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Salinity is the concentration of salts in water. Because the percent composition of the major ions in seawater is constant, we will measure salinity as molarity of chloride ions. Salinity is an important characteristic of natural waters, and in fact is the key factor in determining what species of plants and animals will be found in a certain area. It is usually expressed in ppt of total dissolved salts, and ranges from 0 ppt in fresh water to 35 ppt in the open ocean. This corresponds to about 0.0M - 0.5M of chloride ions. In this lab you will employ a classic method to determine the salinity of water samples, one first devised by Robert Boyle for the British Navy in the early 1700's. This method is based on two precipitation reactions. The main reaction is a double replacement reaction between silver nitrate and sodium chloride, in which silver chloride precipitates. The idea is to add enough silver nitrate to the saltwater to precipitate out all the chloride ions as silver chloride. In order to know exactly when all the chloride ions have been precipitated we will "spike" the saltwater samples with potassium chromate. Potassium chromate also reacts with silver nitrate, to produce a red-orange silver chromate precipitate. The key is that the silver ions prefer to combine with the chloride ions rather than the chromate ions. Thus,the silver chromate ppt. will not appear until all the chloride ions have reacted. By measuring exactly what volume of silver nitrate is needed to reach the endpoint, where the first hint of an red-orange ppt. appears, we can use a simple proportion to determine the amount of chloride in the sample, and hence its salinity. This method, of quantitatively adding a solution of known concentration to a solution of unknown concentration until an endpoint is reached, is called a TITRATION. |
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Procedure: Part I: Checking Accuracy with a Standard Solution 1. Rinse a clean buret with distilled water three times, and then once with a SMALL amount of 0.014M silver nitrate solution. Then fill the buret with 0.014 M AgNO3 (aq). Run a small amount of this solution through the buret (to eliminate any air bubbles at the tip), and discard this solution. Record the volume of solution in the buret to the nearest + 0.1 mL.. 2. In a small, clean erlenmeyer put exactly 10.0 mL of the stock 0.01M NaCl (aq) solution. Then add 2-3 drops of potassium chromate indicator. 3. Slowly add AgNO3 to the contents of the erlenmeyer, while continually swirling the erlenmeyer. At first you will see a white ppt: this is silver chloride. STOP as soon as the first permanent red-brown appears. (This is silver chromate: it only appears when all the chloride has reacted.) Record the new volume of solution in the buret. 4. Discard the solution in the erlenmeyer, and rinse it out with distilled water. 5. Repeat this titration (steps 2-4) twice more, so that you have 3 trials for the stock solution. Be careful not to let the volume in the buret drop below the 50 mL mark! Part II: Class Unknown 6. Titrate 10.0 mL of the "unknown estuarine water" provided by your instructor with the 0.014M AgNO3 (aq) solution. Use potassium chromate as your indicator. 7. Repeat tfor at least three trials total, or until you are satisfied with your precision. |
Analysis:
You're on your own!
