snopes.com: Coca-Cola Acids

That Snopes web page says there's approximately 12 grams of phosphoric acid in a gallon of Coca Cola syrup.

Assuming a mix ratio of 5:1, one gallon of syrup will make 6 gallons of soft drink.
How many glasses of soda does a 5 gallon bag in a box syrup yield if each glass is 16oz? - Yahoo! Answers

One US gallon is 3785 ml. and assuming that Coke is the same density as water, that means there's 12 grams of phosphoric acid in 22,710 grams of Coke, or a concentration of phosphoric acid in the soft drink of about 5 one hundredths of one percent by weight.

(Since Coke is heavier than water because of the dissolved sugar in it, the actual percentage would be even lower.)

So, while there is phosphoric acid in Coca Cola, the amount is so tiny that Coke can't be used as a substitute for phosphoric acid.