Determination of calcium content in eggshell

The determination of calcium content in eggshells is an experiment. Its purpose is to detect the calcium content in eggshells. It is determined based on zinc oxide as the reference standard. It is a common laboratory experimental learning method. For students, it is not only required to master the measurement standards, but also to understand some precautions when using some experimental equipment.

Determination of calcium content in eggshell

1. Experimental purpose:

1. Understand the method of obtaining Ca 2+ from chicken shells.

2. Master the calibration method and operating conditions of EDTA solution and the principle and method of titrating Ca2+.

3. Learn to use atomic absorption spectrophotometer.

2. Experimental Principle:

1. (1) Principle of EDTA calibration: The calibration of EDTA with zinc oxide as the reference substance is carried out in a buffer solution of hexamethylenetetramine at pH 5-6 using xylenol orange as the indicator.

Under this condition, xylenol orange is yellow, and its complex with Zn2+ is purple-red because the complex formed by EDTA and zinc ions is more stable. When zinc ions are titrated with EDTA standard solution to sp, xylenol orange is displaced and the solution changes from purple to yellow, which is the end point.

EDTA titration principle: When pH>12.5, Mg2+ generates Mg(OH)2 precipitate. After masking the magnesium ions with the precipitate, EDTA is used to titrate the calcium ions alone. The calcium indicator appears red with calcium ions and has high sensitivity. When calcium ions are titrated at pH = 12 ~ 13, the end point appears as the indicator itself in blue.

(2) Principle of determination of calcium ions by atomic absorption spectrophotometry: Atomic absorption spectrophotometry is a method in which a hollow cathode lamp of the element to be measured emits light with a characteristic spectrum of a certain intensity and wavelength. When it passes through a flame containing the ground state atomic vapor of the element to be measured, part of the light of the characteristic spectrum line is absorbed, while the unabsorbed light is irradiated onto the photodetector through a monochromator. By detecting the intensity of the absorbed light of the characteristic spectrum line, the content of the element to be measured in the sample can be obtained.

First, prepare a series of standard solutions and use an atomic absorption spectrophotometer to measure the absorbance (A) of each standard solution to obtain a standard curve of A~c. Then measure the absorbance of the test solution and obtain the content of the component to be measured through the A~c curve. )

3. Experimental steps

Method 1. EDTA titration

1. Dissolution of eggshell: Take an eggshell, wash it, remove the inner membrane, dry it, grind it and weigh its mass, then put it in a small beaker, add 10ml6mol/L HCl, and heat it over low heat to dissolve it.

Then transfer the solution in the small beaker into a 250ml volumetric flask, dilute to volume and shake well.

2. (1) Calibration of EDTA standard solution: a. Preparation of 0.1 mol/L EDTA standard solution: Weigh 1.9 g of EDTA disodium salt and dissolve it in 150-200 ml of warm deionized water. After cooling, add it to the reagent bottle, dilute to 500 ml, and shake well.

b. Preparation of zinc standard solution: Accurately weigh 0.2 g of analytical pure ZnO solid reagent, place it in a 100 ml small beaker, first moisten it with a small amount of deionized water, then add 2 ml of 6 mol/L HCl solution, and gently stir with a glass rod to dissolve it. Transfer the solution quantitatively into a 250 ml volumetric flask, dilute to the mark with deionized water, and shake well. The concentration of the zinc ion standard solution was calculated based on the weight of ZnO.

c. Calibration of EDTA standard solution: Use a pipette to draw 25.00ml of zinc ion standard solution, add 1-2 drops of 0.5% xylenol orange indicator into a 250ml small beaker, add 20% hexamethylenetetramine solution until the solution turns a stable purple-red, then add 2ml; then use c(EDTA) = 0.01mol/EDTA standard solution to titrate until the solution changes from purple-red to bright yellow, which is the end point, and record the volume of EDTA solution consumed.

Repeat the titration three times according to the above method. The range is required to be less than 0.05 ml. Calculate its accurate concentration based on the volume of EDTA solution consumed during calibration. (2) Ca2+ titration: Use a pipette to transfer 25.00 ml of the solution to be tested into a conical flask, adjust the solution pH to >12.5, shake well, add 5 drops of calcium indicator, and titrate with EDTA standard solution until the solution changes from wine red to pure blue. Repeat the titration 3 times and record the volume of EDTA solution consumed.

Method 2: Determination of calcium content by atomic absorption spectrophotometry: a. Prepare a series of calcium standard solutions. Accurately pipette 2.00, 4.00, 6.00, 8.00, and 10.0 ml of calcium standard solution into 25 ml volumetric flasks respectively, dilute to the mark with deionized water, shake well and set aside. b. Prepare the sample solution. Accurately transfer 5.00 ml of the calcium ion solution in the 250 ml volumetric flask into a 25 ml volumetric flask, dilute to the mark with water, shake well and set aside. c. Use atomic absorption spectrophotometer to measure the calcium content in the sample through the calcium standard working curve