Hach Calcium test kit- A little advice needed

[mkirda]

Ok, Craig and Randy...

Just finished using the kit. First of all, the color in the beginning is NOT pink as the instructions say, but a shade of purple. At 25 drops of EDTA, it is just a deeper shade of purple. Not blue, but purple still. I doubt the calcium level is at 500+.

I'm not sure about it, and wanted some advice on it. Craig, you said somewhere else that you diluted the sample, but then used the digital titrator. Using the kit as is, what do you suggest?

BTW, Craig, I have to say that the Phosphate test is great. The Alkalinity test also seems good- a quick question here... I used the lower range, needed 48 drops of titrant, which equals 240mg/l.
I'm used to the meq/l measure- Is there a conversion?
Or is mg/l equivalent to meq/l here?

Regards.
Mike Kirda

 

[randy holmes-farley]

Remembger, the Hach kit reads im ppm CaCO3, not Ca++. You have to multiply by 0.4 to get ppm Ca++. So you need at least 40 or so drops of EDTA.

Keep going and you'll see true blue!

 

[mkirda]

Remembger, the Hach kit reads im ppm CaCO3, not Ca++. You have to multiply by 0.4 to get ppm Ca++. So you need at least 40 or so drops of EDTA.

Keep going and you'll see true blue!

Ok, will try again... Thanks, Randy.

Do you know the answer on the alkalinity units?

You know, I have another question too...
As far as cleaning the test tubes/parts... Is rinsing with water, then with rubbing alcohol, then airing dry upside-down reasonable?

Regards.
Mike Kirda

 

[mkirda]

Remembger, the Hach kit reads im ppm CaCO3, not Ca++. You have to multiply by 0.4 to get ppm Ca++. So you need at least 40 or so drops of EDTA.

Keep going and you'll see true blue!

Ok, Randy, I did it again two ways:

First, added 60 drops, equivalent of 480 ppm Ca+ ion. Still purple, but blue is hinted at...

Next, from: http://fins.actwin.com/reefkeepers/reefkeeper3.html

"Hach Test kit details:

Calcium:
Cat. No. 1457-01, Model HA-4P, $47.50, 100 test Dilute your sample 2:1 (Distilled:Saltwater). Each drop of titrant will equal 24mg/l of Ca++.
Dilution saves titrant, and yields clearer results with sufficient accuracy. "

Added two vials of RO water, then salt water. Added two drops of acid, mixed. Water becomes murky white. Added chemical pouch and mixed. Does not mix clear- murky and obviously not mixed- almost like liquid flakes in the water... Got the same with my LaMotte kit before...

Anyway, start dispensing EDTA titrant. After 30 drops, nada. Not blue. If I remember the conversion properly, mg/l = ppm, so this is over 600 ppm Ca++...

I started adding titrant about 0.5 ml at a time. After about 2 ml, it turns blue.

I'm still doing something wrong. Have to be. My water chemistry cannot be that out of whack without major precipitation going on...

Any ideas appreciated.
Regards.
Mike Kirda

 

[randy holmes-farley]

The Alkalinity test also seems good- a quick question here... I used the lower range, needed 48 drops of titrant, which equals 240mg/l.
I'm used to the meq/l measure- Is there a conversion?
Or is mg/l equivalent to meq/l here?

As with other Hach kits, the alkalinity units are ppm CaCO3. You need to divide ppm CaCO3 alkalinity by 50 to get meq/L alkalinity. This article describes these units and other things about alkalinity:

http://www.advancedaquarist.com/issues/ ... mistry.htm

 

[randy holmes-farley]

IME, the calcium test turns a very clear blue. Keep going past 60 drops. You'll get there. When I do the test (I haven't in quite some time) I would add the fiorst 40 or so very fast to save time. Dilution is fine and saves reagent. It may even increase accuracy (I'm not sure) but it clearly decreases precision.

I presume that you are adding base, not acid? KOH? The precipitate is magnesium hydroxide, that takes it out of the picture for the titration. Don't worry about cloudiness.

 

[randy holmes-farley]

Is it working for you yet?

 

[mkirda]

Is it working for you yet?

Randy,

To be honest, I haven't had the time to try it again.
Maybe this Sunday...

Will let you know.
Regards.
Mike Kirda

 

[mkirda]

Is it working for you yet?

Hi, Randy.

I think I've gotten the alkalinity to work. mg/L = ppm at this point, so you need to use the higher end in order for it to work right.

As far as the orthophosphate and the low-range nitrate, the results on my tank are essentially zero. There was no difference at 0 on the color wheel...

The Calcium test is still a bit problematic, however.
I tested the sample the first time. The instructions say the sample should turn pink. It turns purple. Added 40 drops of EDTA titrant- it just is stuck between purple and blue...

So I mixed up a couple of gallons of fresh IO and RO water. Got the samples to an identical SG level based on my handheld refractometer (1.024 in this case...)

Ran the test, but goofed and added the packet before I added the drops. Poof, nice pink color... Hmmm. Added 45 drops of EDTA, seems sorta blue. Tests out to 360 ppm Ca. Not bad for freshly mixed salt water...

