Exceeding Salifert Test results range - linear, addition?

[user 246304]

I feel like I should know this but not surprised I don't. Just curious to get some numbers down on our alkalinity and so got a Salifert test which I plan to do, along with H2SO4/bromcresol green titrations.

The test gives meq values for up to 1.0 ml of known titrant addition; I had to refill the syringe and I ended up with an additional 0.07 ml of the titrant. I presume it's obviously linear, but don't know. In other words, I know I added 1.07 but I don't know their alkalinity meq = 5.59 (1.0 ml titrant) + roughly 0.84 (based on an intermediate correspondence of 0.06 and 0.08 of added titrant). So, I came up with a total meq Alk of 6.43, and Total alk. 321 ppm.

Was this correct? I'm friends with the guy who is at least high up in our water utility. On New Year's told me we switched around 12/20 or so to another, single well (we draw from 2 wells, in summer). He told me Alk is about 318, so this gives some comfort that my figure of 321 is money. Then again, I may have it all wrong so this figure is meaningless.

 

[cire]

Yes, you are correct.
It's probably easier in your case to do the test using 2ml of your water and 2 drops of indicator and double the result.

Looks like you've highly alkaline water.

 

[ajdelange]

Yes, it's linear in acid use but reciprocal in sample size:

alk_mEq =1000*mL_acid_used*acid_normality/mL_sample

Thus, where alkalinity is high you can
1)Just add extra acid as you did
2)Use stronger acid
3)Decrease the sample size

Decreasing the sample size means to use less than the usual volume. It may well be convenient to dilute the reduced sample to normal volume with DI water so that you have a big enough volume to judge the color change, for example. Note that the dilution does not have to be exact.

You can find the normality of the acid furnished by Salifert as

Normality = 5.59*mL_for_normal_sample/1000

based on their instructions indicating that 1 mL of acid corresponds to alkalinity of 5.59 mEq/L in their normal sample size.

 

[user 246304]

OK, great, thanks AJ. I was actually thinking of a larger sample size to get a better resolution, so was a bit surprised to get this result. Yep, really alkaline, and I knew that going in just off of water quality reports. Thank you on this further info, will try it again and see what I come up with (and thanks for the better understanding).