First ... CALM DOWN! We were giving chemo and kcl long before pumps were available. It can be done.
I can't answer your question about chemo certification ... I suspect that, as an employee of a private practice, you're covered there, legally. BUT you're still responsible for giving the chemo safely. Perhaps experienced oncology nurses can give you references to study. Meanwhile, use your PDR whenever you have to give chemo ... it would take just minutes to look up the specific drug and skim the precautions section. It can't hurt to investigate the availability of certification courses in your area. Even though your employer doesn't require it, it's great on the resume and would make you feel more confident regarding your skills.
As for the potassium, 20mEq over 3+ hours is pretty safe. You need to find out how many drops per cc is delivered by your IV sets. From there you can figure out how many drops/minute you need to set your iv in order to deliver 500cc over 3.5 hours. Convert 3.5 hours into minutes and divide that number by 500 = cc/minute ... multiply by #drops/cc on your infusion sets = #cc/minute you need to deliver. From there, it's just a matter of counting drops and adjusting the roller clamp until you get the right rate.
Here's how I would do it:
3.5x60=210 minutes, div by 500=0.42cc/minute, if your sets are 60 drops/minute, then 0.43 x 60=25.2 drops/minute.
Round that down to 24, divide by 4 = 6 drops/15 seconds. Just adjust the roller clamp until you can count 6 drops in 15 seconds and your infusion will run in a bit over 3.5 hours. (I rounded to a number that was divisible by 4 ... easier to count over 15 seconds and multiply x 4 than to count over one minute. Also, it's impossible to count a fraction of a drop. I rounded down rather than up on principle ... a hair slow is theoretically safer than a hair fast. You have a big margin of safety here, since most facilities consider 20mEq over an hour to be safe.) Watch those decimal points and, yes, I used a calculator!
The key to working without pumps is monitoring. Try to check your rates frequently ... it can change with position changes, even with implanted ports. You especially don't want to have them speed up. Always remember that you don't have alarms to tell you when there's a problem.
Meanwhile, you are correct to worry about safety when giving chemo and kcl by gravity. It really isn't ideal. Many pharmacies have infusion pumps available to rent ... my hospice unit used them for over a year. You might mention your concerns to the oncologist, she may not have considered the liability issues involved.
There are also flow meters available that can be attached to the infusion set and will allow you to set a flow rate by turning a dial. If your employer doesn't have them, she may be willing to order them as an alternative to renting pumps.