For the heck of it, ran the original sample again in this way. Hmm... Much more pink than purple... Turned bluer, much bluer than the original at around 40 drops of EDTA. This tests out to 320 ppm, a bit low...

It almost seems like I'm doing it right. However, I am most definitely reversing the order that the instructions tell me to put the regents in... Something tells me that this can't be good.

Being that it is already nearly 11pm, I'm going to put off another round of testing the two samples (in the right regent order) until tomorrow evening.

More to follow.
Regards.
Mike Kirda

 

[mkirda]

Is it working for you yet?

Hi, Randy.

Second round tonight...
For the record, this is for Hach test kit Model HA-4P, Cat. No. 1457-01...

I followed the instructions to the letter on three samples:

1) IO freshly mixed yesterday
2) RO water
3) Tank water sample

All turn more purple than pink. #2 was easy, taking one or two drops to reach blue.

#1 took roughly 50 drops to reach a reasonable blue color. Adding more beyond this seemed to make no color change (up to 68 drops...)
#3 took roughly 35-38 drops to reach a similar shade of blue, but even so, the shading was different than sample #1. Adding more didn't change the color more towards #1 though- The weird shade was still there. There is no true clear blue end point as far as I can tell.

Any ideas?
(Couple of assumptions here: Instructions for Calcium hardness do not indicate that you are to mix the samples after adding two drops of 8N Potassium Hydroxide or after adding the CalVer 2 powder. I have been. Can I assume that this is not a mistake?)

Regards.
Mike Kirda

 

[randy holmes-farley]

The numbers that you are getting do not seem unreasonable to me. Your tank appears low in calcium.

As far as the color, well, that may depend on the lighting and what other stuff is in your water.

You can try diluting the tank and IO samples with Ro/DI water to see if the endpoint is clearer.

Mixing is appropriate as you did it.

 

[mkirda]

You can try diluting the tank and IO samples with Ro/DI water to see if the endpoint is clearer.

Randy,

Someone following this discussion (Tom is his name) and I have been comparing notes on the Hach Calcium test off-board.

One question he asked of me I couldn't answer. Maybe you can.

When diluting the sample 2:1, do you do the rest of the steps on the entire 3 vial amount, or do you mix the three vials, then use just one vial full of the diluted sample? (Is that clear?)

In other words, the vial seems to be 6ml. Do you mix 12 ml of RO water and 6ml of sample, then do the test on the entire 18 ml? Or would you mix it, then do the test on just a 6ml portion of the diluted sample?

Also, if possible, please confirm that at this 2:1 dilution, one drop EDTA titrant equals 24ppm Ca ion.

Thanks.
Regards.
Mike Kirda

 

[randy holmes-farley]

When I suggested dilution, I meant remove half of the calcium ions along with half of everything else. Dilution without removal might help, but it isn't what I intended.

So mix 1 cup RO/DI water with 1 cup tank water. Then test that mixed water using the standard conditions of volume and everything else. Then at the end, multiply the result by 2.

 

[rskibum]

When you do a dilution: Take the Final Volume,
and Divide that by the Initial Volume you diluted with, Then multiply that number by the concentration you read in the diluted sample.

So in your case The Final Volume is 3 Vials, And the Initial Volume is 1 Vial
So you have preformed a 3x dilution

So if you read 100ppm in the diluted sample your water contains 300ppm

You should mix the dilution, then use one vial's worth to run the test.

The test kit might also have a optimum range. Check the literature. If it for example says the range is 1 - 100 ppm. It is probably most accurate at 50ppm, the middle of the range. So try to make your dilution come out at 50ppm.

 

[rskibum]

So, I read the link earlier in this thread, to the article about alkalinity in reef aquariums. I found it interesting, it helped me to remember a few things.

:idea: I'm thinking a dilution wouldn't be as accurate. Help me understand this Randy. In a 'normal' colormetric or other test where the calibration curve is linear, what I described in my previous post, or yours (1:1 , result * 2) would work just fine. But I'm thinking that since pH is a logarithmic function, the dilution wouldn't work the same?

Am I making any since here?

Ryan

 

[randy holmes-farley]

Ryan:

The Hach kit, and others, are clearly not as precise when diluted because the individual drops correspond to larger differences. However, the Hach calcium kit only claims to be accurate to 400 ppm Ca++ ion (or some similar value), so there is some concern as to it's accuracy in many tank situations.

The desire to dilute then trades precision for a possible gain in accuracy.

I personally have not noticed any significant change in the final vlaue when I've diluted my tank water before the Hach calcium kit.

 

[Habib (Salifert)]

Let's make things a bit more complicated.

Most calcium test kits are either calibrated with freshwater or with seawater standards.

Most of the calcium test kits I have tried are sensitive to seawater matrix effects.

In general if a calcium kit is calibrated with freshwater then it will give approx. 10-15% higher value when used with seawater.

This means that for such a kit a measured value of 400 ppm could mean a true value of 350 - 360 ppm.

I have never tried the Hach kit but it could be interesting to know if this kit is sensitive to such matrix effects and if it is then it is IMO good to know how it was calibrated.

In case anyone is interested I will be glad to give two simple procedures to test the kit's (or any other kit's) sensitivity to such matrix effects.