When is titration Complete?

[brad]

Doing any titration, I add a bunch of drops before any color change persists after mixing, then it changes 1 drop at a time for a bunch more drops, then it stops changing no matter how much I add. When is it complete?

Most of the kit directions say "until the vial turns orange or pink" or "until a clear blue color is reached". In those cases, should I stop when a casual observer would call the color "orange", "pink", "blue" etc?

I remember in chemistry in class, we were told a titration isn't complete until the color can't change any more. Unless your familiar with the end color, you have to add 1 drop past where it stops changing and remember where you were at.

Which is correct? This makes a big difference, 30ppm Ca or 1dhk alkalinity or more.

[Bpb]

If you’re willing to sacrifice a test, pump a full syringe or two in. You’ll see the definite end color. I abide by the classic chem class titration technique. Once color starts shifting. Go one drop at a time. Take note of your value each drop. Once two drops in a row yield zero color difference, the first of those two is your end result reading


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[bertoni]

The alkalinity test will be complete when the color shifts to the suggested endpoint and stays there for at least 10-15 seconds or so. For calcium kits, sometimes you will need to wait quite a while (up to a full minute) after the first color change. At that point, you can complete the titration. I had to use this method for the IO calcium kit. The Salifert was faster for this reaction.

[Mrramsey]

Originally Posted by brad

Doing any titration, I add a bunch of drops before any color change persists after mixing, then it changes 1 drop at a time for a bunch more drops, then it stops changing no matter how much I add. When is it complete?

Most of the kit directions say "until the vial turns orange or pink" or "until a clear blue color is reached". In those cases, should I stop when a casual observer would call the color "orange", "pink", "blue" etc?

I remember in chemistry in class, we were told a titration isn't complete until the color can't change any more. Unless your familiar with the end color, you have to add 1 drop past where it stops changing and remember where you were at.

Which is correct? This makes a big difference, 30ppm Ca or 1dhk alkalinity or more.

I've heard it both ways. I too am struggling with this. If I go to the point where the color shifts and stays my readings would indicate a normal range. Which tests are you using? I am using Red Sea Pro.

I am also in Hudson and would be interested in getting together to compare notes / technique.

[OzIA]

I've found the Red Sea alkalinity test color degrades over time. It seems to reach an nice red end point when the reagent bottle is first opened but after some air is in there, it tends to be more orangish. I've never thought it reaches the red color they list as the endpoint. The Calcium and Magnesium tests have better results (more consistent end points) throughout the life of the tests.

If I reach a stable color for 30 seconds to a minute and then starts to bleed back, I feel like I reached the correct end point.

[Dan_P]

Yes, the chemistry rules for titrations still apply for the kits. The difficulty is judging when the color change has stopped because you are starting with one color that transitions to another color which you have not seen before. For some kits the blended color of the initial and final colors may not appear different from the final color to your eye.

As someone has already mentioned, perform a titration, but keep adding titrant once you reach what you judge to be the end point. You may see a subtle diiference after a drop or two. Do this a couple times to train your eye to discern the final color.

Another thing to try is to look at a highlight, those white spots or lines that appear near the bottom of the test vial. Because they are so bright, you may be able to discern a red violet from a blue violet better than when viewing the solution color through the side. You will just have fiddle with your viewing angle to find what works for you. A piece of white paper held behind the test vial is another way to get more bright light through the solution.

[homer1475]

When I see red to blue I stop. Salifert ALK kit gives the same exact number as a hanna. If I continued to add until it stopped turning back My alk would be off the scale.

Directions do not say to keep adding until the color stops(even though this is non typical for titration kits). The directions say to stop as soon as the color change is noticed.

[Reefiez]

The Red Sea Magnesium pro test kit is a little different.

According to the instructions, the end point is the FIRST time it turns blue. It will turn purple again after a while, but you shouldn't add anymore titrant. I didn't know in the beginning and was "testing" 1500 or 1600 ppm mg.

I swapped to Salifert for MG now, and the end point is instantaneous and unambiguous, so I'm pretty happy with that.

[brad]

Originally Posted by homer1475

When I see red to blue I stop. Salifert ALK kit gives the same exact number as a hanna. If I continued to add until it stopped turning back My alk would be off the scale.

Directions do not say to keep adding until the color stops(even though this is non typical for titration kits). The directions say to stop as soon as the color change is noticed.

Directions needlessly clarify not worrying about bubbles at the top of the titrating syringe, tell you not to drink the chemicals, and other stupid things. The instructions look like they were written by a 5th grader for use by a 2nd grader.

If I follow the directions literally, I stop when the color starts to change, not when it stops. But I have to wonder if this is correct.

I got 7 replies from people who seem less than certain of the correct endpoint, but all seem smart enough not to drink reagents. The instructions need work.

[Mrramsey]

Originally Posted by brad

Directions needlessly clarify not worrying about bubbles at the top of the titrating syringe, tell you not to drink the chemicals, and other stupid things. The instructions look like they were written by a 5th grader for use by a 2nd grader.

If I follow the directions literally, I stop when the color starts to change, not when it stops. But I have to wonder if this is correct.

I got 7 replies from people who seem less than certain of the correct endpoint, but all seem smart enough not to drink reagents. The instructions need work.

The directions specifically say:

"As the color in the vial approaches the end color as shown on the instruction card, add one drop at a time swirling after each drop, until the end color is achieved."


If I do it until the color first changes to the orangish / red color from blue my alk reads low (6.3), if I continue until it turns a color that is darker reddish orange I am at an alk of 9. They don't exactly define what "end color" is other than matching the card color which it will never match.

[RCS82]

Originally Posted by homer1475

When I see red to blue I stop. Salifert ALK kit gives the same exact number as a hanna. If I continued to add until it stopped turning back My alk would be off the scale.

Directions do not say to keep adding until the color stops(even though this is non typical for titration kits). The directions say to stop as soon as the color change is noticed.

I get the same results with Hanna and Salifert as well. I did 2 tests with the Hanna checker yesterday, one with old reagent and one with brand new reagent, and they were 7.5 and 7.8. Can't remember which one was which but they were close. Did the Salifert test and saw the color change was at 7.7 I think, and that's close enough for me. Sometimes I will add another drop just to see if the color change will continue but it doesn't really.

[bertoni]

Originally Posted by brad

If I follow the directions literally, I stop when the color starts to change, not when it stops. But I have to wonder if this is correct.

Kit designers have some decisions to make about when to call the titration complete, which also depends on the exact reagents used. If the directions say to stop when a color change begins, then that's where you should stop. The Salifert and IO kits said to stop the calcium test when the color turned blue, although the first big color change was to purple.