I have two Pitbulls, one female and the other is male. The female is in her 12th day of heat. Since her first day, I separated my two dogs but they can still see each other. The male is always crying and whining because he wants to mount her and he is not eating nor playing nor enjoying his walks, plus we can't sleep at night because of his crying. I was wondering if I got the special diaper for female dogs which stops the blood from falling to the ground, does this prevent him from mounting her and eventually making her pregnant?
There are several aspects you need to consider.
Will a diaper prevent pregnancy?
No. Don't underestimate the creativity of your dog or his sharp teeth. He can simply tug the diaper away or destroy it. If you don't want puppies, never let your female be unsupervised with another dog.
Will it stop him from mounting?
No. It's not the blood that causes this behavior, it's the hormones the female dog is excreting right now. Those hormones are primarily shed in the urine, but also via the skin. Your male dog will still know very well that the female is in heat.
How long will it last?
The first stage of heat (that includes bleeding) can last up to 17 days. After that phase, your female can still get pregnant. It's followed by a phase called "the standing heat" (another 9 days on average) that is signaled by the blood becoming lighter and watery. This is the highly fertile phase of heat. Then follows a several week long "post-estrous" phase where your dog cannot get pregnant anymore.
Please read more in any of these articles:
What should you do?
If you don't want puppies, don't let your dogs get together under any circumstances, I'd even suggest not letting them see and smell each other.
If your dogs manage to mate, they'll get stuck together at the rear end for up to 45 minutes. If that's the case, the act already happened and separating them will not prevent pregnancy. It will only cause both dogs severe pain.
If you don't want to breed with your female, you should get her spayed after her heat is over (please note the date of her heat and speak to your vet to determine a suited date for the operation. Spaying too soon after the heat or even during heat is too dangerous because it can cause excessive blood loss.)
Spaying your female dog actually has health benefits, as described here, but the longer you wait (in terms of how many heats she has), the smaller those benefits become.