Four methods are given. The student should choose which two are to be performed. It is recommended that the student consider doing the Gravimetric Method of Determination (p.20) and one Volumetric Method of Determination. Each of the four methods has pitfalls and strong points.
Preparation of the Standard Silver Nitrate Solution
Dissolve the sample of silver nitrate supplied in 1000 ml. of distilled water in a brown glass bottle. The sample will contain approximately 17 grams and will yield 1000 ml. of 0.1 M AgNO3.
CAUTION: Your brown bottle cap must be plastic and not aluminum-lined.
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GRAVIMETRIC DETERMINATION OF CHLORIDE
NOTE: Do 4 samples at the same time! YOU WILL NEED FOUR FILTER CRUCIBLES THAT HAVE REACHED CONSTANT WEIGHT BEFORE YOU CAN FILTER YOUR PRECIPITATE.
TO DISSOLVE THE UNKNOWN, USE 0.1 M HNO3 AS LISTED!
IF YOU USE CONC. HNO3, YOU MUST START OVER. YOU BLEW IT!
Dry the Unknown for 2 hours at 160o C. Weigh 0.5 g samples of unknown chloride into 600 mL beakers. Dissolve in 100 mL 0.1 M nitric acid. Precipitate silver chloride using 0.1 M silver nitrate (90 mL but avoid large excess). Keep the precipitate of silver chloride out of bright light, and in the dark until you filter the AgCl. Digest the precipitate 1 hour over low heat (or overnight). Add 1 drop silver nitrate solution to check for complete precipitation (if precipitation is not complete add 50 mL more AgNO3 and digest 30 more minutes). Filter through a constant weight filter crucible using good decantation procedures and rubber policemen. Wash several times with 0.1 M nitric acid until all excess silver nitrate is removed (check wash solution with dilute HCl if desired). Place filtrate in waste Ag bottle on back bench. Pre-dry precipitate at 105O C for 2 hours to prevent occlusion of water, then dry to constant weight at 160o C, (purple coloration may occur at this high temperature but this should have a negligible effect). Cool in desiccator and weigh silver chloride. The precipitate must also achieve constant weight.
TO GET AN ITEM TO CONSTANT WEIGHT: Constant weight is defined as that state when a substance will not change its weight on heating. This is achieved by heating the object. Weighing the cooled object and then heating again, cooling again, and weighing a second time. If the two weights agree within 0.5 mg., the object is said to be at constant weight. If not then the heating, cooling and weighing process is continued until two successive weighings do agree within 0.5 mg. Use the final weight in the calculations.
CALCULATION: Calculate percent chloride in sample.
CAUTION: Remember to treat all chemicals with respect.
VOLUMETRIC DETERMINATION OF CHLORIDE
THREE METHODS ARE GIVEN
OBJECTIVES: which apply to all three volumetric
1. The student will use the method of aliquots in this determination.
2. The student will treat the known and unknown by the same method.
3. The student will use the technique of precipitation titration.
4. The student will use the transfer pipet.
Method of Aliquots
The volumetric methods of determining chloride often use the method of aliquots. The method of aliquots is one in which one sample is weighed and then parts of this dissolved sample are taken. This involves the use of the volumetric flask and the transfer pipet. The chief disadvantage of using the method of aliquots is that weighing errors cannot be detected. You either do well or blow all 4 samples.
In each method the student will standardize the silver nitrate solution. The molarities vary enough from method to method to require standardization by each method. All three methods require a standard chloride solution and an unknown chloride solution.
Preparation of Standard Chloride Solution: Weigh out 40 millimoles of dry sodium chloride (approx. 2.3 grams) into a 250 mL. volumetric flask. Dilute to the mark. Shake well. Store the solution in the stoppered volumetric flask. (If two volumetric method are to be done, plan to use 20 mL aliquots in each volumetric method.)
Preparation of Unknown Chloride Solution: Weigh out approximately 2.3 grams of the unknown chloride (dry) into a 250 mL volumetric flask. Dilute to the mark. Shake well. Store the solution in the stoppered volumetric flask. (If two volumetric method are to be done, plan to use 20 mL aliquots in each volumetric method.)
Recovery of Silver: In all methods place your titrated silver in the silver chloride waste jar and return unused silver nitrate to the instructor.
ADSORPTION INDICATOR METHOD
Specific Objective: In the adsorption indicator method the student will use the concepts of buffering and peptization of precipitates.
Treat known and unknown alike. Transfer 25 mL of the chloride solution to a 250 mL Erlenmeyer flask with a transfer pipet. Add 100 ml. of distilled water. Add 1 spatula of dextrin. Add 5 mL of pH 5 acetate buffer(use dispensing bottle). Add 3 drops of dichlorofluorescein indicator solution. Titrate with silver nitrate to the first permanent appearance of the pink color of the indicator. Calculate the molarity of the silver nitrate for the known. Calculate % Cl for the unknown after calculating the molarity of the silver nitrate from the known.
Specific Objectives: In the Mohr method, the end point will be determined by the formation of a secondary colored precipitate.
Treat known and unknown alike. Transfer 25 mL of the chloride solution to a 250 mL Erlenmeyer with a transfer pipet. Add 100 ml. of distilled water. Add 10 drops of 1% potassium chromate (the indicator). Titrate with 0.1 N silver nitrate until the first dirty yellow or muddy red. Be consistent with the color you take as the end point. The known and the unknown are done the same way. Calculate % Cl for the unknown after calculating the molarity of the silver nitrate from the known.
Preparation of Potassium Thiocyanate Solution:
Dissolve approximately 9.8 grams of KSCN in about a liter of water. Mix well. Store in a conditioned glass bottle.
Standardization of KCNS:
Accurately measure 25-30 mL of silver nitrate into a 250 ml. Erlenmeyer flask. Add 100 mL of water. Add 2 mL of con. nitric acid. Add 2 mL of ferric indicator. Titrate with the KCNS until a permanent reddish brown persists. Calculate the ratio of AgNO3 to 1 mL of the KCNS. If the molarity of the silver nitrate is known then the actual normality of the KCNS can be calculated.
Treat known and unknown alike. Transfer 25 mL of the chloride solution to a 250 mL Erlenmeyer with a transfer pipet. Add 100 mL of distilled water. Add an excess of silver nitrate (use 5 to 10 mL more than used by the other method, or use 50 mL, if you are starting with this method). Add 2 mL of concentrated nitric acid. Use a clean 250 mL filter flask. Using suction filtration, filter the resulting mixture. Wash the precipitate with 3 10-mL washes of 0.1 M HNO3. You need to titrate the filtrate and washings. Plan to titrate in the filter flask. Add 2 mL of ferric indicator. Titrate the excess silver with standard thiocyanate until the color of the color of the ferricthiocyanate complex is permanent for 1 minute.
CALCULATION: Calculate percent chloride in sample for the unknown and the molarity of the silver nitrate from the known titrations. Include a spread sheet for the calculations. Remember to calculate each titration separately and then to average the three, four or five runs. After finishing remember to compare the results from the two or more different methods.
CAUTION: Remember to treat all chemicals with respect.
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