[Making your own grenadine]

Hi, I made my first batch of grenadine a few months ago and will never go back.  It was very simple:  1 cup juice, 1/4th cup sugar, reduced to 1/2cup, and bottle.  No lemon, no rose water, no vanilla.  Planning to try those soon.

But I am someone who will have a week of drinking XYZ and then a week or two of drinking something else, and I'd like to KNOW that my grenadine will be refrigerator stable for at least a few months.  Can anyone tell me how much vodka or Everclear I should add to be certain that I'm going to kill lingering bacteria?  I know it will be sweet, and I will probably make it even sweeter using some of these above recipes.  But I like knowing that the alcohol is going to kill anything that might try to grow.  If I have 8oz of liquid, will 1oz be enough?

Ideally I'd like to know the biological or chemical ratio, something that will apply for non-grenadine liquids as well.  Bitters, extracts, etc. 

I also posted this elsewhere, apologies, before finding this thread -- seems like this will be a good place to find experts.  :-) 

Thanks so much for any and all help!!! 

[Making your own grenadine]

First time poster here, thank you for reading:  I've recently become a convert to making homemade grenadine, with pomegranate juice and sugar.  It's tasty, but a bit time consuming and I'd like to add some vodka as a preservative.  Ideally, I'll add the minimum required to retard/eliminate spoilage.  Can anyone tell me how much vodka would be needed to preserve 1 cupful of grenadine?  It starts out as 2 cups pomegranate juice and 1/2 cup sugar, then reduces by about a half.  So it's pretty sugary.  Just not enough (I suspect) to keep indefinitely. 

Thank you!!!!