[NotThePainter]

I want a higher accuracy from my API Nitrate test kit. I know I should dilute the water and then multiply. But with what? I can think of three different ways to dilute

1. Measure out 50ml of tank water, and test
2. Add 45ml of RO to 5ml of tank water, and test
3. Add 45 ml of RO to 5ml of tank water, extract 5ml of this mixture, and test

The first makes sense since the reagent is diluted. The 2nd one makes sense since the test sample is diluted. And the third make since the test sample is diluted but the reagent isn't.

What's the proper way?

 

[jeffkrol]

How do you calculate serial dilutions? + Example

You multiply the original concentration by the dilution factors for each dilution. A serial dilution is any dilution in which the concentration decreases by the same factor in each successive step. In serial dilutions, you multiply the dilution factors for each step. The dilution factor or the...

socratic.org

.
Lets see if I remember this right...

Take .2 ml add it to 1 .8 ml to get a 10x dilution.
i.e 50 to 5. 5x10 is 50.
Assuming initial sample is 2ml.

 

[Deanna]

The API NO3 test is not the greatest. You will get much better delineation with the Salifert kit (may not need to dilute). If you decide to use the Salifert kit, you can use this dilution detail for increased sensitivity:

80% RO water, e.g.; 8ml RO / 2ml tank.
Use same 1ml sampling, 4 drops & 1 spoon.
Multiply results by 5

Incidentally, with the Salifert kit, and many others, the meniscus makes reading more difficult (reflection/light distortion issues). You can eliminate the meniscus (to a point) by spraying a silicone lubricant into the vial and wiping it clean (it will remain on the surface). It won't affect the test and will last for many tests. Once you notice the meniscus returning, repeat the process.

 

[Immortal1]

Much higher accuracy with the Salifert NO3 test. In my opinion, the best part of that test kit is the supplied cup.
When viewed from the side it magnifies the reading by a factor of 10 (i.e., your sample contains 5ppm - looking thru the side of cup it's 50ppm).
In my case, I dilute the tank water first. 1 part tank to 3 parts RO.
Then I take my sample as directed, add the chemicals, wait, and when I look thru the side of the cup it's a perfect 25ppm on the chart.
25 / 10 = 2.5
2.5 * 4 = 10, which is what I was hoping to get.

 

[Deanna]

We should point out that the Salifert NO3 test we are talking about is the "Profi" kit, not the freshwater kit. IMO, the freshwater kit is not as good due to the apparatus used (the titration method is the same).

 

[Savetheplants]

This is a good one. I would say option number three but I never thought about option number one. I have tried using small measures of water preparing samples but I think option number three is better.

 

[NotThePainter]

This is a good one. I would say option number three but I never thought about option number one. I have tried using small measures of water preparing samples but I think option number three is better.

I did option 3 and it worked out pretty well. I was able to measure about 50ppm before the water change and the, after, it was hard to tell, since the color was > 0 and less than 5 so I ran it again with 4:1 dilution instead of 9:1 and it was right on 5 so that meant 25ppm, which is what I would expect after a 50% water change.

 

[Savetheplants]

I did option 3 and it worked out pretty well. I was able to measure about 50ppm before the water change and the, after, it was hard to tell, since the color was > 0 and less than 5 so I ran it again with 4:1 dilution instead of 9:1 and it was right on 5 so that meant 25ppm, which is what I would expect after a 50% water change.

I spoke to a technical support representative at the LaMotte company a couple of months ago. He said a nine part dilution is the limit to diluting samples. I just started testing after water changes last month. I always used to test before water changes. Option 3 is good because I think it's more accurate than trying to prepare a small sample.