I'm testing with the standard indicator solution, however instead of using 25% phosphoric acid I'm using 2% w/v sulfumic acid crystals in tap water. At any rate when I run my test for 15 minutes with the pump on, the sulphite levels are lower than expected. Once I titrate the indicator solution back to neutral from magenta, I run the test further with what's left of the wine sample and leave it for another 15 minutes and its still turning the indicator solution magenta. It seems that all the SO2 is not liberated in fifteen minutes. Can I just run the test longer, maybe for 1/2 hour? Or will this somehow make the test inaccurate.