RSP Alkalinity Test - Question

[steelfin]

Is anyone using the Red Sea Pro Reef Foundation test kit?

 

I have read through the instructions and have viewed the RSP youtube videos, but I am still not clear as to when to stop the test and take the readings...

 

The test uses a titration method. THe video shows the solution turning blue (much darker than my test kit). At which time you continue adding the solution until it turns pink (according to the video).

 

However my kit does not turn pink but rather orange. The test cards have colors but they are not even close to the color that the solution turns....

 

So the question is, when do I stop the test and take the reading? As soon as the solution turns from blue to orange?

 

Or do I continue to add solution until the color is much deeper orange?

 

Any advice?

 

Thanks

 

SF

[flypenfly]

Once you do it a few times, you start to recognize the shade of orange which means stop. For me, it's the deeper orange that is on my card. Make sure you're near a good bright white light source.

[Veng]

It's a titration curve, so what happens is something like this:

 

Swirl for a second or two to make sure the solution is uniform. Make sure you are under a natural light source and not some 20K+ tank light source as this can screw up your ability to read the colors, 6500K is recommended.

 

I look for the point where adding an additional drop doesn't cause a significant color change and read one drop less (to account for the extra drop that did nothing).

[LarryMoeCurly]

^ What he said.

 

I use the Red Sea test kit. I add 0.4 mL like it says to get it to the starting color. Then I add enough titrant to get it to turn light pink/orange. IIRC, you have to add a lot more to get it become much darker.

[flypenfly]

IMO the light pink/orange doesn't match the color on the card or youtube video.

[markalot]

My Pro kit turns pink/red, not orange.

 

The color match is not important, when it stops changing color you are at the top of the curve Veng posted above, that's the important point. I note the point at which a strong color change has taken place, then continue to add drop by drop until I fail to notice any additional color change. Generally the KH is just below the last color change.

[flypenfly]

Then why on earth would they print a specific color on their instruction card? That seems really dumb of them.

[Veng]

Then why on earth would they print a specific color on their instruction card? That seems really dumb of them.

Because it's a pictorial instructions, not the full manual. All it is saying is it goes from blue to a red, nothing more. It's a titration, not a coloration test.

[flypenfly]

From their calcium tiltration test, they specifically say that the color turns to violet then to blue which is when you're done. I assumed this was the same thing where it goes from the light color to the much darker color on their chart.

[steelfin]

Thanks for all the replys...I guess I need to play around with it a little more...

 

Is there a solution with a known alkalinity that I can test against?...This would allow me to see the color change at the know ALK level...

 

Thanks

 

SF

[LarryMoeCurly]

You may be able to use a known concentration of baking soda (NaHCO3) if you are quite good at stoichiometric calculations and maybe the henderson-hasselbalch and a few other equations that I have long forgotten.

[Veng]

If you were crafty and did some googling, you could probably use saturated lime water /kalkwasser as a known reference.