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Buffers, and pH, and Diffusion oh my The pH of a solution is the measure of the concentration of charged Hydrogen ions in that given solution. A solution with a pH lower than seven is considered to be acidic. A solution with a higher pH is a base. It is very important for organisms to maintain a stable pH. Biological molecules such as proteins function only at a certain pH level and any changes in pH can result in them not functioning properly. To maintain these constant pH levels, buffer solutions are used. A buffer solution can resist change to small additions of acids or base’s.
A good buffer will have components that act like a base, and components that act like an acid. Diffusion is random movement of molecules or other particles, resulting in even distribution of particles when no barriers are present (David Sadava, 2011). Diffusion always occurs from areas of high concentration to areas of low concentration, and leads to uniform distribution of solutes. Molecular weight plays a large role in diffusion. The first experiment we did was to simply test the pH of white grape juice, 7-Up, white wine, seltzer water, milk of magnesia, sodium bicarbonate, and Maalox.
We hypothesized that the white grape juice, 7-Up, white wine, and seltzer water would be acidic while the others would be basic. This is because I know that the more acidic 7-Up and white grape juice have been known to rot and corrode teeth. The second experiment we conducted was to test for chloride ions and starch. We did this by adding drops of silver nitrate and iodine to Sodium Chloride, starch and distilled water. Our hypothesis for this experiment was that the silver nitrate would react in some way with the sodium chloride and the starch, but not with the water.
The third experiment we conducted was to test the buffer zone of a buffer solution. We used a 2 pH buffer solution and steadily added hydrochloric acid, then sodium hydroxide as a base. Our hypothesis was that we would reach the buffering potential with the hydrochloric acid but the sodium hydroxide would be buffered out and wouldn’t show significant change in pH. The next experiment we did was to test the diffusion of ions through an agar solution. We filled four holes in the agar with silver nitrate, sodium chloride, potassium bromide, and potassium ferricyanide.
We hypothesized that the potassium ferricyanide would diffuse the furthest away from its origin, then silver nitrate, then potassium bromide, and the sodium chloride would diffuse the least. This is because the molecular weight of potassium ferricyanide is much great than that of the other molecules and smaller, lighter molecules diffuse faster than larger, heavier ones. The last experiment we did was to test the diffusion of starch and sodium chloride solution through a dialysis bag that is submerged in water.
Due to the massive size of starch molecules I hypothesize that when we add iodine to the water no reaction will occur. However, when we add silver nitrate to the water with the sodium chloride dialysis bag we will see the same reaction we got in the test tube. Methods For the first experiment we poured white grape juice, 7-Up, white wine, seltzer water, milk of magnesia, sodium bicarbonate and Maalox into separate test tubes. We then took our digital pH reader and tested each substance for its pH making sure that we wiped off the electrode thoroughly before we tested each solution. We then recorded our results in a graph.
For the second experiment we filled six test tubes: two with 10ml starch (using pipets), two with 10 ml sodium chloride, and two with 10 ml of distilled water. We then added three drops of iodine to each solution and observed its results. We then added silver nitrate to each solution and observed its results. In the third experiment we put 40 ml of a 2 pH buffer solution into a beaker and out it on a mixing platform. We then added 1 ml of HCL then recorded the pH. 1 ml of HCL was added and retested a total of 10 times. Each pH measurement was recorded in a graph. We then did the same procedure, but with 1ml of NaOH solution.
The next experiment was to test diffusion in agar solution. A petri dish with a layer of solidified agar had four holes punched in it using a No. 5 cork borer. Three holes were punched in a triangle shape with the fourth hole directly in the middle. There should be 15 mm between each outside hole and the middle hole. The three outside holes were filled with one drop each of potassium bromide, potassium Terri cyanide, and sodium chloride. The middle hole was filled with a drop of silver nitrate. It is very important to make sure that none of the holes overflow. After each has been filled allow to sit for an hour and observe the results.
