EXPERIMENT 1

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1 EXPEREMET 3 Complex-Formation Titrations with EDTA Procedure Preparation of Standard 0.0 M EDTA Solution Dry about 4 g of the purified dihydrate a H Y.H O (ote ) at 80 to remove superficial moisture. Cool to room temperature in a desiccator. Weigh (to the nearest milligram) about.0 g into a 50mL volumetric flask (ote ). Use a powder funnel to ensure quantitative transfer; rinse the funnel well with water before removing it from the flask. Add 50 to 00 ml of water (ote 3) and swirl periodically. Dissolution may take 5 min or longer. When the entire solid has dissolved, dilute to the mark with water and mix well (ote 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 otes:. Directions for the purification of the disodium salt are described by W. J. Blaedel and H. T. Knight, Anal. Chem., 954, 6(4), 74.. 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 0.0 M can be prepared and standardized by direct titration against a Mg + 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 50-mL volumetric flask, dilute to the mark with water, and mix thoroughly. Transfer mL aliquots to 50-mL conical flasks; add to ml of ph0 buffer and 3 to 4 drops of Erio T or Calmagite indicator to each. Titrate with 0.0 M EDTA until the color changes from red to pure blue (otes, ). Express the results as percentage of Mg + in the sample. Calculate the purity of magnesium sulfate in the sample. ( MgSO 4 :Fw = 0.37 ) otes: 0

2 . The color change tends to be slow in the vicinity of the end point. Care must be taken to avoid overtitration.. Other alkaline earths, if present, are titrated along with the Mg + ; removal of Ca + and Ba+ can be accomplished with (H 4 ) 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. M (Sufficient for 90 to 00 titrations.) To 3.7 g of a H Y*H O in 50 ml of distilled water, add an equivalent quantity (.46 g) of MgSO 4 *7H O. Add a few drops of phenolphthalein, followed by sufficient 0. M aoh to turn the solution faintly pink. Dilute to about 00 ml with water. The addition of a few drops of Erio T to a portion of this solution buffered to ph 0 should cause development of a dull violet color. Moreover, a single drop of 0.0 M a H Y solution added to the violet solution should cause a color change to blue, and an equal quantity of 0.0 M Mg + should cause a change to red. The composition of the original solution should be adjusted with additional Mg + or H Y - until these criteria are met. The Determination of Calcium by Replace Titration Weight gram sample containing calcium chloride to a clean 50mL volumetric flask, dilute to the mark with water, and mix thoroughly. Transfer 50.0mL aliquots to 50mL conical flasks, add the sample with a few drops of HCl, and boil gently for a few minutes to eliminate CO Cool, add 3 to 4 drops of methyl red, and neutralize with 0. M aoh. Introduce ml of ph 0 buffer, 3 to 4 drops of Erio T or Calmagite, and titrate with standard 0.0 M a H Y to a color change from red to pure blue (ote). Express the results as percentage of Ca + in the sample. Calculate the purity of calcium chloride in the sample. ( CaCl :Fw = 0.99 ) ote: The color change is sluggish if Mg + is absent. In this event, add to ml of 0. M MgY - before starting the titration.

3 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 H C COOH H C H C HOOC C H H 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 Y -,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 : 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 - +HIn - (3-) 紅色藍色置換法 Mn+ + MgY- +In3- MYn-4 + MgIn- (3-) 指示劑為 EBT( Eriochrom Black T ) 其結構如下 :

4 OH OH HO 3 S O 三使用器材及藥品使用器材 : 乾燥器含乾燥劑 50mL 量瓶 50mL 滴定管藥品 : a H Y*H O (EDTA) Mw = 37.5 吸量管 50mL 錐形瓶玻棒 EBT 指示劑 MgSO 4 *7H O Mw = Methyl red 指示劑 CaCl Mw3 = 0.99 PH 0 buffer solution HCl 0.M aoh R.O. H O 四實驗步驟 A: 0.0M EDTA 標準溶液的製備 :. 取約 4g 的 a H Y.H O 於 80 中烘乾, 以除去表面的水, 並在室溫中於乾燥器內冷卻 ( 由助教完成 ). 以秤量瓶精稱.0g, 倒入 50mL 燒杯中加 50-00mL 的水, 攪拌使固體溶解 3. 將以完全溶解的液體倒入 50mL 量瓶中, 乙水稀釋到標線 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) = (.0*0.997)/37.5 * (000/50) (3-3) B: 以直接滴定法測定鎂離子的含量. 精秤 MgSO 4 *7H O 樣品克並以 50 ml 水溶解後, 移入 50mL 量瓶中, 以水稀釋到標線並徹底混合. 以吸量管取 50mL 上述溶液到 50mL 錐形瓶中 3

