Adding baking soda - sodium bicarbonate (NaHCO3)is a quick and cheap method to reduce acidity in foodstuffs- but beware of the sudden release of gas (CO2) and foaming.
Strictly speaking any alkaline substance can be added to reduce acidity in solution. The trick is keep the taste at least "pleasant", if not "authentic", without rendering the drink dangerous. Sodium bicarbonate (sodium hydrogen carbonate) usually makes reasonable tasting organic acid salts that don't greatly detract from the flavour. Some of these organic acid salts are actually used as food flavourings in their own right. Which in itself can present the same problem in a different form. The adjusted brew itself might not be excessively acidic but one can still taste vinegar, lemon juice or sour apples.
Some prefer adding magnesium carbonate, calcium carbonate or potassium carbonate as they say these ions can produce a preferable taste but IMHO they sometimes can also produce a "haziness" in the brew. Calcium carbonate (chalk) is often locally obtainable and magnesium carbonate is a known food additive (E504). Both are of "reduced solubility" in water.
Finding out how much to add is an important step and the only real way is to take a small but measured quantity of the cider and add a SMALL measured amount of (say) bicarbionate to it and stir. The cider should foam if it truly is acidic. Leave it to stand for a minute and then taste it. If still too tart/acidic then add another small but measured(weighed?) amount and repeat. If too much was added initially then start again with fresh cider and much less bicarbionate. As a guide try starting with 150ml (a cup full) of cider and something like "half a pea's worth" of bicarbonate as a tiny amount on the tip of a teaspoon. If you can find an accurate measure or have some accurate scales then use that. When a suitable amount to add has been determined - "do the math" to scale up the amount needed for the entire batch. However start off by adding half of the calculated bicarbonate to the cider, stirring and waiting. Test the taste, if still very tart/acidic then add half of what remains - and so on. IT IS A GOOD IDEA TO LEAVE THE CIDER A SOMEWHAT MORE TART TASTING THAN BLAND TASTING, as acidity usually decreases with standing and bottle conditioning. IMHO cider needs a long time (eg one year) to condition well.
I don't recommend this method generally - only in cases where the brew is so acidic that it is unpleasant to drink. Even then it is preferable to let the cider condition correctly to reduce the acidity. A lot of biological processes go on on the bottle/barrel during conditioning - especially with the tart tasting malic acid.
But sometimes this method produces a useful drink when all other methods have failed - and it has saved several demi-johns of West Country farm cider from being poured away.
