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A little farming town is complaining that a local lake is experiencing an excessive growth of algae. The residents are concerned the algae will deplete the oxygen and kill other marine life in the lake. Your group has been sent to determine if the phosphate from the fertilizer is the cause. You have found through research that the simplest phosphate is orthophosphate. You also found that when it reacts with ammonium molybdate (NH4Mo7O24.4H2O), a yellow acid forms. With this information, you can do a colorimetric analysis to find the concentration of phosphate in the lake. Keep in mind that a phosphate reading of 1.0ppm or less is excellent, one of 1.1-4ppm is good, one of 4.1-9.9 is fair and a reading of 10ppm is poor.

Determine how much phosphate is present by using the following method:

First the spectrophotometer must be standardized. Do so by creating the following standards with a 20ppm (mg/L) phosphate stock solution.

1. Blank

2. 2.0 ppm, final volume 100ml

3. 5.0ppm, final volume 100ml

4. 8.0ppm, final volume 100ml

5. 10.0ppm, final volume 100ml

For each of the standards follow the following procedure:

First measure 25.0ml of the standard into a 50ml volumetric flask or a graduated cylinder. Then determine if the standard is acidic by adding 1 drop of phenolphthalein indicator to the solution. If the solution turns pink, then add 6M HCl drop wise until the solution turns clear. Then dilute the solution to 50ml. Measure out 25ml of this solution and add to 50ml volumetric flask and add 10.0ml vandate-molybdate reagent using a volumetric pipet. Add distilled water carefully to increase the volume to exactly 50ml. Wait at least ten minutes for the yellow color to develop fully. Zero the spectrophotometer with your blank and measure the absorbance of the solution at 420nm. Using the measured absorbance values create an absorbance vs. concentration graph. For the concentration use the original concentration of phosphate before reagents were added.

Next test the water for presence of phosphate. Do so by obtaining a filtered sample of the water. If there is still color in the water, shake 50ml of the water sample with 200mg of activated carbon for 5 minutes and filter. Then analyze the sample by using the same procedure as used for the standards. If the sample has an absorbance greater than that of the most concentrated sample, then dilute the original sample 1:10 and repeat the procedure. Use your results to find the concentration of phosphate in the water.

Pre-Lab Questions:

1. Draw the structural formula for phosphate.

2. How do you calibrate a spectrophotometer?

Post Lab Questions:

1. Draw a standard curve using the recorded data, then find the best fit line, its equation, and r2.

2. Using the standard curve, what was the concentration (ppm) of phosphate in the water sample?