In our last issue, we pointed out that some auditors now require that the user make his or her own concentration chart for our E-58D hydrophilic emulsifier. The apparent
reasoning behind this is that there is no guarantee that the refractometer used by Met-L-Chek and the refractometer in the user’s possession will give the same results. So let’s discuss how to make a chart that will be useful.
First, it is relevant to understand that the relationship between the refractometer reading and the E-58D concentration is a linear one. This means that if you have one point for the concentration and its associated refractometer reading, you can draw a straight line from the zero point on the graph paper through the measured point, and you will have what is needed. While this is true, it poses a couple of practical problems. The first one of these is that if a hand held refractometer is used, it is sometimes
difficult to get an exact reading with it, because the scale divisions are in increments of 0.2, and the dividing line that marks the scale is sometimes a bit fuzzy. In practice, this is not a problem, because the effect is very small, but it can become a problem during an audit. That leads us to the second practical problem, which is that of an audit. Auditors have varying degrees of technical education, and it is possible that an auditor does not understand that the relationship is linear, in which case he or she may want to see a graph with more than one measured point on it. And, as pointed out above, if the graph has been made with a single reference point, the auditor may ask that the point be defended as accurate.
So we will discuss how to make a useful graph that can be defended if necessary. What is needed is a container that will hold an exact amount of liquid. A kitchen
measuring cup will not do. We suggest a volumetric pipette of 20 ml size. With this, you can measure exactly 20 ml of a liquid. You will also need a container that will hold
perhaps 600 ml of liquid.
Now, using the pipette, measure exactly 20 ml of E-58D into the 600 ml container. Now, using the same pipette, and without cleaning it, measure into the same 600 ml container, 40 ml of water. This will require two 20 ml portions. Mix the liquids thoroughly, and the result is 60 ml of E-58D at a concentration of 1/3, or 33.333%. Now take a reading with the refractometer and record it. Next, add one more 20 ml portion of water to the mixture, and mix it thoroughly. Now the mixture is one part E-58D and three parts water, or 25% exactly. Take a refractometer reading and record it. Add one more 20 ml portions of water, and mix. The result is one part of E-58D and four parts of water, or 20% exactly. Record the refractometer reading for this. You can continue this process as long as you wish, recording the refractometer readings and the
concentration each time, but when you get to a 10% solution it is probably sufficient. You can use the following chart as a guide.
When all of the readings have been taken, place the points on a graph and connect the dots, using a ruler. We have made this exercise in our laboratory, and the results are shown below. Remember that these points were developed using our refractometer. Your results will be either identical or very similar, depending upon your refractometer.