Department of Chemistry University of Kentucky EXPERIMENT 2 Gravimetric Analysis of a Soluble Chloride SAFETY AND LABORATORY TECHNIQUE NOTE Throughout this experiment, avoid getting silver nitrate solution on your hands (or any other part of your body that you slop it on). If you do, rinse it off immediately. If you don't and then expose your body to light, the skin will turn black and might peel off in a couple of days. Essentially this is due to the photographic process; the black material coloration due to tiny silver metal particles: AgCl + h Ag + Cl. [When very finely powdered, all metals appear black.] Don't worry, you won't die or lose the limb. This is rather more evidence of sloppy technique than it is of a health hazard. However, if you are at all concerned, seek medical advice and/or treatment. UNKNOWN Submit a clean, dry, labeled glass weighing bottle to the instructor so that your solid unknown chloride sample can be issued. Your name (not initials), section number, and your locker number should be written legibly on this flask with a fine-point permanent marker pen. The bottle must be dry inside and out. The bottle must be turned in immediately after you have checked into the laboratory, because you will have to dry the unknown for at least 1 hour in a 110o oven and cool it in your desiccator prior to use. Drying overnight or for several days will not harm the unknown. The solid unknown should be a free-flowing powder. If it is not, very carefully crush the lumps with a heavy rod or your spatula prior to putting it into the oven. Place the weighing bottle into a clean, dry, labeled 250-mL beaker with its lid tipped sideways. Cover the beaker with a watch glass and place in a drying oven. TIME REQUIRED FOR COMPLETION Three to four laboratory periods. BACKGROUND Gravimetric analysis is based on the quantitative isolation of the desired constituent - the analyte of interest - from the sample in highly pure form or in some combined form and weighing the isolated constituent. The desired constituent is usually isolated or separated by precipitation. From the weights of sample and precipitate, the percentage of the constituent in the original sample can be calculated. CHE 226 - Analytical Chemistry Laboratory 11 Gravimetric ChlorideDepartment of Chemistry University of Kentucky Precipitation is effected by inorganic or organic precipitating agents. Two common inorganic precipitating agents are silver nitrate, which is used to precipitate halide ions such as chloride, and barium chloride, which is used to precipitate sulfate ion. Potassium, ammonium, rubidium, and cesium ions can be precipitated by sodium tetraphenylborate. In all of these precipitation reactions, the product is a salt because it is formed by reactions between cations and anions. Thus the bonding is ionic or electrovalent. Organic precipitating reagents contain functional groups that combine with inorganic ions to form insoluble salts. The organic reagent may contain groups such as carboxyl or hydroxyl that ionize to form anions that combine with cations to form insoluble salts. In this reaction, the bonding is also electrovalent. Some organic reagents contain nitrogen or oxygen that can combine with metal ions by forming covalent or coordinate bonds. Other organic reagents may contain two or more functional groups that can combine with a single cation to form a ring structure. Such a reagent would be called a bidentate ligand if it contained two functional groups, a tridentate ligand if it contained three, and so forth. The product formed between a cation and some multidentate ligand to form a ring structure is called a chelate. The three major requirements for a good gravimetric analysis are that (a) the reagent will react only with the analyte of interest to form a precipitate, (b) it forms one and only one product with the analyte and (c) that the analyte precipitates quantitatively from solution, that is, >99.99% In aqueous solution silver ion undergoes the following reaction with chloride: Ag+(aq) + Cl-(aq) ↔ AgCl(s) Silver chloride is a relatively insoluble compound with a solubility product Ksp = [Ag+][Cl-] = 1.8x10-10 An excess of silver ion is added so that the chloride concentration at equilibrium will be negligible. If enough silver nitrate solution is not used, the precipitation will be incomplete, resulting in a substantial error and a low value for the % Cl in the sample. PROCEDURE Preparation of Filter Crucibles 1. Clean and dry three porcelain filter crucibles (see note below). Make sure crucibles are marked so they can be distinguished from one another. Use a permanent marker, not a paper or tape label. 2. Dry crucibles in the oven at 100-110 °C for one hour or overnight. The crucibles should be put in a labeled beaker and covered with a watch glass when in the oven. 3. Cool the crucibles in a desiccator for 20 minutes and weigh. CHE 226 - Analytical Chemistry Laboratory 12 Gravimetric ChlorideDepartment of Chemistry University of Kentucky 4. Repeat Steps 2 and 3, this time oven-drying for only 20 minutes. 5. Repeat this procedure until the mass of each crucible agrees to within 0.3 mg between weighings. Note: Cleaning Procedure for Porcelain Filter Crucibles (a) Scrape any large amount of gray or purplish precipitate in the crucible into the waste container for Solid AgCl. (b) Thoroughly wash the inside and outside of the crucible with a test tube brush and a small amount of soapy water. Soak off and remove all paper or tape labels. (c) If there is a gray or white residue in the crucible, add about 100 mL of water to a filter flask to dilute the rinse acid or base when it comes through. Mount the filter crucible in the filter flask, add 1-2 mL of concentrated NH3, cover the crucible and let it sit for a few of minutes, then apply a gentle vacuum to pull the resulting solution through the fritted bottom. Follow by rinsing with large amounts of distilled water from a wash bottle. You may need to repeat this procedure 2 or 3 more times. NOTE: Always break the vacuum first before you turn off the aspirator water. Make sure to cover any container that has acid or base in it with a watch glass! (d) If a dark stain remains, empty the contents of the filter flask in the appropriate waste container and add a few mL of conc. HNO3 to the crucible. Cover the crucible and