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Buffer Capacity of Water

What's in your water?

Depending on where you live and the water conditions in your area, there could be a large variety of minerals dissolved in your water supply. Most of these will be harmless, but they do have an effect on the water’s ability to buffer pH. For example, reverse osmosis water has very little ability to resist (buffer) changes in pH while water saturated with minerals (tap water) has a larger capacity to resist change in pH.  A commonly known buffer that we have all used after a particulary spicy meal for acid indigestion is an antacid like TUMS.  Antacids buffer or neutralize the acids in your stomach.  We use TDS (or total dissolved solids) to measure the amount of dissolved solids a liquid.

So lets talk about the TDS.

TDS is the total dissolved solids in water and indicates the amount of minerals in the solution. While this is a useful number to know and understand, TDS does not indicate what minerals are in the water and can only tell you the gross amount of solutes in the water. For instance, you could have two water samples with a TDS of 150, one is filled with calcium and magnesium and the other is filled with sodium and sulfate which could mean a world of difference. So it is important is to find out what actual minerals are dissolved in the water.

We actually put together an experiment to demonstrate to you the relationship between the TDS and the buffer capacity of water.

The Experiment

We started off with three categories of water. Reverse Osmosis purified water (RO), OptiPure mineral infused water (OptiPure MA) and tap water (TAP). We prepared a sample of each water source and then measured the initial conductivity and pH. We then added drops of lemon juice (a mild citric acid with a pH of 2.5) until the ph of the water became similar, to illustrate the differences of the buffer capacity of each type of water.

Tap Water - Total Number of drops until similar: 29

Tap water has a large buffer capacity, and as such it takes many more drops of lemon juice to change the pH of tap water than RO water. Each drop of the mild acid is buffered by the calcium and magnesium in the water until the capacity to buffer is exceeded. 

OptiPure Mineral Water - Total Number of drops until similar: 15

Mineral enhanced water still takes quite a few drops to change the pH, much more than RO water, however it takes fewer drops to change the pH than Tap water. This is due to the mid range amount of buffer capacity indicated by the higher TDS.  Note that the initial pH is very high, but one drop of acid quickly drops the pH to 7.1, compared to the one drop in tap water dropping the pH from a starting point of 8.31 to 7.85.

RO water - Total Number of drops until similar: 7

The water purified with reverse osmosis has a very low TDS and low buffer capacity and takes only a drop or two before it significantly changes pH, showing just how important minerals are in buffering pH.


The higher pH of OptiPure RO Water with Mineral Addition has very little impact on the ingredient water due to it's lower buffer capacity.  One drop of an acidic solution in 100 ml of this water quickly brought the pH into the neutral range whereas straight RO water after one drop went acidic.


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