Question

What is the purpose of the acid added in the beginning of the experiment?

CHM3120C Analytical Chemistry Lab Lab 7. Spectrophotometric Determination of Iron in Mole's Salt with e-Phenanthroline Required Reagents and Supplies 50 mL of 0.0005 -0.0006 M of Moher's solution unknown. 10 of Hydroxylamine hydrochloride (NH2OH, HCT, 500 ml of phenantholine solution contains 05 of-phenanthroine monohydrate). 83 g of sodium acetate (NOAC3H20) in 500 ml of water (for 1.2 M of phl 5 buffer som minimum 3 of 99.9% Mole's safem ammonium sulfate hexahydrate FeSOLINISOL6H20)20 mL of conc. sulfuric acid, Ocean Optics, Kime and parafilm Introduction: The bidentate ligando-phenantholinol. 10-phenantholiereacts quantitatively with iron (ll) to form a very stable, intensely red colored complexion. The complex with iron (III) is pale blue and much weaker color. In order to the alm o fron in a sample by spectrophotometric means, it is therefore essential that all be present in the ferrous state (1 ) Fell) + 3 phent → Felphen 323 (Red.ms-508m) Spectrophotometric measurements will be made prepares a set of standard solutions of su an Ocean Opties spectrometer. Each group s taining neactly known amount of II. Procedure: Start from step . a 10% (w/v) lilydroxylamine hydrochloride sufficient for about 1 0 measurements: Dissolve 10 g of NH2OH, HCl in about 100 mL of distilled water b. 0-phenantholine solution (sufficient for NO to measurements: Dissolve 0.5 g of - phenanthraline monohydrate in about 500 Lower Warm sigly if necessary. Each milliliter is sufficient for no more than about 0.09 mg of Fe. Must be fresh, so do not prepare more than needed; it darkens on standing and must then be discarded. G. Sodium acetate buffer, 1.2 M ufficient for so to measurements: Dissolve 166 of NOA 3H20 in 1 L of distilled water d. Standard iron solution, 0.01 mg/mL. Please follow the procedures below. . d.1. Preparation of stock standard: Obtain a sample of Mah'sult s ammonium sulfate hexahydrate. [FeSO4,(NH4)2SO4.6H2O: MW=392.16g melj DO NOT DRY Mehr's salt! Using the weigh-by-difference technique, accurately weigh out about 0.07 -0,08 g of the salt into a Il volumetric flask. Dissolve i 50 ml of distilled water that contains 1-2 ml of concentrated sulfuric acid". dilute to the mark with distilled water to the mark wal with parafilm, and mix well . Do not add too much H2SO4, or the buffering capacity of the sodium aetate, to be added later, may be exceeded. . d-2. Preparation of working standards, blank and unknown solutions. Prepare all of the following solutions at least 30 min before spectr o metric measurements are performed e. The six working standards are prepared by volumetrically delivering the indicated volumes of the dilute stock solution 100 ml. volumetric flasking abune. Again, make sure that the burette is precised with the solution being diluted. Be sure to calculate the exact ron concentration of each working standard Treat each flask as follows (e may refer the table below. using a graduated cylinder, add 1 ml. of 10% hydroxylamine hydrochloride to reduce and maintain allion in the state), 10 ml of 0.02 Mo-phenantholine, and 10 ml of acetate buffer (The latter solution is added because full color development occurs only at about pH 5.) Dilute to the mark with deionized water and mix well.

Answer 1

Ans : To prepare the standard solution of Mohr's salt, it is necessary to add dilute H​​​​​​2​​SO​​​4 to prevent the oxidation of Fe​​​​​​​​2+ ions of Mohr's salt to Fe​​​​​3+ ​​​​​​ions.

If you do not add dil. H​​​​​​2​​​SO​​​4​​ , you will observe the Mohr salt will become yellowish in colour and can't be used for further titration.