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Daniel Lunk
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In this video, Christine Muth of the North Carolina School of Science and Mathematics demonstrates how to measure the total dissolved solids of a water sample using a Vernier conductivity probe.


Christine Muth (00:00)
Hello my name is Christine Muth, I’m from the North Carolina School of Science and Math, and I’m going to demonstrate how to measure total dissolved solids.
Total dissolved solids is a measure of the concentration of ions dissolved in a sample of water. It’s also very similar to the concept of conductivity. And in fact we’re going to demonstrate the use of the Vernier conductivity probe as a way to measure total dissolved solids in a sample of water.
So to start, you want to have your Vernier probe in some distilled water or it could even be tap water. And you want to set the probe — in this case with the conductivity probe, there’s three different ranges it can measure: 0 to 200 microsiemens, 0 to 2,000 microsiemens, and 0 to 20,000 microsiemens. If you know that your sample is going to be somewhere between 0 and 200 microsiemens, it’s actually most accurate to choose that smaller range. But if you’re not sure, and you think your sample might range into the thousand — into a thousand or more, you would want to choose the 0 to 2,000 range. The 0 to 20,000 microsiemens range would probably only be used for a saltwater sample. Since I’m dealing with fresh water and I’m not sure what range my samples are in, I’m going to choose the 0 to 2,000 microsiemen range.
Next you need to connect the probe to your Labquest. Now in this case I have a Labquest, you could also use a Go!link connected to a laptop, or you could use a CBL connected to a graphing calculator. So there are different options available to you. In this case I’ll demonstrate it with a Labquest, but the procedure is basically the same.
The next thing I need to do is connect my conductivity probe to my Labquest, and I’m going to insert it in channel one. There’s actually several channels to choose from, so I’m just going to choose channel one.
So next you would turn on the Labquest, or laptop or graphing calculator, depending on which method you’re using.
First I need to identify to the Labquest that I’m using a conductivity probe. So I’ll use this little pen, and I’ll click on sensors, and then sensor set-up, and right now the Labquest is building a list of all the possible sensors I could be using. And my sensor is plugged into channel one, so I’ll use the channel one drop-down menu, and I’ll scroll down to conductivity, and within conductivity, I need to choose the 0 to 2,000 microsiemen conductivity meter. Then I hit "OK."
Now I need to calibrate my probe. So I’m going to click on “sensors,” then “calibrate,” and then the only probe that’s in there is the channel one conductivity probe, so that’s what I select, and I’m going to click on “calibrate now.” And it automatically puts the cursor into a box that says, "Reading one known value." So the first known value I’m going to use is the probe in air. So holding my probe in air, the correct value would be zero microsiemens. So I’m going to type in “zero” and select “keep.”
Now I need to read a second known value, and so for my second known value I’m going to use a known conductivity standard, and so this standard has 500 milligrams per liter total dissolved solids and 1,000 microsiemens per centimeter conductivity. Since I want to measure in total dissolved solids, which is in milligrams per liter, I’m going to place this probe into that solution and then, since I’m doing total dissolved solids, I’m going to type in 500, which is the correct reading for total dissolved solids. I’m typing in 500. I choose “keep,” and then "OK."
And there’s one final step I need to do, so now I can take my probe out, and I can wash it with distilled water, or deionized water. But one more thing before I read my sample that I need to do is I need to tell the Labquest that I want to measure my data instantaneously rather than have it keep the data over time and make a graph.
So to do that I’m gong to again select “sensors,” and this time “data collection.” Then I choose the drop-down menu under “triggering disabled” and place a check mark in the box that says "enable triggering," and then hit "OK."
So now I’m measuring instantaneously total dissolved solids. So now that my probe is calibrated, I’m simply going to place it into my water sample, and I’m getting an instantaneous measure of total dissolved solids. So the units I’ll actually read it in are milligrams per liter. This is set up for conductivity, but I’ve calibrated it for total dissolved solids. So this is giving me a reading of 250 milligrams per liter total dissolved solids.