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					                                     INSTRUCTION

          Determination of chloride and sodium chloride in foodstuff
                              (Mohrs method)'

The chloride content is analysed by direct titration of preparing food sample with silver nitrate
solution with chromate indicator.
This method is to determine the amount of common chloride present in foodstuff ( milk,
butter, soup powder).

Reagents
1. The Carrez reagent :
       Carrez I – potassium ferrocyanide K4Fe(CN)6 x 3H2O, 15% solution (m/V);
       Carrez II - zinc sulphate ZnSO4 x 7H2O, 30% solution (m/V);
2. 0.1N solution of silver nitrate AgNO3;
3. Potassium chromate solution K2CrO4, 10% (m/V);

Preparation of the food sample.
Soup powder.
Weigh 2 g of sample (with precision of 0.01 g) in a 250 ml measuring flask; moisten with
150 ml hot distilled water (about 800 C)and mix. Cool and dilute with water to the mark. Mix
again and filtrate.
Powdered milk
Weigh 20 g of sample (with precision of 0.01 g) in a 250 ml measuring flask; moisten with
150 ml hot distilled water (about 400 C) and mix. When cool dilute with water to the mark and
mix.
Measure 50 ml of prepared milk to measuring flask (250ml). Add 100 ml distilled water, 5 ml
Carrez I reagent and 5 ml Carrez II reagent. Solution mix, dilute distilled water to the mark,
mix again and leave for 15 minutes and filtrate.
Milk
Measure 50 ml of prepared milk to measuring flask (250ml). Add 100 ml distilled water, 5 ml
Carrez I reagent and 5 ml Carrez II reagent. Solution mix, dilute distilled water to the mark,
mix again and leave for 15 minutes and filtrate.

Determination of chloride in sample.

Measure 25 ml of soup filtrate or 100 ml of milk filtrate to conical flask. Add 1 ml potassium
chromate solution and mix. Titrate with 0.1N silver nitrate solution to a final orange colour.
Do a blank determination (25 or 100 ml water + 1 ml 1 ml potassium chromate solution).
Repeat the titration procedure at least 3 times. Individual results shouldn’t vary more than 0,2
ml.

Calculations
Calculate chloride content from the following formulas:

       For soup powder:    Cl = (V1 – Vo) x 0.003546 x 1000 / m [g/100g sample],
       For powdered milk : Cl = (V1 – Vo) x 0.003546 x 1250 / m [g/100g sample]
       For milk:           Cl = (V1 – Vo) x 0.003546 x 250 / 50 [g/100ml sample]
where:
V1 – volume of silver nitrate required to titrate the sample [ml];
V0 – volume of silver nitrate required to titrate the blank determination [ml];
0,003546 – mass of the chloride [g] corresponding with 1 ml 0.1 N silver nitrate solution;
m – mass of sample [g]

Calculate sodium chloride NaCl content in 100 g or 100 ml of product.


REPORT CONTENTS:
  - filled up analysis report;
  - description of the aim of the exercise;
  - shown calculations of individual results;
  - discussion of results and errors;
  - comparison of the experimental data with references.
                              ANALYSIS REPORT

           Determination of chloride and sodium chloride in foodstuff
                                (Mohrs method)'
Name:                                                         Date:




Product:


   1. Determination of chloride and sodium chloride in foodstuff.

A. Blank determination

                     Volume of AgNO3          Average Vo [ml]
                         Vo [ml]




B. Chloride determination

m – mass of     Volume of      Content of Cl        Average content       Content of NaCl
sample [g]       AgNO3      [g /100g or 100 ml]          of Cl          [g /100g or 100 ml]
                 V 1 [ml]                         [g /100g or 100 ml]
Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound
with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the
ocean and of the extracellular fluid of many multicellular organisms. As the main ingredient
in edible salt, it is commonly used as a condiment and food preservative.

