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PRECIPITATION TITRATION BY THE METHODS OF VOLHARD AND MOHR
1Johan Nicolas Bermudez: 20092150011; 1 Cristian Gonzalez: 20092150024;
1 Fabian Barreto: 20102150007
1Group :08
2 Álvaro Jiménez
1,2 Universidad Distrital Francisco José de Caldas
1Estudiantes Análisis Químico Inorgánico, 2 Profesor

Bogotá, D. C., 09 de Octubre de 2012

ABSTRACT: The following report presents theresults of the determination of the concentration of HNO3 through the process of precipitation by the methods Volhard and Mohr with a known concentration of 0,99193N Ag2CrO4
KEY WORDS: Precipitation titration, chloride ion, Mohr, Volhard, titration curve, the final volume, stoichiometric point, the potential of chloride, chloride concentration.

INTRODUCTION

Much of the precipitation reactions,complications and formation of weak ionic meet the requirements necessary to be applicable to titration, although in many cases there is no right for a visual indicator to show its endpoint. Potentiometric methods metric, amperometric are generally suitable for all reviews of this type. The objective of this lab is to determine the concentration of chlorides and chlorides potential through threemain methods are: Mohr and Volhard.


1. THEORETICAL FRAME

1.1 Chloride Ion: Distributed in nature, mainly as part of the sodium chloride in water, the chloride content can be 250 mg / L and is usually found next to the sodium cation where ions are predominantly calcium and magnesium. In oceanic waters the sodium chloride content is 2.6 %.

Chloride is good for health and preservingthe acid base balance in the blood, contributes to the adsorption ability of potassium and helps the blood to carry carbon dioxide.

1.2 Precipitation: to determine the chloride precipitation titrations are used, that is to say, titrant reacted in this case is the AgNO3 to the analyte which is the NaCl through the following methods:

1.3 Determination of silver

a. Mohr Method: The finalpoint in this method is determined by the first formation of an orange-red precipitate of silver chromate that forms when the precipitation of silver chloride is complete, the indicator is a solution of potassium chromate.

Cl-(ac) +Ag+ (ac) →AgCl2(s) (white precipitate)

CrO4 -2 + 2Ag → Ag2CrO4 (orange-red precipitate)

Silver chloride is less soluble than silver chromate; the lattercan’t be formed permanently in the mixture until precipitation of the chloride ion to form silver chloride has been reduced chloride ion concentration to a very small value. Any portions of the silver chromate is formed temporarily, due to local excess of silver ion as you add the reagent to the mixture, is transformed by reaction with chloride of silver chloride dissolution:

Ag2CrO4 + 2Cl - →CrO4 -2 + 2AgCl

b. Volhard Method:

The reaction is:

Ag+ + SCN - → AgSCN (white precipitate)

When this reaction is complete, the first portion of added CN gives rise to a red color with ferric nitrate solution used as an indicator:

Fe+3 + SCN - → Fe (SCN)-2 (red solution)
Ferric nitrate indicator is used in high concentration to ensure the immediate formation of adetectable amount of red complex by adding the first over after the point thiocyanate reaction stoichiometric precipitate.

2. CHEMICAL REACTIONS

Mohr Method

AgCl(s) → Ag+ + Cl-

Ag2CrO4 (s) → 2 Ag+ + CrO4-2

Volhard Method
SCN- + Ag+ → AgSCN (s)
SCN- + Fe+3 → Fe (SCN)+2
3. RESULTS

The results of the concentration of AgNO3 and KCN that we found are inthe next data table:

Method | NaCL | [NaCl] | AgNO3 | [AgNO3] | SCN |
Mohr | 5ml | 0,9919N | 10 mL | 0,99193N | |
Volhard | 5ml | 0,9919N | 1.3 mL | 0,99193N | 5.6 ml |
Data Table 1: Concentration of AgNO3 and KCN

Results of the test, sample problem:
Method | Positive | Negative |
Mohr (For Cl) | X | |
Volhard (for Ag) | | X |

4. ANALYSIS

In the method of Mohr,...