.. on this, we calculated the parts per million and the grains per gallon for each water sample. Finally, we took an AA reading for each sample. This gave us absorption values and concentration values for each of the two main metals we were observing; Ca++ and Mg++. We then plotted a graph of Atomic Absorption Standards. These were values given to us by the AA operator. These values helped us to calibrate the machine.
The parts per million that we find will be based on plugging in the reported absorption value into the resulting curve from the graph of these values. The resulting concentration was used as the final value for the hardness for that particular sample. All calculations and conclusions were done based on these final values obtained for the concentration of Ca++ and Mg++. For more detail, refer to full in depth procedure as directed by: Penn State Version of.. Chemtrek August 1996 – July 1997; Stephen Thompson; Prentice Hall; Englewood Cliffs, NJ 07632; 199 RESULTS Molarity x (100g CaCO3 / 1 mole CaCO3 ) x (1000 mg / 1g) = Xmg/1000g = ppm Grains/Gallon = ppm /17.1 Example: (1.6 x 10 -3 moles / 1 Liter) x (100g CaCO3 / 1 mole CaCO3 ) x (1000 mg / 1g) = 160 ppm 160 ppm/17.1 = 9.35 grains/gallon Serial Titration Results Name: # Molarity Parts Per Million Grains Per Gallon Samir Sandesara 1 1.6 x 10 -3 160 9.35 Andy 2 1.6 x 10 -3 160 9.35 Ben 3 1.2 x 10 -3 120 7.01 Tom 4 1.8 x 10 -3 180 10.5 Table #1: This table displays the values obtained by serial EDTA titration of the water samples. Conversion Factors Given by AA operator: Ca++ = 2.5 Mg++ = 4.2 Ca++ x 2.5 = CaCO3 hardness ppm value Mg++ x 100 x 4.5 = Mg CO3 hardness ppm value *NOTE: the Mg++ is x 100 because it was diluted before it was put into the AA.
Example: Ca++: 27.52 x 2.5 = 68.8 ppm 4.02 g/gal Mg++: .251 x 100 x 4.2 = 105.42 6.16 g/gal Atomic Absorption Values Name : # Abs Mg++ Abs Ca++ AA ppm Mg++ AA ppm Ca++ ppm Mg++ ppm Ca++ g/Gal Mg++ g/Gal Ca++ Samir 1 0.2270 0.5923 0.251 27.52 105.42 68.8 6.16 4.02 Andy 2 0.2041 0.5493 0.225 25.10 92.40 62.75 5.40 3.67 Ben 3 0.3633 0.5800 0.401 26.83 168.22 67.07 9.88 3.90 Tom 4 0.2673 0.5589 0.295 25.65 123.90 64.11 7.24 3.75 Table #2: This table displays the values obtained from AA analyzation, and shows the hardness of the water as contributed by each individual element. Absorbency Values Parts Per Million 0.000 0.0 0.125 0.1 0.403 0.5 0.716 1.0 Absorbency Values Parts Per Million 0.0000 0.000 0.0142 0.493 0.0262 0.985 0.0536 1.970 0.2360 9.850 0.4540 19.700 0.9230 49.250 Floor Number Hardness (ppm) 1 174.3 2 159.1 3 235.5 4 188.0 DISCUSSION The final hardness values were obtained by graphing the AA Standards on the previous page and then plugging in the absorption values give by the AA (Table #2). This is the grey line that appears in both graphs. When this line was extended down from the point of intersection, it was able to read the ppm value at that point. The ppm value for both Ca++ and the Mg++ were then summed to attain the final hardness of the water. The other numbers above reveal much about the water in Hamilton Hall. Looking at the final hardness values that were attained, it is clear that the two upper floors had harder water than the lower floors.
However, table #2 shows that the concentration of Ca++ decreased overall as the water climbed higher in the dormitory. What was unexpected was that the concentration of Mg++ actually increased as it climbed higher. As of present, I have no rational scientific explanation for this. The only possible explanation I could possibly think of is perhaps there is something within the plumbing that contains Mg and the further the water travels in it, the more dissolves of the Mg dissolves. Aside from that, there does not seem to be any possible explanation.
What is also interesting is that with the exception of the #3 sample, the hardness values attained from the AA were very similar to those attained by serial EDTA titration. These indicates a low source of error and gives support to my numbers. Even more support is added to the numbers when the ppm values are added up in Table 2. These values, for the most part, also seem to be in a relatively tight “ball park” of the final AA values. Given that the accuracy of serial titrations is 10 ppm, it is extremely safe to say that my numbers are correct.
A brief overview of the numbers seems to show that there is indeed a trend, and the more in-depth look at the numbers shows that they all seem to back each other up. This seems to imply a that most if not all of the results are quite accurate and precise. CONCLUSION Upon completion of this lab, it can be said that the data supports only half of the original hypothesis. Yes, the Ca++ did seem to decrease as the water got further from the source and climbed higher in the dormitories. However, the Mg++ did not. Instead it did quite the opposite and showed a general trend of increasing in concentration as it got further away from the source and higher in the dormitories.
Perhaps a viable explanation could be attained if studies were done on the plumbing inside the building. Perhaps there is a high concentration of magnesium in the solder used to hold the pipes together. Perhaps it is not in the pipes but rather perhaps the people on the upper floors get up later and therefore at the time of collection, the water in the upper floors had been sitting longer than that on the lower floors. In either case,. More investigation would have to be conducted in order determine what caused the unexpected results.
In light of this discrepancy, the overall accuracy of the lab was very good. The numbers all seem to back each other up and correlate very well. As was mentioned in the previous section, the precision and accuracy with which this lab was carried out seems to indicate that there is very little source of error. The only one that was possibly flawed was sample #3. This could have been due to an error in the dilution or any other factor.
Since I personally did not carry out that portion of the experiment, I cannot be sure. However, the other 3 samples provide more than ample ammounts of accurate information. Overall, it seems that the lab was quite well done. The hypothesis would have to be revised and as of this point, without further investigation, it would have to be reformulated to say that only the Ca++ would decrease in concentration whereas the Mg++ would increase. REFERENCES 1) Brown, Theodore L.
et al; Chemistry The central Science; Sixth Edition; Prentice Hall, Englewood Cliffs, NJ; 1994 2) Stephen Thompson; Penn State Version of..Chemtrek; August 1996 – July 1997; Prentice Hall; Englewood Cliffs, NJ; 1990 3) Internet Resource; http://www.kinetico.com/hard.htm.