Hi,
Before bottling some red Zin, I tested the SO2. The titret shows the
SO2 is 100 ppm. I know that titrets are not really accurate for red
wine but in the past I have used them and not had readings like this
before. I have not tasted it yet but there is no sulphur oder.

PH is between 3 and 4, maybe 3.5. (I only have some test paper strips)

The test was just after racking wine that had been fined. It was
racked into a just sulfite washed carboy. The solution was 1 tsp for
about 2 gallons of water and I drained it but did not rinse it.

Could that be the cause of the high reading?

Note that I did not add any sulfite in the process and thus why I am
unclear why the reading is so high unless some wines have higher
concentrations than others.
The alcohol measures on my viometer at 15% but I do not know how
accruate those little things are.