I read three different labs and tried a couple of them and then took different components from each to make my own titration lab today. Titration is the process of adding a known acid to an unknown base until its neutralized to figure out the strength of the acid, or adding a known base to an unknown acid until its neutralized to determine the strength of the base. The process is pretty straight forward, we started with 10 drops of a base solution in a watch glass, added 2-3 drops of indicator (cabbage indicator left over from last week), then count the number of drops of 0.1M HCl required to neutralize (indicator turns purple again) the base. M1V1 = M2V2, where M1 is the molarity of the base and V1 is the volume (in number of drops of the base – 10 in this lab) and M2 is the molarity of the acid (0.1M in this case) and V2 is the volume of acid (in drops) that we added to the base. I made up three different bases, 0.1 M NaOH, 0.24 M NaOH and 0.06M NaOH. The 0.1 M NaOH was labeled and before we started the lab I asked the students how many drops of 0.1M hydrochloric acid did they think they would need to neutralize 10 drops of the 0.1M sodium hydroxide base and they all realized it would need an equal number of drops since they have the same concentration. I then asked them, what if the NaOH is 0.2M, would you need more or less drops of HCl? They answered more drops of acid (twice as many to be exact) since its more concentrated.
Because these are very strong bases and acids students wore goggles, gloves and aprons. The only tricky part is making sure they clean the pipette before switching to the acid. Its important to use the same pipette since you are comparing drops to drops and different pipettes might give slightly different drop sizes. Students also needed to keep the pipette vertical to keep drop size uniform.
It was a little difficult to determine when to stop dropping acid into the base with the cabbage indicator so we also tried using bromthymol blue indicator (photos above) which is a beautiful deep blue for bases and turns yellow for acids and its a very striking color change so it was a bit easier to tell when the base was neutralized. If I do this lab again I would just have the students use the bromthymol blue. It was also useful to have one student agitating the foil pan to mix the solutions in the watch glass. Some students also used toothpicks to stir between drops.
One thing that had been bothering me about the lab last week is WHY does the indicator change color? So I did some digging and found this web page (www.madsci.org) that had a very nice description of the physics/chemistry going on with cabbage indicator. Bascially the molecule in the indicator responsible for the color is anthocyanin and it likes to add hydroxide ions onto itself, so if a base is added to the indicator, hydroxide ions are released by the base and attach themselves to the indicator molecules (anthocyanin) making them longer. The stronger the base, the more OH-, the bigger the indicator molecules get and that changes the color (wavelength) of light reflected by the solution. The reverse happens for acids with the anthocyanin giving up OH- to react with H+, so the indicator molecule gets smaller, again changing the color.
This was a nice short lab so we spent some time doing some extra problems, calculating molarity and pH. I wanted to do some of these in class because some of the students might not have used logarithms yet and I wanted to show them how to do it on their calculators.
I had the students watch Crash Course Chemistry on pH (on last week’s post) and the one on Buffers before class.
I found this Ted Ed video while looking for videos for the middle school class.