Biological importance
Sodium chloride is essential to life on Earth. Most biological tissues and body fluids contain a
varying amount of salt. The concentration of sodium ions in the blood is directly related to the
regulation of safe body-fluid levels. Propagation of nerve impulses by signal transduction is
regulated by sodium ions. (Potassium, a metal closely related to Sodium, is also a major
component in the same bodily systems).
0.9% sodium chloride in water is called a physiological solution because it is isotonic with
blood plasma. It is known medically as normal saline. Physiological solution is the mainstay
of fluid replacement therapy that is widely used in medicine in prevention or treatment of
dehydration, or as an intravenous therapy to prevent hypovolemic shock due to blood loss.
Humans are unusual among primates in secreting large amounts of salt by sweating.

Production and use
Jordanian and Israeli salt evaporation ponds at the south end of the Dead Sea
Modern rock salt mine near Mount Morris, New York
Nowadays, salt is produced by evaporation of seawater or brine from other sources, such as
brine wells and salt lakes, and by mining rock salt, called halite.
While most people are familiar with the many uses of salt in cooking, they might be unaware
that salt is used in a plethora of applications, from manufacturing pulp and paper to setting
dyes in textiles and fabric, to producing soaps and detergents. In most of Canada and the
northern USA, large quantities of rock salt are used to help clear highways of ice during
winter, although "Road Salt" loses its melting ability at temperatures below -15°C to -20°C
(5°F to -4°F). Salt is also the raw material used to produce chlorine which itself is required for
the production of many modern materials including PVC and pesticides.

Biological uses
Many microorganisms cannot live in an overly salty environment: water is drawn out of their
cells by osmosis. For this reason salt is used to preserve some foods, such as smoked bacon or
fish. It has also been used to disinfect wounds. In medieval times salt would be rubbed in to
household surfaces as a cleansing agent.

Additives
The salt sold for consumption today is not pure sodium chloride. In 1911 Magnesium
carbonate was first added to salt to make it flow more freely. In 1924 trace amounts of iodine
in form of sodium iodide, potassium iodide or potassium iodate were first added, creating
iodized salt to reduce the incidence of simple goiter.
Determination of chloride by Mohrs method.

The reaction of AgNO3 with Cl- may be represented as:

                Ag+ + Cl- = AgCl (s)

Suppose that we have a sample containing an unknown quality of chloride ions. The number
of moles (or weight) of chloride might be determined by titration with a silver nitrate solution
of known concentration, C. The known silver nitrate solution is added to the unknown
chloride solution until we have added exactly the same number of moles of AgNO3 to the
mixture as there are moles of chloride present.
But – We will not establish the equivalence point volume of AgNO3 in the titration of
chloride. The equivalence point is a quantity that exist only in theory. In the world we must
satisfy ourselves with the “endpoint” which is an experimental estimate of the equivalence
point. Our objective in a titration analysis is to match the experimental endpoint as closely as
possible with the theoretical equivalent point.
The Mohrs method analysis for chloride employs a colour-change indicator, potassium
chromate K2CrO4, to signal of titration of chloride ions with a “standard” AgNO3 solution – a
AgNO3 solution of knowing concentration. We will assume that the visual endpoint of this
titration is also the equivalence point and use this assumption to calculate of chloride present
in an unknown sample.

The indicator reaction.
Considered the process of adding AgNO3 solution held in a flask and containing chloride ions.
At the start, when only a small amount of AgNO3 has been added, the solution contains some
AgCl precipitate, a very small concentration of Ag+ ions and an excess of Cl- ions (Cl- ions
are just floating around “waiting” for the addition of more AgNO3). When we have added just
enough AgNO3 to reach the equivalence point the solution in the flask contains AgCl and very
small concentration of Ag+. If we continue to add more AgNO3 the Ag+ ion react with CrO4
ions (indicator) from the following reaction :

                2AgNO3 + K2CrO4 = 2KNO3 + Ag2CrO4 (orange precipitate)

that is the endpoint of titration.

				
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posted:11/30/2011
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