Introduction:

Gravimetric analysis, which by statement is based on the measurement of mass, can be generalized into two kinds; volatilization and precipitation. The quantitative determination of a substance through the precipitation process of gravimetric analysis comprises isolation of an ion in solution via a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of acknowledged composition, and lastly weighing the precipitate and finding out its mass by difference. From the mass and known composition of the precipitate, the amount of the original ion can be found out.

Safety and laboratory method note:

Throughout this experiment, avoid getting silver nitrate solution on your hands (or any other part of your body which you splatter it on). If you do, rinse it off instantly. If you do not and then expose your body to light, the skin will turn black and may peel off in a couple of days.

Necessarily this is due to the photographic method; the black material coloration due to tiny silver metal particles:

AgCl + hv → Ag + Cl

(Whenever very finely powdered, all the metals appear black).

This is instead more proof of sloppy method than it is of a health hazard. Though, if you are at all concerned, search for medical advice and/or treatment.

The Unknown:

Submit a clean, dry, labeled glass weighing bottle to the instructor in such a way that your solid unknown chloride sample can be issued. Your name and matriculation number must be written legibly on this flask by a fine-point permanent marker pen. The bottle should be dry within and out. The bottle should be turned in instantly after you have checked to the laboratory, as you will have to dry the unknown for at least 1 hour in an oven set to a temperature of 110^o C and cool it in your desicator prior to use. Drying overnight or for some days will not harm the unknown. The solid unknown must be a free-flowing powder. If it is not, very cautiously crush the lumps by a heavy rod or your spatula prior to putting it into the oven. Put the weighing bottle into a clean, dry, labeled 250 ml beaker by its lid tipped sideways. Cover the beaker by a watch glass and put it in a drying oven.

Time requisite for completion: Three to four laboratory periods.

Theory:

Precipitation is effected via organic or inorganic precipitating agents. Two common inorganic precipitating agents are silver nitrate that is employed to precipitate halide ions like chloride and barium chloride that is employed to precipitate sulphate ion. Potassium, ammonium, rubidium, and cesium ions can be precipitated via sodium tetraphenylborate. In all of such precipitation reactions, the products are salts as they are made by reactions between the cations and anions. Therefore, the bonding is ionic or electrovalent.

Organic precipitating reagents include functional groups which combine by inorganic ions to form the insoluble salts. The organic reagent might have groups like carboxyl or hydroxyl which ionize to form anions which combine by cations to form insoluble salts. In this reaction, the bonding is also electrovalent. Some organic reagents have nitrogen or oxygen which can combine by metal ions by forming covalent or coordinate bonds. Other organic reagents might have two or more functional groups which can join by a single cation to form a ring structure. Such a reagent would be termed as a bidentate ligand if it contained two functional groups, a tridentate ligand if it contained three, and so on. The product formed between a cation and a few multidentate ligands to form a ring structure is termed as a chelate.

The three main requirements for a good gravimetric analysis are that (i) the reagent will react only by the analyte of interest to make a precipitate, (ii) it forms one and only one product by the analyte and (iii) that the analyte precipitates quantitatively from solution, which is, > 99.99%. In aqueous solution silver ion experiences the given reaction with chloride:

Ag⁺ (aq) + Cl^- (aq) ↔ AgCl (s)

Silver chloride is a relatively insoluble compound by a solubility product.

K_SP = [Ag⁺ Cl^-] = 1.8 x 10^-10

The surplus of silver ion is added in such a way that the chloride concentration at equilibrium will be negligible. If adequate silver nitrate solution is not employed, the precipitation will be incomplete, resultant in a substantial error and a low value for the percentage Cl in the sample.

Procedure:

Preparation of Filter Crucibles:

1) Clean and dry the three porcelain filter crucibles. Ensure crucibles are marked in such a way that they can be differentiated from one another. Make use of a permanent marker, not a paper or tape label.

2) Dry the crucibles in oven at 100 to 110 °C for one hour or overnight. The crucibles must be place in a labeled beaker and covered by a watch glass whenever in the oven.

3) Cool the crucibles in a desicator for around 20 minutes and weigh.

4) Repeat the steps (2) and (3), this time oven-drying for just 20 minutes.

5) Repeat this process till the mass of each and every crucible agrees to within 0.3 mg between the weighing.

=> Note: Cleaning method for Porcelain Filter Crucibles

a) Scrape any huge amount of gray or purplish precipitate in the crucible to the waste vessel for Solid AgCl.

b) Methodically wash the inside and outside of the crucible by a test tube brush and a small amount of soapy water. Soak off and eliminate all paper or tape labels.

c) If there is a white or gray residue in the crucible, add around 100 mL of water to a filter flask to dilute the rinse acid or base whenever it comes through. Mount the filter crucible in the filter flask, add 1-2 mL of concentrated NH₃, let it sit for some minutes and then apply a gentle vacuum to pull the resultant solution via the fritted bottom. Follow by rinsing by large amounts of distilled water from the wash bottle. You might need to repeat this process 2 or 3 more times.