In the final experiment we filled a dialysis bag with starch solution and tied off both ends of the bag so that it is water tight. We then filled a separate bag with sodium chloride and submerge both dialysis bags in two beakers of distilled water. We allowed the bags to sit in the water for 10 minutes. We then put silver nitrate into the water that held the dialysis bag filled with sodium chloride and recorded any changes in the water. We then added iodine to the water that held the dialysis bag of starch and observed any changes in the water. RESULTS Table I. pH values of beverages and medicines BeveragespHMedicinespH
White Grape Juice3. 13Milk of Magnesia (Mg(OH2) )8. 2 7- Up3. 31Sodium bicarbonate (NaHCO3)8. 29 White Wine3. 4Maalox8. 15 Seltzer Water4. 09 As my hypothesis predicted, the beverages, on the right, were more acidic and the medicines were more basic. Fig 1 Buffering Capacity The sodium hydroxide reached the buffering potential at 4 mL. this was contary to my hypothesis. Fig 2 Just for comparison purposes we added NaOH to distilled water and recorded the pH every mL of NaOH added. Discussion In the first experiment, my hypothesis was correct. The beverages turned out to be more acidic while the medicines were more basic.
As we all know soda and too much fruit juice can be bad for your teeth as they can actually corrode the enamel, this was the basis of my hypothesis. Acids release H+ ions into solutions, which is how sodas are carbonated (David Sadava, 2011). I knew that the grape juice, 7-Up, the wine and the seltzer water have acidic characteristics, so by default, they must be acidic. As for the medicines, I knew that bases accept H+ ions, which means that when medicine is added to the stomach acid it drives the reactions going on in the stomach the other way. The second experiment was an introductory for the dialysis bag experiment.
When silver nitrate was added to sodium chloride the solution reacted and became white and milky. It created sodium nitrate and silver chloride. Silver chloride precipitates into a solid, which is what is left at the bottom of the test tube this is because AgCl is one of the few transition metal chlorides that is unreactive toward water. The iodine turned the starch a dark purple which indicates that amylose is present. Amylose in starch is responsible for the formation of the deep blue color in the presence of iodine, which is not soluble in water. This means that to test for starch in a solution you just need to add a drop of iodine.
This is expressed more in the dialysis experiment. This also proved my hypothesis correct. My hypothesis for the buffer experiment was incorrect. I said that the HCL would reach its buffer zone before the NaOH would, however this was not the case. After 4 mL of NaOH was added the buffer zone was reached and the pH level shot up. This, to me, was unexpected; I believed that the acid would alter the pH more than the base would simply because it is an “acid”. This naive hypothesis was not fully researched and was a mental error on my part. We did use a pH of only 2 however, which significantly lowers the buffer zone.
If we would have used a buffer with a pH of 12 the results might be flipped. As the HCL would have exceeded the buffer zone instead of the NaOH. The agar experiment was based on the molecular weight of the solutions in each hole. The potassium Terri cyanide has a molecular weight of 329. 25 which would make it diffuse quicker. The agar gel is still about 98% water, but the gel forms a crisscrossing network of carbohydrate that the potassium Terri cyanide ions must work through. Since the ions must move up, down, forward, and even backwards to diffuse, it takes much longer for them to move away from the source (Jacobs, 1998).
The other molecules are lighter and it’s more difficult for them to get through the agar. The silver nitrate in the middle hole diffuses out and when it hits the diffusion of the other solutions it creates a triangular precipitate around the silver nitrate. My hypothesis was correct; it was based solely off the molecular weight of each solution. Note however that we did not have sufficient time to fully observe the results of the experiment. I do not know how far each solution diffused though the agar. The dialysis bags are a semi permeable membrane and only allow solutions that can fit through it.
Diffusion always goes from areas of high concentration to areas of low concentration. Starch molecules are complex carbohydrates and are very large, which means they did not diffuse through the dialysis bag. This proves my hypothesis, which was based on just how large starch molecules are. The dialysis bags prevent molecules with molecular weights that are greater than about 10,000. Starch is in the 100,000’s. (Jacobs, 1998) Although when iodine was added to the water it diffused into the starch bag turning the starch the same dark purple we observed in the test tubes.
With the dialysis bag of sodium chloride however, the sodium chloride was able to diffuse out of the bag into the water. So when silver nitrate was added to the water it reacted with the diffused starch and created the same effect we saw in the test tube. Bibliography David Sadava, D. M. (2011). Life: The Study of Biology Ninth Edition. Saunderland: Planet Friendly Publishing. Jacobs, C. W. (1998). Diffusion and Osmosis. Retrieved from Henry Ford Community College: http://sciweb. hfcc. net/Biology/jacobs/bio131/diffusion/Diff&Os. html