분석화학실험 | Determination of Ca2+ using on Ion-selective Electrode

 

 

 

TIP

 

 

전기화학의 기본 개념을 이해하고 전기화학분석법 중 하나인 Ion-Selective Eletrode (이온선택전극)을 이용해 우유 속에 녹아 있는 Ca²⁺ 이온을 정량 분석한다.

 

 

 

이온 선택 전극(Ion-Selective Electrode)

용액중의 특정이온에 감응하는 전극으로 어떤 이온의 활량과 그 이온이 생기는 전위와의 사이에 네른스트의 식(式)과 관계가 있는 것을 말한다 이 전극에는 막의 성상에 따라서 고체막전극(pH전극, 유리전극, 염화은전극), 액체막전극(K전극, Ca전극, Cl전극), 가스감응전극(암모니아전극, 전극)이 있다. 이 전극에 따라 시료의 전위차와 표준액농도의 전위차를 측정함으로써 시료중인 이온농도를 구할 수 있다. 임상검사에서는 Na, K, Cl, Ca 등의 측정장치에 사용되고 있다.

 

 

실험 방법

1. Measurement Procedure

1) By serial dilution prepare three standard solutions from the 0.1 M standard. The resultant concentrations should be 10^-2, 10^-3, 10^-4M. Add 2.0 ㎖ of ISA to each 100 ㎖ of standard.

 

2) Place the most dilute solution (1.0×10^-4M) in a 150 ㎖ beaker on the magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is on the mV mode, lower the electrode tips into the solution. After the reading has stabilized, record the mV reading.

 

3) Place the mid-range solution (1.0×10^-3) in a 150 ㎖ beaker on the magnetic stirrer and begin stirring. After rinsing the electrodes with distilled water, blot dry, and immerse the electrode tips in the solution. When the reading has stabilized, record the mV value.

 

4) Place the most concentrated solution (1.0×10^-2M) in a 150 ㎖ beaker on the magnetic stirrer and begin stirring. After rinsing the electrodes in distilled water, blot dry and immerse the electrode tips in the solution. When the reading has stabilized, record the mV value.

 

5) Using the graphing program (Sigma Plot, Origin or Excel), plot the mV reading (linear axis) against the concentration (log axis).

 

6) To a clean, dry 150 ㎖ beaker, add 100 ㎖ of sample and 2 ㎖ of ISA. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Rinse the electrode tips with distilled water, blot dry, and lower the electrode tips in the solution. When the reading has stabilized, record the mV value. Using the calibration curve, determine the sample concentration.

 

7) The calibration should be checked every two hours. Assuming no change in ambient temperature, place the electrode tips in the mid-range standard. After the reading has stabilized, compare it to the original reading recorded in Step 3 above. Reading differing by more than 0.5 mV or a change in the ambient temperature will necessitate the repetition of Steps 2-5 above. A new calibration curve should be prepared daily.

 

2. Analysis of free Ca²⁺in Milk

1) Prepare a powered milk sample by weighing accurately about 5 g of powered milk. Quantitatively transfer to a 100 ㎖ beaker and add an initial 25 ㎖ of hot (～40℃) distilled water. Stir on a hotplate with gentle heating and after 15 minutes (no longer), transfer to a 50 ㎖ volumetric flask. Wash the beaker with two 5 ㎖ aliquots of distilled water and transfer these washings to the volumetric flask. Allow the flask to cool, make up to the mark with distilled water, thoroughly mix the solution, and check the homogeneous nature of the resulting solution.

 

2) Add 200 ㎖ of ISA to 10 ㎖ of each milk sample and mix the solution thoroughly. Repeat the measurement on a fresh sample for each of the different type of milk. Measure the potential. Use your calibration graph to determine the concentration of free calcium ion in the milk samples. Express your answer in ㎎/L. Note : milk samples need to be at room temperature before ISE measurement.

 

3) Compare the concentration of free calcium ion with the value obtained for total calcium. Hence, calculate the percentage of free calcium ion in the milk samples and comment on the results.

 

 

 

[분석화학실험]Determination of Ca2+ using on Ion-selective Electrode 레포트