氧化劑將 I - 氧化成 I 2, 再以硫代硫酸鈉還原 I 2, 而測得待測氧化劑的量 市售漂白水中, 大約分為兩類型, 一種含 H 2 O 2, 另一種含 ClO -, 此兩種化合物都是氧化劑, 因此就可利用碘滴定法來測量氧化劑的量 H 2 O H I - I 2 + 2H 2 O (5-2) 4 H ClO I - I 2 + Cl 2 + 2H 2 O (5-3) 產生的 I 2 分子再以硫代硫酸鈉滴定, 並以澱粉指示劑觀察 I 2 分子的消失而達到分析的目的, 其反應方程式如下 : I S 2 O I - + S 4 O 6 2- (5-4) 3. 實驗器材及藥品 使用器材 : 250ml 量瓶 250ml 錐形瓶 50ml 滴定管 36

37 藥品 : 溴酸鉀 KBrO 3 50ml 硫代硫酸鈉 Na 2 S 2 O 3 * 5H 2 O 6.25g 碘化鉀 KI 3% Ammonium molybdate 3M 硫酸 H 2 SO 4 42ml 澱粉指示劑 30ml 漂白劑 2ml 雙氧水分析藥品 10ml 四 實驗步驟 A: 製備 0.1M 的硫代硫酸鈉溶液 : 1. 加 6.25g 硫代硫酸鈉到 250mL 量瓶中, 再稀釋到 250mL, 並混合均勻 2. 以 0.015M 之溴酸鉀標定硫代硫酸鈉溶液 ( 本實驗之 B 部分 ) B: 標定硫代硫酸鈉溶液的濃度 : 1. 取 25mL 溴酸鉀溶液到 250mL 錐形瓶中 2. 加 2-3g 碘化鉀, 搖晃到固體溶解 3. 加 5mL 3M 硫酸後, 以 0.1N 硫代硫酸鈉溶液滴定到淡黃色 4. 加入 5mL 澱粉指示劑 5. 以 0.1N 硫代硫酸鈉滴定到藍色消失 6. 計算硫代硫酸鈉的濃度 M Na2 S 2 O 3 V Na 2 S 2 O 3 = MKBrQ 3 25(mL) 6 (5-5) 7. 重覆步驟 計算平均值及相對誤差 C: 以碘滴定法測定樣品中雙氧水的含量 1. 依組別領取待測溶液 10mL 到 100ml 量瓶中, 以水稀釋到標線 2. 取 50ml 稀釋液到 250ml 錐形瓶中 3. 加入 8ml 3M 的硫酸 3 g 的碘化鉀及 3 滴 3% ammonium molybdate 溶液 4. 以 0.1N 的硫代硫酸鈉滴定到淡黃色 37

38 5. 加 5ml 澱粉指示劑 6. 以 0.1N 的硫代硫酸鈉滴定到藍色消失 7. 重覆步驟 2-6 一次, 求平均值 8. 計算待測溶液中, 含雙氧水的含量 (mg/ml),fw=34.0 M (Na2 S 2 O 3 ) V (Na 2 S 2 O 3, ml) = ( mmole of H 2O 2 ) 2 (5-6) (mmole of H 2 O 2 ) (100/50) / 10 = mg/ml (H 2 O 2 ) (5-7) D: 以碘滴定法測定漂白劑樣品中氯的含量 : 1. 取 2ml 的待測液到 250ml 錐形瓶中, 加 75mL 水 2. 加入 8ml 3M 的硫酸 3g 的碘化鉀及 3 滴 3% ammonium molybdate 溶液 3. 以 0.1N 的硫代硫酸鈉滴定到淡黃色 4. 加 5ml 澱粉指示劑 5. 以 0.1N 的硫代硫酸鈉滴定到藍色消失 6. 重覆步驟 1-5 一次, 求平均值 7. 計算待測溶液中, 氯的含量 (mg/ml), Cl: AW=35.45 M (Na2 S 2 O 3 ) V (Na 2 S 2 O 3, ml) = (mmole of Cl 2) 2 (5-8) (mmole of Cl 2 ) / 2(Volume of sample) = mg/ml (Cl 2 ) (5-9) 五 注意事項 1. 本次實驗須加入 KI, 重量不必過於精確, 只需每次加入量差不多為三克即可 2. 本次取藥頻繁, 請同學們互相合作, 勿拖延時間 38

39 八 實驗數據與結果 A: 製備 0.1N 的硫代硫酸鈉溶液 : 硫代硫酸鈉重 硫代硫酸鈉濃度 B: 標定硫代硫酸鈉溶液的濃度 : 溴酸鉀體積硫代硫酸鈉體 (ml) 硫代硫酸鈉濃度平均值相對誤差 1 2 C: 以碘滴定法測定樣品中雙氧水的含量, 未知物編號 稀釋後分析液使用體積 硫代硫酸鈉體積 (ml) 原分析液濃度 (mg/ml) 平均值 相對誤差 1 2 D: 以碘滴定法決定漂白劑樣品中氯的含量 : 分析液體積硫代硫酸鈉體積 (ml) 分析液濃度 (mg/ml) 平均值相對誤差 1 2 九 問題與討論 問題 : 一 在本實驗中, 確認下列物質一莫耳相當於多少當量? A: KBrO 3 B: H 2 O 2 C: NaClO D: Na 2 S 2 O 3 二 實驗中加過量的碘化鉀有何用意? 三 滴定結束後, 分析液放在桌上一段時間後, 又會變回藍黑色, 是何原因? 是否須要以硫代硫酸鈉再滴定到當量點, 以找出正確的滴定結果 討論 : 請針對本次實驗的結果與收穫加以探討 39