My memory needs refreshment.
1. Do the endpoint and equivalence point of a titration always coincide or are there cases where they don't? And by coincide do they mean occur at the exact same point or just roughly close to each other?
2. When selecting an appropriate indicator, do we select one that corresponds to a ph range anywhere on the vertical region of the graph or does it have to be at the midpoint of the vertical region?
Also, 3. How do you write the reaction between NaOH and oxygen?
1. The end point and the equivalence point are different things. The equivalence point is the point at which reaction has actually completed. The end point, however, is the visual cue indicating that the equivalence point is achieved (e.g. indicator changing colour). Thus, the end point always occurs after the equivalence point; the end point is the result of adding, say, a little more acid than at equivalence, which thus alters the pH such that the indicator will change colour. Generally, the difference between the end point and equivalence point is very small because of the large change in pH generated by the addition of a single drop of whatever you're titrating.
2. Anywhere in the vertical region is fine. The equivalence point point does occur in the middle, but - again - the idea is that in a single drop you will most likely span the entire range of that vertical region, so it doesn't matter where on that region coincides with the indicator range.
3. Hmm NaOH and oxygen? Is this in relation to why it cannot be a primary standard? If so, you may rather be looking for its deliquescence (absorption of moisture) or its absorption of CO2 (as opposed to oxygen).
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