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This experiment was done to determine the amount of chloride ions present in a known Sodium Chloride sample and an unknown sample by Argentometric method. Silver nitrate solution of 0.2 M was added slowly to the aqueous solution of 0.50 g test sample while continuously stirring and the chloride was precipitated as silver chloride. This precipitate was filtered, dried and weighed. Then the average percentage of chloride present in the known NaCl and the unknown sample was determined to be 65.40% and 24.98% respectively.
Introduction of gravimetric determination of chloride
The use of silver nitrate to form the precipitate of silver chloride and gravimetrically determine the amount of chloride present in the sample is one of the most common methods employed in lab. Any inorganic compound containing halogen can be made into aqueous solution and treated by silver nitrate to obtain the precipitates of silver halide. The reaction involved is as follows:
Ag+ + X− (aq) → AgX (precipitate)
where X− = Cl−, Br−, or I−.
In this lab, chloride salt was used as test sample. The salts (both known and unknown) were treated with silver nitrate to obtain white precipitates of silver chloride. The silver chloride was initially formed as colloidal solution. After subsequent heating, nucleation process was dominated by particle growth and the precipitates separated out as coagulated mass.
During this procedure, the pH of the solution is kept slightly acidic by addition of Nitric acid. The acidic pH eliminates the interference by anions (like carbonate and sulphides) of weak acid (www.wikipedia.org). Otherwise, these anions can from sparingly soluble silver salt in neutral pH. Similarly, the silver nitrate is also added in a little excess to enhance the formation and diminish the solubility of silver chloride (Le Chatelier’s Principle). Moreover, the combined action of moderately high pH and slightly excess silver nitrate promotes the coagulation of the silver chloride precipitate by formation of dielectric layer (Skoog, 317-319).
During the experiment, if the precipitate of silver chloride is exposed to light, it photo-decomposes to give elemental silver and chlorine gas. The elemental silver (violet color seen in precipitate) combines with other chloride ion to from silver chloride. However, due to the chloride that escaped as gas, the product is lost and that gives low yield (www.cooper.edu). Although, in practice, the amount of silver chloride nitrate that decomposed is negligible, it is better to avoid exposure to direct light. The photodecomposition formula is as follows:
2 AgCl (s) → 2 Ag (s) + Cl2 (g)
3 C12 (aq) + 3 H2O (l) + 5 Ag+ (aq) → 5 AgCl (s) + C1O3-(aq) + 6 H+(aq)
Materials and Methods:
Three 400 mL Beakers
Three fritted funnels
Sodium Chloride (0.50 g)
Unknown sample (0.50 g)
Silver nitrate solution (0.2 M)
Nitric acid (concentrated and 6 M)
Ammonium Hydroxide or Ammonia in water (6 M)
Three 400 mL beakers and three fritted funnels were cleaned, dried and labeled properly.
By using the analytical balance 0.50 g of sodium chloride (NaCl) salt was weighed and dissolved in 100 mL of distilled water in each beaker.
Slowly, with continuous stirring, 0.2 M of silver nitrate was added to each beaker until the coagulation of silver chloride was observed.
Then, 3 to 6 mL of 6 M of nitric acid was added to each beaker.
The mixtures in the beakers were heated to digest the solution for ten minutes.
Small volume of silver nitrate was added periodically while heating the beakers until the solution stopped becoming cloudy upon the addition of silver nitrate, confirming that the reaction was complete.
The precipitate of each beaker was filtered through the pre-weighed fritted funnel. The precipitates were washed up to eight times with distilled water mixed with 0.5% of 6 M Nitric acid. (1 mL concentrated HNO3 in 200 mL distilled water).
The funnels along with their contents were wrapped in the aluminum foil and placed in the oven at 120oC for ninety minutes.
The funnels were taken out and placed in desiccators to cool to room temperature.
The mass of the fritted funnels and their respective contents were determined by using analytical balance and subtracting the weight of fritted funnel from that of fritted funnel and its contents combined.
All the above procedures were repeated fro the unknown sample.
Cleaning fritted funnel
Concentrated nitric acid was allowed to stand in each funnel for five minutes.
Using a vacuum the acid was drawn out and disposed in proper waste container.
Each crucible was rinsed with three portion of water. Then, 5 mL of 6 M ammonium hydroxide was added, and after five minutes it was drawn out through the filter.
Each crucible was rinsed with six to eight portion distilled water.
The crucibles were placed in oven at 110oC for about ninety minutes to dry them to constant weight.
The funnels were taken out and cooled in the desiccators.
The crucibles were weighed, three times each, to ensure they give constant mass. (If not, heating and drying would’ve been repeated.)
Sample weight = 0.050 g
Gravimetric factor (GF)= Cl− formula weight/AgCl formula weight = 35.45/143.3214 = 0.2473
Percentage of Chloride = Wight of AgCl precipitate weighed (g) * G.F. * 100
Sample weight (g)
Discussion of gravimetric determination of chloride:
The percentage of Chloride in the known sodium chloride salt and the unknown sample was determined to be 65.40% and 24.977% respectively via gravimetric method. In theory, the percentage of chloride in sodium chloride salt is 60.66%. The variation in the percentage of chloride in the known sample may have been resulted by impurity in the sample or due to the random error during experiment.
This experiment is the classical example of the precipitation reaction where the formation of dielectric layer of silver chloride stabilized by nitrate ions takes place. Upon the addition of silver nitrate in the aqueous solution of the salt, the formation of the silver chloride occurs as a colloid. Slightly excess silver nitrate is added and also nitric acid is added to raise the concentration of the nitrate ion in the solution for two reasons. The first reason is to prevent the solubility of the silver chloride precipitates that are formed as precipitate. The second reason is to enhance particle growth and the formation of the dielectric layer (Skoog, 317-319).
The precipitates formed are formed as colloids. Since colloids are stable molecules, it is necessary to heat, stir and add more electrolyte (nitrate) to obtain particle growth by the process of adsorption. The colloid obtained is made of silver chloride as primary adsorption layer and nitrate ions make up the counter-ion layer to stabilize the system. Continued heating ruptures the counter ion layer and promotes stabilization by the nitrate ions produced by the addition of slightly excess silver nitrate and nitric acid (Skoog, 317-319). However, addition of too much silver nitrate is avoided to prevent the co-precipitation of the silver nitrate. Nitric acid is used in the reaction mixture as well as to wash the precipitates formed. Washing with nitric acid ensures that the precipitates will be preserved because washing with water may dissolve it (Lab Handout).
The precipitate thus obtained was weighed to determine the percentage of chloride present in the starting sample. This process can be employed quantitatively to determine the purity of the sample. In many cases, precipitation reaction of silver halides is qualitatively used to identify whether the inorganic compound consists of halides. In this lab however, the silver nitrate was used to quantitatively determine the percentage of chloride present in the samples. The known sample of Sodium chloride seems to have impurity because it gave more percentage of chloride than theoretically expected. But, the identity of unknown sample was unknown and jus by analyzing the percentage of chloride ion nothing can be said about its identity.
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