Qualitative and quantitative analysis of potassium, sodium, calcium and magnesium in tap water

Summary

This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.

Operation method

Qualitative and quantitative analysis of potassium, sodium, calcium and magnesium in tap water

Principle

Standard addition method is commonly known as the standard increment method, is to test the sample is divided into equal amounts of five points of the solution, add the concentration of 0, C0, 2C0, 3C0, 4C0 standard solution and C x, C0 + CX, 2C0 + CX, 3C0 + CX, 4C0 + CX (C0 ≈ CX) diluted to a certain volume, under fixed conditions to determine the absorbance, the addition of the concentration of the element to be measured as the abscissa, corresponding to the absorbance as a vertical coordinate to draw the absorbance-concentration curve, extend the curve and the extension of the horizontal coordinate line intersected at CX, the distance between this point and the origin of the horizontal coordinate, that is, the sample in the element to be measured. Absorbance as the vertical coordinate, absorbance - concentration curve, extend the curve and the extension of the horizontal coordinate line intersected at CX, the distance between this point and the origin of the horizontal coordinate, that is, the concentration of the element to be measured in the sample. The use of the method should be noted: 1 the method is only applicable to absorbance and concentration into a linear region, the standard curve through the origin of the curve. 2 in order to obtain accurate extrapolation results, at least 4 points (including the standard specimen itself is not added), while the first time to add the standard concentration (C0) is best to add the concentration of roughly equivalent to the specimen, and then according to the 2C0, 3C 0, 4C 0, respectively, three copies of the preparation, the last to add the standard solution C0 is too large or too small will increase the concentration of the test solution. C0 is too large or too small will increase the concentration of the test solution (CX) reading relative error or absorbance reading relative error will affect the accuracy of extrapolation.3 standard addition method can only eliminate physical interference and slight chemical interference unrelated to the concentration, but can not eliminate the concentration-related chemical interference, ionization interference, spectral interference, background and other interference. In short, the measurement should be within the linear range, the line is not straight can not be extended, followed by the addition of standards should be appropriate, otherwise the slope of the line is too large or too small are introduced into the error.

Materials and Instruments

TAS-990 Atomic Absorption Spectrophotometer Potassium, Sodium, Calcium, Magnesium Standard Solutions Volumetric flasks Pipettes

Move

1. Qualitative identification of potassium in tap water

(1) In test tube A, take 5ml of 5ug/ml potassium standard solution.

(2) Take a sample of tap water in test tube B.

(3) When the instrument is adjusted, spray the potassium standard solution in test tube A and read the absorbance value A=? Spray wash with deionized water to the background

(4) Spray a sample of tap water in tube B for qualitative identification and a rough estimate of the K content without taking interference into account

2. tap water sodium semi-quantitative analysis

(1) take three test tubes A B C, A tube to take 5ug / ml of standard solution, tube B to take a sample of tap water, tube C to take 1:1 tap water and 5ug / ml of standard mixture, and then the water is mixed with 5ug / ml of standard solution.

(2) Under the same condition of instrument adjustment, measure the three solutions above.

(3) under the condition of no interference or very small interference C value should be 1/2 of the sum of A and B, otherwise the interference is serious, when the interference is not big, its effect can be ignored, with the proportion method to find out the approximate content of sodium in tap water. This method is often used in the standard addition method, preliminary estimation of the content, to prevent the slope of the straight line is too large or too small.

3. Standard curve method for determination of magnesium content in tap water

(1) Prepare a standard series in the linear range according to the standard curve method.

(2) Make a preliminary estimate of the magnesium content in the sample according to the method in 2(3) above. Take a suitable sample of water in a test tube and dilute it accurately to 25 ml with distilled water.

(3) Adjust the instrument according to the conditions of the determination of the standard series, and then determine the magnesium content of the water sample 4 Standard incremental method for the determination of calcium content in tap water

(4) Adjust the instrument according to the measurement conditions, and then estimate the calcium content in the sample according to the method of 2 (3) (2) Preparation of the standard incremental method series

(5) Measurement of absorbance

Note: When the above standard curve method is used to measure an unknown sample, the absorbance should be in the middle of the curve.

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