Alk titration

[Sweetaz]

I'm trying to figure out how to test for alk using sulfuric acid titration and a pH probe. I have access to testing equipment used for water treatment. The Hach procedure we use calls for 50mls sample, .02N sulfuric acid solution added to a buret, and titrated to color change using BCG. Then you multiply mls of acid used by 20 to get ppm CaCO2 equivalents.

I used 50ml tank water sample and titrated the acid to the sample to a pH of 4.2. This took 6ml of the acid solution. So, 6x20 =120 ppm. Does this sound right?

 

[Larry L]

@JimWelsh

 

[JimWelsh]

I'm trying to figure out how to test for alk using sulfuric acid titration and a pH probe. I have access to testing equipment used for water treatment. The Hach procedure we use calls for 50mls sample, .02N sulfuric acid solution added to a buret, and titrated to color change using BCG. Then you multiply mls of acid used by 20 to get ppm CaCO2 equivalents.

I used 50ml tank water sample and titrated the acid to the sample to a pH of 4.2. This took 6ml of the acid solution. So, 6x20 =120 ppm. Does this sound right?

Your math is correct. I suggest the endpoint in seawater should be pH 4.5, not 4.2. You have 2.4 meq/L alkalinity, or 6.72 dKH, or 120 ppm as CaCO3.

 

[WWIII]

http://www.reefedition.com/a-diy-alkalinity-test-by-randy-holmes-farley/

 

[Sweetaz]

Your math is correct. I suggest the endpoint in seawater should be pH 4.5, not 4.2. You have 2.4 meq/L alkalinity, or 6.72 dKH, or 120 ppm as CaCO3.

http://www.reefedition.com/a-diy-alkalinity-test-by-randy-holmes-farley/

4.5 endpoint you say? The article WWIII posted says that as well, but in this(older) article, also by RHF, he says 4.2 https://www.advancedaquarist.com/2002/2/chemistry

 

[WWIII]

4.5 endpoint you say? The article WWIII posted says that as well, but in this(older) article, also by RHF, he says 4.2 https://www.advancedaquarist.com/2002/2/chemistry

Ghl kh director uses 4.5 as well. I know fresh water is different. I'm sure Jim or Randy can explain the difference and why 4.5 is used.

 

[JimWelsh]

It's complicated. There's the theoretical, and then there's the empirical. Empirically, I've been doing a lot of these manual titrations lately, and finding the inflection point, and calculating the pH at the inflection point. It is generally within 0.05 pH units (and usually even tighter than that) of pH 4.5, in my current experience. I do go into all the theoretical stuff, and came to a conclusion that the theoretical endpoint is between 4.2 and 4.3 here: Salifert KH/Alk testing help