# DIY Stir Plate for Titrations



## Chiumanfu (Mar 27, 2013)

I got my Vinmetrica SC-100A SO2 Analyser a few months ago and I've been playing around with it, getting used to the instrument and the testing process. It is a beautiful addition to the winemakers tool kit and I think it should be high on everyones wishlist. The one thing I didn't like is the constant swirling motion that is required during titration. Swirling and holding the electrode with one hand and dropping the titrant solution with the other always felt like a "rub your belly, pat your head" move to me. So in true DIY fashion I turned to the internet to see how difficult it would be to make my own magnetic stir plate. Turns out, it's not that difficult and I had all the parts I need laying around already.

I do not take credit for coming up with this brilliant idea... Many people have done this before me, so rather than redocument the procedure, I'll just link to the tutorials that I followed and add my own comments where appropriate.

The following link shows the general assembly. The only thing I do not agree with is his choice of stir bar. I tried with a metal bar and it tended to bounce around inside the test beaker. The problem is, if the metal bar works it way to the end of the magnet, it starts moving in an arc instead of spinning. Instead I used another magnet which stays dead center the entire time and does not wander.
Instructables.com - Magnetic Stirrer

The following link shows very much the same thing except the author added a potentiometer to vary the stir speed. This is pretty important because the normal working speed of these fans is a little fast for our small volume test samples. The fan I had just happen to have has a variable speed adjustment already so I was good to go. A lot of fans intended for high end gaming or home theater computers have this feature. Also, the larger the fan is, the slower the RPM usually. Another solution would be to use an adjustable output voltage power source. One of those Universal AC/DC Wall Warts from Radio Shack would be ideal. Just make sure the one you have can deliver more than 200mA. If you're really stuck, take a look at those old cell phone chargers that I know all you have a small collection of. They tend to be 5V-7.5V which may end up being the perfect voltage for a "non-violent" stirring action.
Instructables.com - Cigar Box Stir Plate/

The following link shows where to get the magnets from. I'm pretty sure everyone has (or knows someone who has) a dead hard drive that can be cannibalized for the neodymium magnets. The multi platter hard drives have a smaller rare earth magnet in the head arm as well. That is what I used as a stir bar.
Instructables.com - Pulling apart a desktop hard drive to get rare earth magnets

The following image shows my testing setup. Vinmetrica SC-100A on the left. Stir plate with titration beaker and electrode in the middle and various pipettes on the right. Don't forget the pH meter in the back which is crucial for SO2 testing.






I built the stir plate as simple as I could. Maybe one day I'll put it in a proper project box with on-off switch and all that fluff. I used bolts and nuts to create a standoff so the magnet has room to spin. I used a CD case cover as a top plate. I broke the vanes off the fan so it doesn't blow air during testing. This also lets it spin more freely because it isn't pushing air into the table. There is one spot at the exact mid way point between the north and south pole where the stir bar magnet will want to sit. Mark this point and make sure it lines up with the center of the fans spinning axis. Make sure the magnet is centered to avoid unbalancing the fan as that will lead to unwanted vibration. The magnets inside the fan itself will make our magnet want to slide around so keep an eye on it. Use crazy glue to stick the magnet to the fan. It's wise to rough up the face of the fan so the glue has something to stick to. 





Here you can see my stir bar. It's not so much a bar but more like a stir block. I experimented with shoving the magnet inside a plastic straw to create more of a bar shape but that just made the stirring action to great. With just the tiny little block spinning in there, it doesn't create a huge vortex but still keeps everything mixing well. Here you can see it's natural resting spot in the center of the north and south poles. When you first start the testing, before you fill the beaker with wine, you have to place the beaker on the center of the stir plate and drop the stir bar in. If it aligns itself with the middle spot, you're good to go. If it aligns itself with one of the poles at the side of the magnet, then you have to coax it into the middle until it snaps into place. It's easier to do that it is to describe. 





Here is my solution for an electrode holder. Basically four tie wraps arranged in an X. This keeps the electrode out of harms way and you can concentrate on the titration rather than holding the electrode at a safe level. The electrode tip can easily be damaged if you let it contact the spinning stir bar.





There you go. A setup that can be used for SO2 and TA titration that has virtually no cost if you have a junk PC lying around somewhere.


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## Norske (Mar 27, 2013)

SWEET! Thanks again chi for going the extra mile in sharing this!


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## Chiumanfu (Mar 27, 2013)

Just my way of giving a little back to this fantastic group of winemakers that have taught me so much!


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## ibglowin (Mar 28, 2013)

Pretty cool idea! 

You may want to pick up a "real" stir bar from Amazon etc. They are only a buck or two and are sealed in Teflon.


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## Chiumanfu (Mar 28, 2013)

I had considered that but the magnet I found in the hard drive works perfectly... and it was already encased in plastic. Just lucky I guess.

If you do buy a real stir bar, make sure to get the smallest one possible. The Vinmetrica test samples are so small and you really don't want to create a big vortex or the electrode tip will fill up with air bubbles.


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## ibglowin (Mar 28, 2013)

Chiumanfu said:


> and it was already encased in plastic. Just lucky I guess.




As long as you are tossing the sample away in the end it makes no difference really.


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## roblloyd (Mar 29, 2013)

Cool thanks. I have some computers that I was about to get rid of too. And about 20 hard drive magnets - that's how I wipe drives for clients. More fun than a program....


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