5 3. 加 ml P H 0 的緩衝溶液, 並加 3-4 滴的 EBT 指示劑 4. 以 0.0M EDTA 溶液滴定到顏色從紅色到正藍色 5. 重覆 -4 步, 並計算平均值 6. 計算鎂離子在樣品中的濃度 (w/w %) 及 MgSO 4 *7H O 的純度 (Mg Aw = 4.305) 溶液中鎂離子的重量 M EDTA V EDTA (ml) w/w % = = 00 % (3-4) 溶液中 MgSO 4 *7H O 的重量 M EDTA V EDTA (ml) 純度 = = 00 % (3-5) C. 以置換法測定鈣離子的含量. 精秤 CaCl 樣品克以 50 ml 水溶解後, 移入 50mL 量瓶中, 以水稀釋到標線並徹底混合. 取 50mL 上述溶液到 50mL 錐形瓶中, 以數滴的鹽酸酸化 3. 緩慢沸騰數分鐘, 以除去二氧化碳 4. 冷卻後, 加入滴 methyl red, 並以 0.M aoh 溶液中和之 ( 從紅色變為黃色 ) 5. 加 ml P H 0 的緩衝溶液及 -ml 0.M Mg/EDTA 溶液及 3-4 滴 EBT 指示劑 6. 以 0.0M a H Y 溶液滴定到從紅色變成純藍色 7. 重覆 -6 步驟, 取平均值 8. 計算鈣離子在樣品中的濃度 (w/w %) 及 CaCl 的純度 (Ca Aw = ) 溶液中鈣離子的重量 M EDTA V EDTA (ml) w/w % = = 00 % (3-6) 溶液中 CaCl 的重量 M EDTA V EDTA (ml) 純度 = = 00 % (3-7) 4

6 五實驗數據與結果 A. 溶液的製備與濃度 : EDTA 重, g (W) EDTA 濃度 (M) MgSO 4 *7H O 重, g (W) MgSO 4 *7H O 濃度 (M) CaCl 重, g (W3) CaCl 濃度 (M3) B. 以直接滴定法決定鎂離子的含量 : 數據紀錄 MgSO 4 *7H O 體積 (ml), V MgSO 4 *7H O 樣品重 (mg), M3*V3*Mw3 滴定前刻度滴定後刻度 EDTA 體積濃度換算含公式鎂含量 (%) 平均值相對誤差 MgSO 4 *7H O 純度平均值相對誤差 5

7 C. 以置換法決定鈣離子的含量 : 數據紀錄 CaCl 體積 (ml), V3 CaCl 樣品重 (mg), M3*V3*Mw3 滴定前刻度滴定後刻度 EDTA 體積濃度換算含公式鈣含量 (%) 平均值相對誤差 CaCl 純度平均值相對誤差六問題與討論問題 : 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 + / 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 6

15分析實驗九水的硬度及牛奶中鈣的定量.doc

15分析實驗九水的硬度及牛奶中鈣的定量.doc 實驗九水的硬度及牛奶中鈣的定量一目的測學習 EDTA 的配製標定及應用於水的硬度及牛奶中鈣含量的檢圖 9-1 滴定裝置與器材 - 1 - 二原理 A. EDTA 配位根 : 許多金屬離子和提供電子對的物質形成配位化合物 (coordination compounds) 或稱為錯合離子 (complex ions) 提供電子對的物質稱為配位根 (ligand), 需具有一對或一對以上的未分享電子對