Aim: to determine the iron content of a tablet by the visible spectrophotometry using the external standard method

Introduction/theory:

The external standard methodThe external standard method uses standards that are prepared separately from the sample. External standards are used to calibrate instruments and procedures when they are no interference effects from matrix components in the analyte solution. A series of the external standards of known concentration is prepared. In this experiment standard solutions prepared from ammonium ferrous sulphate and thiocyanate are used to form a complex which absorbs light. This complex is read and has maximum absorbance at 490 nm and all absorbance readings are measured at 490 nm.

To produce the coloured complex which absorbs light the ferrous ions that are present in the tablet needs to be converted to the ferric form. Fe3+ ions react with thiocyanate to form a blood-red colored complex which characteristically has a high molar absorptivity:

Fe3+(aq)+SCN- (aq) [FeSCN]2+(aq)

The ferric ion is the analyte and to determine the amount present more easily it is converted to the intensely colored thiocyanate complex. The ferric iron-thiocyanate complex is more stable than what would have been formed by FeS, whichis present in the tablets. The employment of nitric acid ensures that the ferrous ion would be oxidized to the ferric ion, thereby allowing for a more stable complex to be formed with the thiocyanate.

Calibration is accomplished by obtaining the response signal (absorbance, peak height, peak area) as a function of the known analyte concentration. A calibration curve is formed by plotting the data or using a suitable mathematical equation such as the slope-intercept form used in the least square method. The step in this method is the prediction step, where the response signal from the sample is used to find unknown analyte concentration from the calibration curve or best fit equation. The concentration in the original bulk sample is then calculated from the concentration of analyte by applying the appropriate dilution factors from the sample preparation step.When the external standard method is used it is assumed that the same responses will be obtained when the same analyte concentration is present in the standard. Thus the calibration functional relationship between the response and the analyte concentration must apply to the sample as well. Usually, in determination the raw analytical response is corrected by measuring a blank. An ideal blank is identical to the sample but without the analyte. In practice, with complex samples it is too time consuming, or impossible to prepare an ideal blank and a compromise must be made. Most often a real blank is either a solvent blank, containing the same solvent the sample is dissolved in, or a reagent blank containing the same solvent and reagent used in the sample preparation.

Reagents: Concentrated nitric acidConcentrated sulphuric acid 1M sulphuric acid1M ammonium thiocyanateAmmonium ferrous sulphate

Method: Preparation of the Iron Tablet for analysisPlace iron tablet in a 100 ml beaker in a fume hood and use a measuring cylinder to add 5 ml of concentrated nitric acid. Allow the tablets coating to break down and its contents to dissolve. You may help this process by using a stirring rod to carefully crush the tablet and stir the solution. Do not inhale any of the brown fumes that may be evolved. (NB: iron tablets sometimes contain filler materials that may not fully dissolve in acid)Iron tablets usually contain ferrous sulfate, with iron present as Fe2+ ions. Since Fe2+ does not form a coloured complex with thiocyanate. The nitric acid is added to oxidise all the Fe2+ to form Fe3+ ions.After any reaction subsides, add 5 ml concentrated sulphuric acid and transfer the iron solution to a 250 ml volumetric flask, rinsing the beaker with distilled water a few times and transferring the washings to the volumetric flask.Make up to mark with distilled water, stopper the flak and mix well, and then allow any undissolved matter to settle.Use a pipette to transfer 20 ml of iron solution to a 100 ml volumetric flask. Add 10 ml 1M sulphuric acid and make up to the mark with distilled water. This diluted solution will be used for spectrophotometric analysis.Preparation of standardsUsing the given salt (ammonium ferrous sulphate) weigh out sufficient mass into a 100 ml beaker to make 100 ml of a solution containing 1000ppm Fe. Place the beaker in the fume hood, add 5ml concentrated nitric acid and stir until all the salt is dissolved. If brown fumes are evolved, wait until they subside, then add 4ml concentrated sulphuric acid then quantitatively transfer the solution to a 100 ml volumetric flask, and make up to mark with distilled water. This is solA.Pipette 10 ml of solution A into a 100 ml volumetric flask and dilute to mark with water. This is solution B

External standard methodPrepare a blank and a series of standard by placing 0, 1, 2 ,3 and 4 ml of solution B in separate 50 ml volumetric flasks. Using a measuring cylinder, add 5 ml of 1M sulphuric acid to each flask.Pipette 10 ml and 25 ml of diluted sample solution into separate 50 ml flasks. Add 5 ml of 1M sulphuric acid to each.Note the time and add 5ml of 1M thiocyanate to the first flask and make up to mark with water. After 2 minutes add 5ml of thiocynate to the second flask and continue in this manner until thiocyanate is added to each standard and sample.Fifteen minutes after the first addition of thiocynate, measure the absorbance of each solution at 490 nm in the same 2 minute intervals.

Results:

Table 1: The Absorbance Obtained for the Iron Standards and the Samples Made From the Tablet.SolutionsVolume of Solution BConcentration of Fe in solution B (mg/L)Absorbance at 490nm

Blank0 mL0.000

Standard 11 mL0.052

Standard 22 mL0.073

Standard 33 mL0.491

Standard 44ml0.673

1.0 mL Diluted Sample 0 mL-0.190

1.0 ml Diluted Sample 0 ml-0.159

1.0 ml Diluted Sample 0 ml-0.190

Mass of the Iron Tablet:Mass of beaker = 23.46gMass of beaker + tablet = 24.08g

EXERCISE 1. Complete the table (include ALL calculations)2. Calculate the percentage by mass iron in the tablet (include ALL calculations)3. Calculate mean and standard deviation4. Write a discussion and a conclusion5. (use excel to do this lab)6. (print this lab)