Hach CA test kit - what am I doing wrong?

[stevenp]

Just got my new Hach test kit:
145701, Hardness (Total & Calcium) Test Kit, Model HA-4P MG-L, Drop Count Titration, 20-400 mg/L, 100 tests

I'm trying to use the dilution method. Here's what I'm doing/getting:

1) Fill jar with 6 parts RO/Di water and one part tank water, total volume equaling one measuring tube (1/7 tube tank water, 6/7 tube RO/Di).

2) Add 2 drops of solution 8n, Potassium Hydroxide.

3) Add one packet of CA indicator

4) Add drop wise EDTA titrant

It takes 11 drops to change from pink to blue, so 11 x 7 = 77, 77 x 20 = 1540 mg/L or ppm CA. I know this can't be right.

I tried a different approach by using one full measuring tube of tank water, then added 6 tubes of RO/Di. I added the two drops of Potassium Hydroxide, then the packet of indicator. As soon as I added the indicator the sample immediately turned blue. Can't even get a reading.

What am I missing here?

 

[penguinsix]

You should be reading the number of ml you are using. if around .20-.25 then you are in the good range, higher or lower then you are too high or low in ca. Don't know what ml amount equals what ppm, sorry but im not that good , alternatively i could just look at everyone's tests in the spreadsheet i have and tell you.

 

[Greg Hiller]

Josh,

He's running the test (I think) with the original reagent that came with the kit, not the homemade stuff the club is using.

I just dug my old directions for the kit out. If you are using the reagent that came with the kit and have diluted the sample as above, you performed the calculation correctly...except, you are getting a reading in mg/l (ppm) of calcium carbonate. Most people in the hobby use numbers in terms of calcium. To get this number multiply by 0.4 (has to do with the weight fraction of calcium carbonate that is calcium). 1540 * 0.4 = 616 ppm. High, but within believability.

To increase the accuracy of the test you should get a hold of a 1 ml syringe and add the reagent drop wise using that. Continue to use the sample dilution as you have done. Then multiply the number of ml used to get the color change by 854 for ppm calcium.

 

[Greg Hiller]

Also, the club has some samples of water with known concentrations of calcium and magnesium. If you come to a meeting we can give you a small sample to test your kit against.

 

[stevenp]

Thanks Greg and Josh. Which of the two dillution methods I list above are you using at the meetings? Are you using a total of 1 measureing tube? Or are you using a total of 7 measureing tubes?

If I'm at 600ppm then my Salifert is WAY off.

 

[Greg Hiller]

You are supposed to dilute your sample to 1/7th it's original concentration in DI water. Then follow the test kit instructions in exactly the same way as if you had not diluted your sample.

 

[stevenp]

Greg, unless I'm misunderstanding, both ways produce a sample of tank water that has been diluted to 1/7 of its original concentration. The difference being the total volume of the diluted sample.

Should the total volume of the sample being tested equal one, or seven, measurement tubes?

If you're supposed to follow the directions as if the sample was not diluted, then I would assume you're suppose to use the "7 tube" approach. When I tried this, the sample went blue as soon as I added the indicator.

 

[penguinsix]

use 3 ml of tank water and 18 ml of DI. Then mix and use the vial that should have come with it to get the sample (a little above 5 ml) to do the test with. If you still need help with this tomorrow I'll be more than happy to help.

 

[Greg Hiller]

What Josh said. You must understand that this is a titration. You put X atoms of calcium into the tube and then add EDTA until it 'sucks' up all the calcium. If you dilute the sample, but still end up adding the same number of calcium atoms to the sample tube, then you will not get an accurate reading.