Salifert Ca test instructions changed

[WheresTheReef]

If you're like me, you usually grab the test kit contents out and toss the box and instructions without a second thought. I just opened a new Ca test today and noticed that the 2nd step changed from 8 drops to 10 drops. Not sure how much error is introduced with 2 fewer drops, but just wanted to give a heads up.

 

[Origami]

It might be worth trying both approaches and seeing if there's a difference in outcomes.

 

Here's what I think is going on, though. The test kit is using a method known since the 50's to measure calcium concentration by EDTA titration. In this method, I think that the NaOH in solution Ca-2 is used to stabilize pH and to precipitate out magnesium (which will interfere with the calcium measurement since it complexes with EDTA just like the calcium ions do). If that's the case, then the amount of Ca-2 used, as long as it's sufficient to meet these objectives, is not critical to the calcium measurement. 

 

You can learn more about this method of calcium determination here. Matching the method to the Salifert directions, I'm guessing that Ca-1 is the Patton-Reeder indicator (dye that complexes with the calcium ion); Ca-2 is sodium hydroxide; and Ca-3 is a diluted EDTA solution.

[WheresTheReef]

It might be worth trying both approaches and seeing if there's a difference in outcomes.

 

Here's what I think is going on, though. The test kit is using a method known since the 50's to measure calcium concentration by EDTA titration. In this method, I think that the NaOH in solution Ca-2 is used to stabilize pH and to precipitate out magnesium (which will interfere with the calcium measurement since it complexes with EDTA just like the calcium ions do). If that's the case, then the amount of Ca-2 used, as long as it's sufficient to meet these objectives, is not critical to the calcium measurement.

 

You can learn more about this method of calcium determination here. Matching the method to the Salifert directions, I'm guessing that Ca-1 is the Patton-Reeder indicator (dye that complexes with the calcium ion); Ca-2 is sodium hydroxide; and Ca-3 is a diluted EDTA solution.

Thanks for the info. Didn't seem to make much difference. I ran each test 2x alternating between 8 and 10 drops for step 2. I drew the water sample just prior to each test.

 

8 drops : 420

10 drops: 415

8 drops : 420

10 drops: 420

[Origami]

The results are reasonable and seem to confirm that the sodium hydroxide is mostly being used to lock up the magnesium so that it doesn't add to the calcium measurement. If you continued cutting back on the drops, or if your magnesium levels were unusually high, you'd probably begin getting higher (and inaccurate) calcium measurements.

[YHSublime]

I often wonder if it matters as sometimes I think an extra drop slips in. Thanks for burning through a test for the sake of research! Tom, thanks for the wealth of information. 

[WheresTheReef]

I often wonder if it matters as sometimes I think an extra drop slips in. Thanks for burning through a test for the sake of research! Tom, thanks for the wealth of information.

While in this case I saw no difference, I still believe consistency is important. Another test step on this or another test kit may result in drastically different results. There is already some inherent error in our kits. Those errors can be exacerbated through messy measurements.

[Origami]

I often wonder if it matters as sometimes I think an extra drop slips in. Thanks for burning through a test for the sake of research! Tom, thanks for the wealth of information. 

 

An extra drop of Ca-2 (which seems to be sodium hydroxide) won't matter given the way the test is probably designed. It's being used to lock up (i.e. remove) the magnesium so that it's not measured by the test. That's why I suggested that the OP try it both ways and report back. Just to attempt confirmation that the Salifert test was probably implementing this particular test method in its product.

 

You see, as I understand the test, it works something like this:

 

• Ca-1 (Salifert's dry reagent) is apparently the Patton-Reed indicator - a dye that's used turns red in the presence of free alkaline earth metals (2+ ions like strontium, magnesium and calcium; and all present in your water). It turns blue in their absence (or when they're sequestered). It doesn't bind these ions very strongly, so they can be pulled away to react or complex with other atoms/ions (such as EDTA). 
• EDTA (Salifert's Ca-3 reagent) will basically sequester these 2+ ions so they're not availalble to the dye.
• Your water has an abundance of magnesium and calcium, and traces of strontium. Magnesium, in fact, is present at levels 4-5x the calcium.. So, to measure the calcium, you need to lock up the magnesium.
• Ca-2 (Salifert's second reagent) is Sodium Hydroxide. It is added so that it reacts with the magnesium ions to form magnesium hydroxide (i.e. milk of magnesia), a solid precipitate. This makes it unavailable to both the EDTA and the dye. A few extra drops of this reagent have little effect because once you've bound up the magnesium, it's done. All youi're doing at that point is stabilizing the pH.
• What remains in the water (and therefore available to the EDTA and the dye) is mostly calcium. There's a trace of strontium, but it's accepted as a small measurement error. 
• You then titrate with a known concentration of EDTA. This is the Ca-3 reagent. It's concentration is known and this is where precision becomes important. Each drop that's added locks up some of the remaining calcium and strontium ions, lowering the concentration available to interact with the dye. As long as there are free calcium and strontium ions available to the dye, the dye keeps its red color. Once it's all been locked up, however, the dye turns blue. So, drop by drop, you're locking up these ions and looking for a color change. When you see it, you read off how much you used from the syringe and cross check it with the table to get an estimate of calcium. 

There will be a quiz.