You definitely want a low purine diet...low uric acid is what you want. However, you did say the ph was high...meaning more alkaline? Not sure then because my experience was with high acidity levels. ACV does help lower the acidity level...but then again you said a high number, meaning more alkaline LOL!!! Maybe I'm no help...but I do know from experience that the prescription crap helps but is no good in the long run as far as nutrition is concerned. Do your research online but diet change is the answer, and remember that the vet crap is NOT medicated food...there is no magic in it...just different ingredients with usually lots of corn, corn gluten, chicken fat, etc. Lower protein, lower phosphorous is the goal as was mentioned. No organ meats...too full of purine levels. Lots of water is a good thing also to wash out the bladder. Add it to the food. Remember to frequently let your dog out to urinate, once again to flush out the bladder. There are holistic stores that offer good supplements for the urinary system as well. Homemade meals or foods such as Sojos grain free are alternatives....just have to add an appropriate type and level of protein, the rest is already made up...just add water. Good luck!!
Lots of questions and advice here:
Dog Food (Canine Diet and Nutrition) - Answers by Marie -Pet Chef & Dog Vitamin Expert
Oh yeah, you can buy ph strips to test your dog's urine level yourself at health food stores, etc.