Alkalinity titration endpoint pH & Salifert endpoint color

[GoVols]

I give up. You do you.

Yep

But thanks for the help

 

[JimWelsh]

Yep

But thanks for the help

EDIT: Snark deleted.

 

[hart24601]

It took me a while to get it too.

Think more about the liquid dispensed as that is what you are measuring. The bubble at the top doesn’t matter. Pull to 1ml and let’s say the end point you used 0.4 ml. None of that was air, the air was at the base of the plunger.

 

[GoVols]

It took me a while to get it too.

Think more about the liquid dispensed as that is what you are measuring. The bubble at the top doesn’t matter. Pull to 1ml and let’s say the end point you used 0.4 ml. None of that was air, the air was at the base of the plunger.

If your tip is held tight the same way each time, then the bubble size matters in how much reagent you have, and would effect the # on the syringe at the endpoint.

Never meant to be a debate.

 

[JimWelsh]

Never meant to be a debate.

Well, you are debating someone with considerable bona fides when it comes to titrations. But I'll make one more attempt to explain it.

I have a pair of 50 mL Class A burettes I use to do various titrations in my home lab. They were used extensively in developing the Neptune Trident. When ever I do a titration with one of them, I always fill them to somewhere below the "0.00" mark. I then record what that level is, say "0.50" mL on one titration. If the sample I'm titrating will need 25.00 mL to complete the test, then the final reading of the burette will be 25.50. The difference between the beginning reading at the bottom of the "bubble" in the top of the burette will be 25.50 - 0.50 = 25.00 mL. Let's say I repeat the titration, but this time I only fill the burette to the 0.75 mL mark. Then, the titration will still take 25.00 mL to complete the test, so this time the reading at the end will be 25.75 mL. The volume of reagent delivered will still be 25.75 - 0.75 = 25.00 mL. In both cases the volume delivered is the same, even though the "bubble" at the top was a different size.

The size of the "bubble" at the top just doesn't matter. What matters is the difference between the beginning reading and the ending reading. In the case of the burette, that is referenced to the bottom of the "bubble". In the case of the hobbyist test kit, it is referenced to the bottom of the plunger, at the top of the "bubble". In either case, the difference between the beginning and ending readings is all that matters.

I apologize if I'm coming across as frustrated, but that's because I am. I have several times provided a lucid, clear explanation of how this works, and how the bubble size doesn't matter, and you have repeatedly asserted that it does matter without any supporting argument.

 

[GoVols]

Well, you are debating someone with considerable bona fides when it comes to titrations. But I'll make one more attempt to explain it.

I have a pair of 50 mL Class A burettes I use to do various titrations in my home lab. They were used extensively in developing the Neptune Trident. When ever I do a titration with one of them, I always fill them to somewhere below the "0.00" mark. I then record what that level is, say "0.50" mL on one titration. If the sample I'm titrating will need 25.00 mL to complete the test, then the final reading of the burette will be 25.50. The difference between the beginning reading at the bottom of the "bubble" in the top of the burette will be 25.50 - 0.50 = 25.00 mL. Let's say I repeat the titration, but this time I only fill the burette to the 0.75 mL mark. Then, the titration will still take 25.00 mL to complete the test, so this time the reading at the end will be 25.75 mL. The volume of reagent delivered will still be 25.75 - 0.75 = 25.00 mL. In both cases the volume delivered is the same, even though the "bubble" at the top was a different size.

The size of the "bubble" at the top just doesn't matter. What matters is the difference between the beginning reading and the ending reading. In the case of the burette, that is referenced to the bottom of the "bubble". In the case of the hobbyist test kit, it is referenced to the bottom of the plunger, at the top of the "bubble". In either case, the difference between the beginning and ending readings is all that matters.

I apologize if I'm coming across as frustrated, but that's because I am. I have several times provided a lucid, clear explanation of how this works, and how the bubble size doesn't matter, and you have repeatedly asserted that it does matter without any supporting argument.

Totally agree when you know exactly how much you need, then the bubble does not matter.

Jim,
Out of absolute due respect that I have for you, just don't be frustrated.

I'll just keep setting the reagent at .9ml and the plunger at 1ml

Thank you

 

[GoVols]

Jim,
Are you just saying to read the # where the reagent starts and then look at the end #

And do the math to determine how much reagent was used to reach the endpoint?

Is that correct?

 

[JimWelsh]

Jim,
Are you just saying to read the # where the reagent starts and then look at the end #

And do the math to determine how much reagent was used to reach the endpoint?

Is that correct?

Yes.

 

[taricha]

Randy did a Reef Question of the day on this. Even many people who know the right answer find it a little maddening.

Reef Chemistry Question of the Day [HASHTAG]#160[/HASHTAG]

I have just drawn tank water into a syringe with a tip (shown below, including a blowup of the barrel). The blue coloration shows where the fluid is.

When I started to draw fluid in, the tip and syringe were empty and the plunger was all the way in. Now I am going to dispense the fluid into my test vial.

How much fluid will be dispensed by pressing the plunger all the way back in?

A. 3.6 mL
B. 2.9 mL
C. 2.2 mL
D. 1.6 mL
E. Cannot be determined because of the air present

Good luck!

.

And the answer is...B. 2.9 mL

Here's how to think about how a syringe works:

When the plunger is pushed all the way in, the leading edge of the plunger aligns with the zero mL marking. That is true of nearly every syringe I've seen. When you begin to use it, the tip and barrel are empty, with the tip containing air and the barrel occupied by the plunger.

As you draw back the plunger with the tip in a liquid, the plunger creates a vacuum between it and the liquid, drawing in the liquid. The movement of the plunger draws in exactly the amount of liquid equal in volume to the travel of the plunger backwards (or upwards). So you can track how much fluid is taken up exactly by the travel of the plunger against the volume markings, regardless of what you see with the fluid (assuming it is functioning properly).
....

More explanation in Randy's answer post.
It's a little hard to look at the liquid level on the syringe markings and ignore it, but that's not what matters. It's the change in the plunger.

 

[homer1475]

While I don't use the ALK kit(used to before hanna, and still do to double check the hanna if I get an "off" reading), I do use the CAl and MAG kits.

I don't know what @GoVols is on about, but every time I draw up reagent with the tip on, it goes to the exact same spot. I draw the plunger all the way up, then depress the plunger to the 1ml mark. The air bubble is the extra space from the tip.

It's common sense, how can it not be?

 

[GoVols]

While I don't use the ALK kit(used to before hanna, and still do to double check the hanna if I get an "off" reading), I do use the CAl and MAG kits.

I don't know what @GoVols is on about, but every time I draw up reagent with the tip on, it goes to the exact same spot. I draw the plunger all the way up, then depress the plunger to the 1ml mark. The air bubble is the extra space from the tip.

It's common sense, how can it not be?

Thank you

 

[David S]

OK
To confuse things more:
I recall hearing, that after drawing in the solution you should have an air pocket of about 14ml, give or take a millimeter.
I also recall a well known Youtuber mentioning that the recommendation he got from Salifert(?) was a 15 ml air cushion.
Sometimes it can be frustrating to get the right amount. It can require several tries.
Been using this method for years.
See no reason to stop.

 

[JimWelsh]

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