More accurate way to determine Alkalinity
After trying to get a accurate reading with my color titration Alkalinity test kit and failing. I can't tell the difference between the slight color changes. My gf gives a different reading than I do because we see the color differently. I just don't think those kits are very accurate. So I figure I have a digital pH meter why not do a HCL titration to saturate the alkalinity and calculate the amount of HCL used to get total alkalinity. Note: I am not a chemist but this is high school chemistry so if you are a chemist please take a look at my method and scrutinize.
The hard part; Making a 0.1 N solution of hydrochloric acid.
Grab a bottle of muriatic acid from your local home depot. Mine sells 20 baume 31.45% w/w HCl. Now we have to calculate the Morality of this solution. Most bottles will have a baume number on them. You will need this. To calculate the total weight of HCl in the bottle we take:
Specific gravity * %of HCl = g of HCl
Specific gravity can be found by 145/(145-baume).
So in my case I have a s.g. of
145/(145-20) = 1.16
Total HCl is 1.16 *1000 * .3145 = 364.82g HCl
Now I can find my morality, the molecular weight of HCl is 36.46g
364.82g/36.46g = 10 M
So in sort, the HCl home depot sells is 10 M. This is way too strong for practical purposes so we want to make a dilution of this for our titration. We are going to need a 0.1 N stock solution of HCl so first we will make a 1 N solution then make a 0.1 N solution from that. You should really make the stock solutions in phases like this for accuracy.
M1V1 = M2V2 we want to make a 1000ml 1 M solution from a 10 M so.
10x = 1 * 1000
x= 100 ml
So we add 100ml to 900ml of DI water (make sure your water reads 0 on your TDS meter.) and we get a 1 N stock solution. The pH of this solution should read 0.1 if your meter can handle this.
Now we do the same thing to make a .1 N solution from this solution. Let’s make a 1000ml of this stuff so we don’t have to do it again for a long time.
0.1*1000/1 = 100 ml. So add 100 ml of the 1N solution to 900 ml of DI water and you will finally have your stock solution!!! Measure the pH of this solution as a check. It should be 1.1 .
Easy part; Testing
Now take a sample of your aquarium water, you can use any volume but I like to use 100ml because my pH meter sits nicely in the cup without me holding it at this level. Stick in your pH meter to your sample. You should get the same reading you do when you stick it directly in your tank. If not start over with a clean sample container. Now draw up some 0.1 N HCl solution in to your 5ml syringe. And start dropping into your sample 0.5 mls at a time and watch your pH meter. You are adding a strong acid so your pH should fall rapidly. Once your pH hits 5 start to slow down and add a drop at a time until your pH hits 4.5 . The reason we are shooting for 4.5 is because this is the point where all available carbonates have been converted to carbonic acid. You would think this would be pH 7 but it’s not as carbonic acid is a weak acid and you have to really saturate it etc. Anyway, once you have reached pH 4.5 you are done and now can calculate your total alkalinity.
Alk mg of CaC03 /L = A x N x 50,000 / mL of sample
Where A = ml of acid added
N = normality of your HCL
So if you added 4ml of 0.1 HCL before you hit a pH of 4.5
4*0.1*50,000 / 100 = 400 ppm CaC03 equivalents or your total alkalinity is 2 meg/L (mg/L / 50 = meg/L).
Very accurate! And cheaaap.