=> NOTE: For all time break the vacuum first before you turn off the aspirator water.

d) If a dark stain remains, vacant the contents of the filter flask in the suitable waste container and add a few mL of concentrated HNO₃ to the crucible. Let it sit for some minutes, apply a gentle vacuum as before, and clean the crucible thoroughly by distilled water.

e) If there is a reddish stain in the crucible, empty it and add some drops of concentrated HCl. This must remove it nicely. Proceed as by HNO₃ above. If the crucible doesn't filter fast (1 to 2 drops per second) after this cleaning, repeat the whole procedure. If it still doesn't drain fast under vacuum, consult by your instructor.

f) After you are totally done. Rinse the vacuum flask and crucible holder carefully with distilled water. Turn on the vacuum and squirt some drops of distilled water down the rubber hose. Put the vacuum flask, crucible holder and rubber hose back in their proper drawers.

=> Hazardous Waste Note: The NH₃ waste (fundamental) container is housed in a separate hood from the HNO₃ and the HCl waste (that is, acidic) containers.

Preparation of the Chloride Unknown Samples:

1. Dry the sample in an oven at around 110 °C for 1 to 2 hours or overnight.

2) Weigh out precisely, by difference, three parts of the dried sample of around 0.5 to 0.7 g each to within ± 0.1 mg. NOTE: Never transfer chemicals within the balance.

3) Dissolve each part in a clean, separate, labeled 400-mL beaker, by employing 150 mL of distilled water to which around 1 mL of concentrated nitric acid has been added.

4) Put a watch glass cover on each beaker.

Precipitation of Chloride by Silver Ion:

1) Heat the chloride solutions to gentle simmering on the hotplate and keep each one hot till AgCl is completely precipitated. Precipitate the chloride from one unknown solution at a time.

2) Go to the hood having the stock silver nitrate solution and carefully pour around 80 mL of the solution into a 100 ml graduated cylinder. [Each and every unknown will need around 50 to 70 ml to precipitate completely. Don't fill any container larger than 100 ml by the stock silver nitrate solution. Silver nitrate is extremely expensive please don't waste it.

3) Stirring constantly by a glass stirring rod (do not make use of a magnetic stirrer and stir bar), add the silver nitrate from the graduated cylinder in around increments to your first unknown solution till the precipitation of the silver chloride is complete. To check for complete precipitation, silver nitrate should be added in small quantities and strongly stirred. Allow the precipitate to settle a bit and add some more silver nitrate solution (no stirring yet). If the solution becomes cloudy, keep on adding. If the solution remains clear, then add a total of around 5% more silver nitrate solution than you have added to that point.

4) Now put the beaker (covered) in your locker for at least one hour to 'digest' before filtering. This reduces the exposure of AgCl to light and subsequent decomposition.

5) Carry out this precipitation and digestion for your other unknown solutions, one at a time. If you have employed a graduated cylinder to monitor the volume of silver nitrate solution employed for your first unknown solution, then you can then estimate regarding how much you require adding to the remaining samples. Simply ratio the volume to be employed based on the masses of the two unknown samples which you weighed out.

Filtration and Final Weighing:

This method must be done separately for each and every sample in turn.

1) After the solution has digested for the minimum of 1 hour, filter the supernatant liquid via a labeled, weighed filter crucible by suction, keeping most of the precipitate in the beaker.

NOTE: For all time break the suction on the flask before turning off the water flow on the aspirator.

2) Test the filtrate in the suction flask for full precipitation via again adding some drops of silver nitrate solution. If your filtrate remains clear, dispose of the filtrate in the suitable waste container.

3) Wash the precipitate by three 25-mL parts of 0.01 M nitric acid (that is, 2 drops of concentrated HNO₃ per 100 mL of water) by using your washing bottle. [A standard-sized washing bottle will hold around 500 ml.] The washings are poured via the filter crucible and the precipitate is left in the beaker. If any of the precipitate has dried on the sides of beaker or the glass stirring rod, scrape them down by a rubber policeman and rinse by small amounts of the wash solution throughout this process to make sure that 100% of the precipitate will be filtered off to the filter crucible.

4) Stir the bulk of precipitate up in a small volume of 0.01 M HNO₃ and quantitatively transfer the precipitate to the crucible.

5) After filtering, put the crucibles in a big beaker covered by a watch glass and dry at for around 2 hours. You can leave the crucibles overnight, if you return the next day and place them in your desicator.

6) Cool in a desicator and weigh.

7) Return them to the oven for around 20 minutes. Then cool in the desicator for around 20 minutes and reweigh. Repeat this step till the mass of a crucible by the precipitate agrees to in 0.4 mg.

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