Urine pH fluctuates throughout the day. Things that affect pH include amount of water drank; the body's acid:base balance; type of food being fed; age of the sample tested; exercise; hydration status; and several other things.
In order to get accurate readings you need to test only a fresh sample at room temperature. PH usually increases as the sample ages. The pH should tested within 30 minutes of sample collection.
On series samples. The testing needs to be done at the same time everyday. This way you will have an accurate trend. Testing should be done several times daily in some cases.
Tests should be run on fasted samples if possible. Type of food will alter the pH. Meat diets will cause acidic urine. Vegetable/grain diets will cause alkaline urine.
Make sure that you testing materials are kept dry, are in date, and are kept in a cool dark place. The quality of the test strips will affect the accuracy of your results. A pH meter is much more reliable then the test strips. Make sure that it is properly calibrated.
If your dog is having issues with urine pH, stone formation, and chronic UTI's, you really need to discuss the treatment and prevention with your vet.
Why would you do this? If your dog is ill, the veterinarian will give you instructions on how to take care of him.
Urine Ph in dogs may vary each day, as the kidneys maintain and adjust electrolyte and acid-base balances in relation to dietary intake, or exercise. Therefore, no specific urine Ph is abnormal, except as it relates to other findings from urine analysis, and to the acid-base and electrolyte balance as a whole.
Urine Ph 7.0 and lower indicates acidic/neutral
Urine in this range is acidic or neutral. If a dog in good health is fed only one main meal each day vs 2-3 times per day, they generally have a Ph in this range. Ideally, 6.2-6.5 is preferred as an indicator of optimum acidity.
If the urine is markedly acidic (lower than 6.0) a systemic problem may exist and further investigations should be undertaken.
Urine Ph 7.0 and higher indicates alkalinity
Alkaline urine may be caused by diets containing high carbohydrates or high percentages of grains. It may also be caused by the addition of excessive amounts of alkalizing medications to the diet.
Consistent alkaline urine may also be caused by a urinary tract infection. Some of the types of bacteria that cause urinary tract and kidney infections produce an enzyme that breaks down to ammonia and turns the urine alkaline.
Consistent alkaline urine in dogs may also cause the formation of a type of kidney stone (crystals); these may also be caused by the indiscriminate use of sulphonamide antibiotics.
Another reason for persistent alkaline urine may be the inability of the kidneys to re-absorb bicarbonate.
Any consistently alkaline urine should be investigated, including a urinary sediment examination.
ETA: BTW the site tells you how to check it.