So one does not get lost here as there are some issues with the article.
What is Alkalinity
http://www.advancedaquarist.com/2002/2/chemistry
From it:
The precise endpoint of a total alkalinity titration isn't always the same pH, but rather depends a bit on the nature of the sample (both its ionic strength and its alkalinity).
For normal seawater, this endpoint is about pH = 4.2. In freshwater it depends strongly on the alkalinity, with an endpoint of pH = 4.5 for an alkalinity of 2.2 meq/L, and pH = 5.2 for an alkalinity of 0.1 meq/L.
Well, the Hach kit was designed for use in fresh water where the pKa of the bicarbonate is much higher than in seawater, and in that situation, it is appropriate. In seawater, however, it is marginal. My tank water took 3.4 meq/L to get down to pH = 5.03, and then an additional 0.4 meq/L to get down to pH 4.00. Consequently, this kit (and others with a similar dye mix) may be
missing out on 10% of the alkalinity simply because it isn't titrating low enough. This difference obviously isn't significant to most reef keepers, but is something to keep in mind when doing such things as comparing test kits to standards (in seawater) or to each other.
Lets look at the plot of 180 ppm for the HACH vs Hanna 240 ppm
So now we need to add 10 % to that HACH reading of 180 = 198
Now the Hanna is reading 18 % to high.
HACH = 198 ppm
Hanna = 240 ppm
240 ppmx 18 % = 43 ppm and 240 ppm - 43 ppm = 198 ppm
Hanna reading - 18 % at ~normal pH and Alk = a more true Alk.
