Determination of the Formula of a Complex Ion

EXPERIMENT 8

Determination of the Formula of a Complex Ion

Introduction

In this experiment the formula of the complex ion formed in solution from copper (II) and EDTA is determined using spectrophotometric measurements. The methods employed are widely used in inorganic and analytical chemistry.

Consider the case where a cation M2 + reacts with a ligand X to form a colored complex MXn2 + :

M2+ + nX MXn2+

An example is the reaction of copper (II) ions with molecules of ammonia to form species such as Cu(NH3)42+. Let us assume that M2+ and X are colorless and that the complex MXn2+ is colored. The problem is to evaluate n to determine the formula of the complex.

The method of continuous variations involves measuring the absorbances of a series of solutions of varying composition at a wavelength at which the complex shows its maximum absorptivity. The total number of moles of M2+ and X is kept constant while the mole fractions of the reactants are varied. The absorbances are plotted against mole fraction and the value of n can be determined from the mole fraction at which maximum absorbance occurs. In Fig. 8.1 an illustration is given in which the peak occurs at a mole fraction of 0.75. This indicates that the formula of the complex is MX32+. The graph is curved near the point of maximum absorbance as shown in Figure 8.1. The departure from linearity is greater the less stable the complex. The straight-line portions of the graph are extrapolated until they cross to determine the mole fraction at maximal absorbance.

The mole-ratio method employs a series of solutions in which the concentration of one reactant, says M2+, is constant and that of the other, say X, is varied. The absorbances are plotted against the ratio of the moles of ligand to metal, as shown in Fig. 8.2. The absorbance increases to a maximal value at a mole ratio of 3: 1 and then levels off. Again, the graph may be curved near the peak and the straight-line portions may be extrapolated until they intersect.

The mole-ratio method and the method of continuous variations are most readily applied when only a single complex is formed between the reacting species. Copper (II) and EDTA react to form a deep blue 1: 1 complex which shows a flat maximum in absorbance in the range 730 to 755 nm. The reaction in a chloroacetate buffer (chloroacetic acid = HX) can be written

CuX2 + H2Y2- CuY2- + 2HX

blue green deep blue

The copper is present as the chloroacetate complex in the buffer and the chloroacetate ligand is displaced by EDTA, since the latter forms a much more stable complex with copper ion.

Procedure

Preparation of Solutions

(a) Chloroacetate buffer. Dissolve 18.2g of chloroacetic acid in 100 mL of water and adjust the PH to about 2 using 5 M NaOH. Then dilute the solution to 250 mL.

(b) Cu2+, 0.0200 M. Weigh accurately a sample of clean copper wire or foil of about 0.32g (0.3175g for exactly 0.0200 M), and place it in a 250mL beaker. Add 5 mL of 1:1 nitric acid and dissolve the copper by warming the solution on a steam bath or over a low flame. Add 25 mL of water and boil the solution for about 1 min. Then add 5 mL of urea solution ( 1g in 20 mL of water) and continue boiling for another minute to remove oxides of nitrogen. Cool the solution in tap water and neutralize the acid with 1:3 ammonia, adding the ammonia carefully until a pale blue precipitate of copper (II) hydroxide is obtained. Now add glacial acetic acid drop by drop until the precipitate redissolves. Transfer the solution to a 250mL volumetric flask, dilute to the mark, and mix by inverting the flask.

(c) EDTA, 0.0200M. Dissolve 1.861g of disodium dihydrogen EDTA dihydrate in about 100mL of water in a 250mL beaker. Transfer the solution to a 250mL volumetric flask and dilute to the mark. Mix the solution thoroughly.

Method of Continuous Variations. Using a 10mL measuring pipet, prepare the following mixtures from the preceding solutions.

Solution

Stir the solutions and measure the absorbance of each at 730 nm using distilled water as the reference. For the measurements at 730 nm the infrared-sensitive phototube and a red filter should be used.

Since the absorbance of the chloroacetate complex is appreciable (as shown by solution 1), the absorbances measured must be corrected to give the absorbance due to the copper-EDTA complex alone. This correction should be made for solutions 1, 2, and 3 as follows:

Ac = Am - × A1

where Am is the absorbance measured, A1 is the absorbance of solution 1, and Ac is the corrected absorbance. No correction need be made for solutions in which sufficient EDTA has been added to convert all the copper to the complex.

Plot the corrected absorbances vs. mole fraction EDTA. From the peak in the graph determines the formula of the complex and report this to the instructor.

Mole-Ratio Method. Prepare the following mixtures form the prepared solutions.

Solution

Buffer, mL

Stir the solutions and measure the absorbance of each at 730 nm using distilled water as the reference. The absorbances of solutions 1, 2, 3, and 4 should be corrected as follows:

Ac = Am - × A1

where Am is the absorbance measured, A1 is the absorbance of solution 1, and Ac is the corrected absorbance. No correction need be made for the other solutions.

Plot the corrected absorbances against the ratio of the volume of EDTA to the volume of Cu2+ solution. Determine the formula of the complex and report this to the instructor.

