Improving Refractometric and Gravimetric TDS Measurement (Part 2)

Method for cross-checking your TDS measurement

Let's continue with instant coffee and the method for TDS calibration.

I tested a few different instant coffee brands in order to find the optimal procedure for TDS calibration. I found some differences in the coffee itself and in the packing but the main issue is the water content. Instant coffee is hydrophilic and will absorb water if left unsealed. Calibration calculations are only possible if the water content in instant coffee is known. Literature research showed that moisture in instant coffee varies from 5-12%. I used a simple method for water content measurement. I added instant coffee to a beaker and heated it at 120-130 °C for 40 min in order to dehydrate it. I measured the weight of the coffee before dehydration and after. The difference in weight is the amount of water evaporated from the coffee. I found that the 2g single sealed packets of instant coffee have the lowest water content around 4.5-4.8%. They are firmly sealed, no moisture can be absorbed, and the water content deviation is smaller. The benefit of using single packets rather than a jar of instant coffee is that you can open them just before use. Here are some calculations based on this method:

I’m the last one who will make marketing for an instant coffee brand, but I have to say that one of the best solutions in this project was using Nescafe classic 2g packs. It looks like Nescafe has great control of water content before packing. This means that the standards preparation for the TDS calibration curve can be easily reproduced in the future without the need to measure the moisture again. The same is valid for everyone who wants to do it as well.

What is the procedure? Dissolve a known amount (in grams) of instant coffee in a known amount (in grams) of water, and you have a TDS standard with a specific concentration in %. Measuring the standard with a refractometer should give you the calculated TDS. Please consider the 4.7% water content in instant water. If you are more familiar with calibration curves, you can make several different standards and construct a calibration curve. Then you can crosscheck or recalculate the TDS measurement of your coffee with the help of the calibration curve to improve the accuracy. The recalculated value will be the most accurate you can get from the TDS measurement.

So what you can do with this information? The main agenda is to have a reliable method to prepare standards and to compare the measurement of your refractometer with something with a known TDS value. Doing so will answer the question, of how accurate is your TDS measurement. Is this what you measure real and reliable? This approach could be easily repeated by everyone at home, and prepared standards could be stored in a sealed container for a few weeks.

So, I’m going to the local market for some more packs of instant coffee. I hope no one will see me doing it. I don’t want people to think I’m drinking instant coffee at home.