Quote Originally Posted by aquarium
But is the reading really 10? Since you got a bit of change at 9, I'd guess that the actual reading is somewhere between 9 and 10, or in the case of 10ppm/drop, somewhere between 90 and 100ppm. Which is why it seems approximate, to me.
If you want an accurate titration (not really necessary for what we are doing with our pools) then you need to use a calibrated burette so you know the exact amount of titrant that has been added.....but when you are close to endpoint you still add titrant drop by drop from the burette and still go to one more drop than necessary to produce the color change (and use the reading on the burette from the previous drop). When we do titrations with a drop count method we are making an assumption that all the drops will be uniform and contain exactly the same measured amount of titrant. This will never happend but we can get a precision that can be within 10 ppm or even less for some tests. More than precise enough for our purposes.