With a pH of between 2 and 3, lemons are highly acidic. Why then are lemons (and limes), i.e. citrus fruits, considered to be alkaline for your body?
The answer is that they are particularly rich in citrate. Once absorbed, citrate can be used by the body to neutralize (buffer) acidity. It is then excreted in the urine, turning it more alkaline. 
Citrate mineral salts are the main thrust behind Optimum Health Vitamins Opti-Alkaline. If you don't want to use fresh lemon juice in purified water, or if you are in a hurry, or particularly acidic you can get citrate in this more concentrated capsule form. (My quick test for determining if someone is acidic is to ask them how much pain they have.)
Metabolic acidosis is epidemic in western society, contributing to all manner of disease.  Turning your body more alkaline is essential for detoxification, and has a multitude of other benefits.
To read more about them see the article links below:
- https://www.sciencedirect.com/science/article/pii/S0085253815570105 Renal handling of citrate