(1) Whatsoever amount you deposit in your PPF account if it is equal to nisab or you are sahib-e-nisab excluding it then zakah is wajib on the deposited amount. One option to pay its zakah is that you pay its zakah every year from your money and the second option is that when you get the amount then pay the zakah of all the years.
(2) Cash money is in place of silver and there is benefit of poor in keeping the silver into consideration hence in that case the gold should be combined with cash amount and the nisab of silver should be taken into consideration and the zakah should be calculated.
(3) In case no cash or silver is available except gold then if it is equal to nisab then zakah would be wajib and in that case the nisab of gold should be taken into consideration.
(4) In this case silver should be taken into consideration.
(5) If you have paid 2.5 gram gold in zakah then you were left with 97.5 gram gold hence in the next year you will have to pay only the zakah of 97.5 gram gold. And if you paid the zakah of 100 gram gold from your money and the gold remained as it was then the next year you will have to pay the zakah of 100 gram gold.
Allah knows Best!
Darul Uloom Deoband