實 驗 八

錯 化 合 物 組 成 的 決 定

一、目的

學習分光光度計的原理及操作，並利用吸收度測量法決定錯離子之組成。

二、原理

當電磁波照射物質時，組成物質的分子會吸收電磁波的能量，如果能量夠大，會將分子的電子從低能階提升到高能階，造成電磁波吸收現象，因不同分子之電子能階不同，吸收波長也不同，此吸收現象遵循Beer’s Law A=bc，暨吸收值和濃度成正比，吸收現象及相關定義如下：

Po→ →P

T = P / Po

A = -log T

= b c

錯離子( complex ion ) 一般為過渡金屬離子與配位子( ligand ) 鍵結而成，如Cu(NH3)42+其通式為：

M2+ + nL → MLn2+

若形成之錯離子具有吸光能力，則可藉由測量其吸收度來決定其化學式。

利用吸收度之量測來決定錯離子的化學式有三種方法：

1. 連續變率法 ( Method of continuous variations )
2. 莫耳比率法 ( Mole-ratio method )
3. 斜率比率法 ( Slope-ratio method )

本實驗僅介紹連續變率法和莫耳比率法來測定Cu2+及EDTA所形成之錯化合物的組成。

1. 連續變率法 ( Method of continuous variations )
2. 假設金屬離子濃度( CM )和配位基濃度( CX )的總濃度C不變，即

CM + CX = C。

1. 在錯離子的最大吸收波長 ( max )下，如果金屬離子與配位子皆不吸收(即吸光係數為零)。
2. 以吸收度對混合溶液中配位子的莫耳分率X作圖。

所得圖形如下：

由外插之三角形兩邊交點所對應之橫座標莫耳分率x ，即可得錯離子的化學式MLn2+中的n值。

因

若 ，

則

1. 莫耳比率法 ( Mole-ratio method )
2. 本法與連續變率法不同處為金屬離子濃度固定不變，配位基濃度依序增加。
3. 錯化合物分子式MLn2+中之n 值決定是以錯化合物吸收度對CL/CM（配位子對金屬離子濃度比值）作圖，所得圖形如下：外插兩直線的交點所對應之橫座標即為n值。

三、實驗步驟：

1. Method of Continuous Variations

藥品與器材

藥品配置

1. 以10 mL吸量管製備下列混合物於50 mL錐形瓶中，配置完成後混合均勻，取8個測光管將測光管洗淨。並且分別用solution 1將預裝此溶液之測光管潤洗兩次，再裝至八分滿。其餘測光管亦須使用solution潤洗兩次。
2. 以JΛsco V-550 分光光譜儀尋找λmax ，操作方法請參考儀器操作手冊，請以Sol’n 4.為樣品，以水做為blank做全光譜掃描，波長範圍800～400nm，以找出錯化合物的λmax。
3. 使用SHIMADZU UV-1201分光光譜儀器操作方法參考儀器操作手冊，以純水為參考溶液(blank)，設定儀器的吸收波長為λmax(上步驟之結果)。測量Sol’n 1-7的吸收值。
4. 因為Cu2+在錯化合物的λmax有吸收，所以Sol’n 1-3的測量值須做修正才能用於作圖，Sol’n 4以後之溶液因已無Cu2+存在，則不需修正。修正值計算公式如下：

Am：測量的吸收值

A1：Sol’n 1的吸收值(即Cu2+的吸收值)

Ac：修正的吸收值

1. 以修正的吸收值對EDTA的莫耳分率作圖，以決定錯化合物的組成。

1. Mole – ratio Method

器材與藥品

藥品配置

1. 以10 mL吸量管製備下列混合物於50 mL錐形瓶中，配置完成後混合均勻，取8個測光管將測光管洗淨。並且分別用solution 1將預裝此溶液之測光管潤洗兩次，再裝至八分滿。其餘測光管亦須使用solution潤洗兩次。
2. 以前一方法的λmax為測量波長，並以水做為blank，在UV -1201 分光光譜儀測量Sol’n 1-7 的吸收值。
3. Sol’n 1-4 的測量值必須做以下的修正：

Am：測量的吸收值

A1：Sol’n 1的吸收值

Ac：修正的吸收值

1. 修正的吸收值對EDTA / Cu2+莫耳數比作圖，決定Complex的組成。

五、實驗數據與結果

I. Method of Continuous Variations

以Sol’ 4掃瞄全光譜之波長的λ max = nm

以Ac對X莫耳分率作圖，圖形用電腦作圖：

Complex 組成：

II. Mole – Ratio Method

以Ac對配位基 / 金屬之莫耳數比作圖，圖形用電腦作圖：

Complex 之組成：

六、問題與討論

問題：

一、 在continuous variation法及 mole-ratio法中，為何前幾瓶的標準溶液之吸收值須做修正，才能正確得到所需結果？

二、 某依金屬Mn+和配位基X-形成錯化合物以continuous variation法分析其組成，如果最高點出現在橫座標為0.8的位置，則錯化物組成為何？

三、 ㄧ般最常見的錯化合物為六配位的錯化合物如FeCl63-，如果以continuous variation法分析其組成，最高點應該落在橫座標什麼位置上？

討論：請針對本次實驗的結果與收穫加以探討