May I use a jalapeño as a substitute for serrano?
Yes, you can actually use just about any other fresh green chile. Just check and make sure that the chile has enough heat. Lately, I've been finding only mild bland jalapenos with almost no heat to them.
The jalapeño has a lovely flavor and generally has 2,500-5,000 Scoville units of heat. The serrano also has a wonderful flavor, but with 10,000-23,000 Scoville units. That's more than twice the heat of a jalapeño. So yes, you can use jalapeños as a substitute for serranos, but the "heat" factor will be substantially lower.
If your recipe calls for you to remove the seeds and veins from the serrano, you can probably use the jalaoeño leaving the seeds and veins in and end up with about the same heat.
And always, use gloves when handling the peppers. I forgot with a serrano this summer and the acid "burnt" my fingers just like it can burn your tongue. Interestingly, the same remedy (milk) helped (soaking my finger in milk, that is).
Whie there may be some acid in a pepper, it's the capsaicin that burned your hand.