How to measure the mineral content of your water

In the context of coffee, measuring your water mineral content can be very useful. If you are crafting water for coffee it can be useful to check for errors or if you are treating an existing source it's necessary to know where you are starting from in order to determine what treatment method will produce results that are desirable.

What is TDS

The most ubiquitous term to describe mineral content is probably TDS or Total Dissolved Solids which is measured in ppm. I personally find the units of ppm to be rather misleading because despite standing for “parts per million” it is actually a measurement of a mass per volume of water, in this case mg/L.

Unfortunately, if you don't have any idea of what is already in your water TDS is not particularly useful, as it doesn't do anything to inform about the ratio of hardness to alkalinity

Electro Conductivity Meters

One of the easiest and most cost effective ways to measure TDS is with an electro conductivity meter. Despite often being advertised as a “TDS meter” these devices pass a small charge through two small electrodes that are inserted into the water and measure the conductance. Pure water is a very poor conductor, and therefore if there are no minerals present the meter will read zero.

If we introduce a mineral such as sodium bicarbonate, the compound will split into a sodium cation (positive charge) and bicarbonate anion (negative charge) when dissolved. This allows the water to carry a charge dependent on the concentration and type of ions present.

Conductivity and TDS are for the most part linearly related so if we measure the conductance we can apply a conversion factor to go between the two. Unfortunately, ions have varying conductivities so most conductivity meters apply a static conversion factor that is derived from the cumulative conductivity of the ions commonly found in drinking water sources.

A close up of the nodes of an EC meter used for measuring conductance

ppm versus ppm as CaCO3

Before we go further, it's important to note that there are many unit options to choose from when describing water mineral content. The SCA and collective coffee community has landed on ppm as CaCO3 as this equates the extraction power of magnesium and calcium by pretending they both weigh the same.

We do this because we are more concerned with the valence charge of ions rather than their weight. As far as coffee is concerned, 40.1 ppm of Ca++ will have a magnitude of effect on your brew similar to 24.3 ppm of Mg++, due to the fact that Ca weighs more than Mg. Converting these to an equivalent weight we get 100 ppm as CaCO3 for both.

Measuring your water TDS

To show this knowledge in action let’s prepare a bottle of Lotus Water and check the TDS using an EC meter. Below is the recipe I will be using.

6 drops CaCl for 60 ppm as CaCO3 of hardness
3 drops MgCl for 30 ppm as CaCO3 of hardness
6 drops of KHCO3 for 30 ppm as CaCO3 of alkalinity

Using an incredibly helpful tool developed by David Seng (link at bottom) we get the following calculated water properties:

TDS: 145.1 ppm
Conductivity: 298.8 µS/cm

Most EC meters automatically apply a conversion factor (K value) automatically of 0.5 or 0.7 to convert from conductivity to TDS. The EC I’m using can toggle between the two or it can simply show the conductivity reading which is what I prefer to use. If your meter does not indicate the K value it’s using you will need to mix up a solution with a known concentration in order to calibrate it.

After mixing up the recipe above I took a measurement and got a reading of 299 µS/cm which is almost exactly our calculated value, however, it is not uncommon for this number to have a small degree of error due to inaccuracies in preparation method or varying temperature of the solution.

An EC reading of our water sample made from Lotus Water

If your EC requires you to use a K value it’s often best to use a value of 0.05 for crafted waters as this gives us a TDS reading of 149.5 ppm which is very close to our calculated value of 145.1 ppm and will work for most other crafted waters as well.

Measuring water using titration

In order to measure hardness and alkalinity separately we need to use a method other than an EC (unless of course your water contains only one or the other). Laboratories often use a method called titration to do so, which is when a solution of a known concentration, the titrand, is added to a solution of unknown concentration, the titrant, until a shift in the indicator is observed, which is most often a change in color.

A relatively inexpensive titration kit that can measure both hardness and alkalinity is the API GH & KH test kit that can likely be found at your local aquarium store or online.

The API titration kit that I prefer to use

The kit includes a 5 mL test tube, a GH titration solution and a KH titration solution. It's use is quite straightforward:

Add 5 mL of the water you want to test to the test tube
Add 1 drop from the bottle marked with GH or KH (whichever you are testing) to the test tub and observe that it dyes the water a specific color
Continue adding drops until a color change is observed, keeping track of the total number of drops required

Each drop until a color change is observed indicates 17.9 ppm as CaCO3 of whatever you are testing. Meaning that if we add 5 drops to a solution before it changes colors then we know the concentration of our titrant is 89.5 ppm as CaCO3 of GH or KH.

Tips and Tricks for Titration

In his book "The Physics of Filter Coffee," Jonathan Gagne describes an excellent technique to increase our resolution by using a larger sample size of 20 mL at the expense of more drops being required. Also, note that the accuracy of the test will increase drastically as we increase the ppm of the test solution until around 100 ppm as CaCO3.

If you happen to have a .001 precision scale laying around you can increase accuracy even further by weighing the drops instead of counting them (which the test kit is assuming a weight for).

Another tip to increase accuracy is to calibrate your kit by using a method to prepare some water that you know will have very little error as there can be small errors from one kit to another.

Final thoughts

Due to the number of units and existing misconceptions about TDS water can be difficult to analyze so if you have any questions feel free to leave them in the comments. While testing water has its uses, my true recommendation would be to focus on your water preparation method.

Whether you are using Lotus Water, one of our competitors, or preparing your own water, testing can be a useful tool, but if we focus on minimizing error in our preparation methods we can be confident that our water will be as we've intended without the need to verify it.

Resources:

David Seng’s Tool

https://www.espressoschool.com.au/coffee-water-calculators/