Experiment 23 Determination of the iron content with Spectrophotometry Purposes 1. Learn to operate 722 Spectrophotometer. 2. Master to determine the iron content by use of sulfosalicylic acid method.

In this experiment, we use Fe 3+ ions to react with sulfosalicylic acid and produce brown [Fe (Ssal) 2 ] - ions under a condition of HAc-NaAc buffer solution (pH=5). Determine the absorbance with λ=460nm. Principles The Lambert-Beer law provides the mathematical correlation between absorbance and concentration. It is usually stated as

The relationship between A and c serves as the basis for the quantitative analysis of a great many substances.

Standard curve · · · · · AxAx · ········· ··········· cxcx λ=460nm c 0 c 1 c 2 c 3 c 4 c 5 A 0 A 1 A 2 A 3 A 4 A 5 Standard Curve The graph should be a straight line · · · · ·

Procedure 1.Prepare standard solutions and a sample solution Prepare a series of standard solutions of different accurately known concentration. Get 6 volumetric flasks (10mL) and use the measuring pipets to transfer solutions into the volumetric flasks (according to the table). Prepare a sample solution of unknown concentration.

Preparation of Fe 3+ standard solutions and a sample solution NO Fe 3+ standard solution/mL unknown 0.2mol/LHNO 3 /mL sulfosalicylic acid solution /mL Buffer (pH=4.7)/mL Add distilled water to mark /mL Final c(Fe 3+ ) c x Absorbances (A)

2. Determine of absorbances (A) According to the usage of 722-type spectrophotometer, use λ=460nm and use blank solution (1#) as reference solution to determine absorbance of each standard solution and a sample solution. Record the results.

3. Draw a calibration curve Draw a working curve. The abscissa is the concentration of Fe 3 ＋ of standard solutions, the ordinate is the absorbance of the standard solution.

4. Determine the [Fe 3+ ] of sample solution According to the determined absorbance of the sample solution, We can look up the concentration of Fe 3+ in sample from the working curve drew by yourself. Calculate the concentration of Fe 3+ in original undiluted sample. [Fe 3+ ] ( original undiluted sample) =[Fe 3+ ] ( diluted sample) ×10