Determination of chloride in a soluble chloride Fajans method. PDF

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Experiment 6 Determination of chloride in a soluble chloride Fajans method. Abstract Fajans method is based on the mechanism of adsorption. For experiment 6 concentration of chloride ion is being determined. In this method silver nitrate is used as a titrant. This method was discovered by Fajan due to lack of suitable indicators when trying to determine the percentage of chloride ions in precipitation reactions. In this method dichlorofluorescein is used as the indicator and the principal reaction of Fajan’s method is silver nitrate reacting with a sample consisting of chloride ions for titration. A precipitate of sodium chloride is produced. This is the reaction at the equivalence point And at the end point no sample containing the Cl ^ion would remain in the flask. The indicator used exists as an anion in the solution and when the excess positively charged silver ion is produced the anion from the indicator is attracted. This changes the color of the solution indicating the endpoint of the reaction. The percentage of chloride ion can then be calculated using stoichiometry. The average percentage of chloride ions in the sample was calculated as 79.53%.

Introduction Fajan’s method is important due to lack of suitable indicators when it comes to precipitation reactions and titrations. Also when it comes to the titration of dilute solutions, the process is very slow and tedious. Furthermore, coprecipitation effects make it difficult for the composition of the precipitate to be known. Despite other methods that have been established the absorption mechanism in Fajans method allows for visual end point detection due to color change. At the equivalence point of this reaction no sample containing the chloride would remain in the flask. In

the next step if one extra drop of silver nitrate is added, that is to say an excess of silver nitrate is added, it would not be reacting with the sample containing the chloride ion because no chloride ion is free for reaction. In that state silver nitrate will convert in the form of silver ion and nitrate ion would form the secondary layer. The primary layer is formed by the adsorption of silver ion on the surface of silver chloride precipitate .Dichlorofluorescein, which is a weak acid will react with silver ion which would produce a pink color which would indicate the endpoint of the reaction and this is when the positive ion of silver reacts with the negative ion of fluorescence on the surface of the silver chloride precipitate. The pink color would appear on the surface of the silver chloride precipitate. Dextrin suspension, which is basically a solution of carbohydrates, was used in this experiment to prevent coagulation of precipitate at the endpoint. The goal was to find the percentage of chloride ions in the sample after calculating the mean of 3 titrations performed.

Method Part 1 of the experiment was not performed in the lab and part two was the determination of chloride ions in the sample .

The first step was to weigh about 0.25 to 0.3 grams of the sample containing the chloride ion and then transfer each sample to three individual Erlenmeyer flasks. The weight of sample 1 was 0.259, sample 2 was 0.253 and for sample 3 it was 0.264 grams. Each sample was transferred to a 250 ml Erlenmeyer flask.

50 mL of distilled water was added to each flask and the sample was dissolved.

To each flask 10 mL of 1% dextrin suspension was also added to prevent coagulation of precipitate at the endpoint

The burette was filled with a standardized solution of 0.1 molarity silver nitrate.

Titration for each sample was performed to the endpoint when the color changed from a pale yellow to a light pink . For sample 1 the titrant volume was 35.7 ml, sample 2 was 34.1 ml and for sample 3 it was 35.7 ml.

Results and calculations Table 1: Titrant Volume of Each Sample

sample 1

sample 2

sample 3

weight of sample

0.259g

0.253 g

0.264 g

initial buret reading

0

0.4

0

final buret reading

35.7 ml

34.5 ml

35.7 ml

titrant volume

35.7 ml

34.1 ml

35.7 ml

number of moles of titrant (silver nitrate): Molarity * Titrant Volume (in L) Sample 1= 3.57*10^-3mol Sample 2= 3.41*10^-3mol Sample 3=3.57*10^-3mol

number of moles of chloride ions: According to stoichiometry, the ratio is 1:1 so for each sample and so, the mol of chloride ion =

mol of silver nitrate

percentage of chloride ions in the sample: Molar mass of NaCl (sample)= 58.44g/mol (Using molar mass of NaCl as commonly used for Fajan’s method)

mass of Chloride ions in sample 1:( 3.57 * 10 ^ -3 )*(58.44)= 0.209g mass of chloride ions in Sample 2: (3.41 x 10 ^ -3)* (58.44)= 0.199 g mass of chloride ions in Sample 3: ( 3.57 x 10 ^ -3)* (58.44 )= 0.209g

Therefore percentage of chloride ions in Sample 1 = (0.209/ 0.259)* 100%= 80.7 percent percentage of chloride ions in Sample 2= (0.199 / 0.253)* 100%=78.7 percent percentage of fluoride ions in Sample 3= (0.209/ 0.264)* 100%=79.2 percent Average percentage of chloride ions in the sample: 79.53%

Conclusion The point of taking a mean of samples is to get as close to the true value as possible. It is noticed that the titrant volume for all three samples are quite close to each other so it can be said that titration was carried out with minimum error at least in terms of human error. During the titration it was difficult to find the endpoint given that it was hard to observe exactly how pink the endpoint had to be given that a single drop could make the difference between a light pink to an intense pink color. Overall the percentage yield of chloride in the sample was above 50% and it can be considered a high yield. The calculations may have error in the sense that it was assumed that the sample was of sodium chloride although, not specifically mentioned in the

experiment. Sodium chloride molar mass was used because that is a sample mostly used when it comes to Fajans method. It was a simple titration to carry out and it was not a slow process. For sample 1 the endpoint detected was a more intense color then for sample 2 and 3 which might have affected the final titrant volume so when it comes to this titration an improvement would be to understand better what the exact color is for the endpoint that we should look out for.

Reference CHEM 3251 Experiment 6 Determination of chloride in a soluble chloride: Fajans method. Lab manual 2021, Mechref ,Yehia pp 1-5...