Alkalinity titration test question

[ss88]

Hopefully someone could answer this for me. I’m not receiving expected results with a sodium bicarbonate/water solution mixed to a dKH of 25.

Recently I started measuring alkalinity using the method of titration described here. Wanted to confirm results against a standard solution.

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

Alkalinity is one of the most important measurements that a reef aquarist can make. It can become rapidly depleted in many aquaria, requiring frequent measurement in order to maintain stable levels.

www.reefedition.com

I’m using a laboratory grade scale, metered pipette, magnetic stir, calibrate pH probe with each measurement, cleaning equipment with the deionized water. Weighing acid solution and sample water.

Here is what I did. Perhaps someone can enlighten me to the issue.

Mixed solution in 5 gallon hdpe container.
8,817.57 grams of RoDi water
6.61 grams sodium bicarbonate

measured out 1000ml/1000 grams of sample solution.

Started titration expecting to add 89.28ml or 87.53 grams of 0.1 N hydrochloric acid. Added 80.80 grams of hydrochloric acid and pH of 4.39.

The only thing i can think is my conversion for volume to weight of .1N hydrochloride acid is incorrect.

Is titration accuracy only limited by equipment accuracy?

 

[ss88]

Do I need to aerate the solution to drive off co2?

 

[Randy Holmes-Farley]

The sodium bicarbonate may have been partly sodium carbonate. It can also have moisture in it. That's one reason I do not usually recommend it as a standard.

"Started titration expecting to add 89.28ml or 87.53 grams of 0.1 N hydrochloric acid."

How did you get those numbers? I understand the 89, but the 87 came from what?

 

[Randy Holmes-Farley]

Do I need to aerate the solution to drive off co2?

No

 

[ss88]

If I remember correctly, I looked up the specific gravity of 0.1N hydrochloric acid. ~1.017

Hydrochloric Acid, Solution, 0.1 N

Hydrochloric Acid, Solution, 0.1 N, 1L

www.thomassci.com

Is this incorrect? If not, what would 100ml of 0.1N hydrochloride acid weigh?

What is a better standard solution for alkalinity?

Thank you for the help.

 

[Randy Holmes-Farley]

The density of 0.1 N HCl (~0.36% by weight) at 20 deg C is close to 1.00 g/mL.

Hydrochloric Acid

<div class="mainContentTitle">Density concentration table (+20°C)</div>

www.mt.com

 

[ss88]

The density of 0.1 N HCl (~0.36% by weight) at 20 deg C is close to 1.00 g/mL.

thanks. Good to know.

Is sodium carbonate a better solution for a reference test?

 

[Randy Holmes-Farley]

thanks. Good to know.

Is sodium carbonate a better solution for a reference test?

yes. It can be baked dry. Sodium hydroxide is also good.

 

[ss88]

Hello again,

Still having issues with pH titration. 0.1m solution or pH probe appears to be inaccurate.

Prepared solution of 6687 grams rodi water with 2.97 grams of sodium hydroxide for 31.52dKH.

Prepared sample by adding 166 grams of solution 31.52dKH in cleaned beaker and further diluted sample with 333.34 rodi water. Sample should have dKH of 10.5~

First tested sample water with red sea alk titration and received value of 10.4dKH. Within margin of error. +-0.14dKH

Sample size reduced to 489.8 grams after removal of 10ml for red sea alkalinity titration.

Titration 4.5pH test ended with 16.1 grams 0.1m hydrochloride acid. Was expecting to end with 18.36.

Suggestion for proper corse of correction, replace pH probe and procure new .1m solution?

0.1m solution was only opened 2 days ago.
pH probe is a year old but calibrates correctly.

Procured .1m hydrochloride acid from Congeriem.com in 1L format.

 

[ss88]

 

[Randy Holmes-Farley]

I seem to get very different values.

2.97 grams of sodium hydroxide (40 g/mole) is 74.25 millimoles

6687 grams of ro/di water at 25 deg C (density =0.997 g/mL) occupies a volume of 6707 mL.

The volume won't change much on addition of NaOH, but we are not making that correction.

Thus, the NaOH solution is 74.25 millimoles/0.6707 L = 110.7 mM or 0.1107 N = 110.7 meq/L

That NaOH solution of 111 meq/l = 310 dKH.

That doesn't agree with the statement:

"Prepared solution of 6687 grams rodi water with 2.97 grams of sodium hydroxide for 31.52dKH."

What am I missing?

 

[ss88]

2.97 grams of sodium hydroxide (40 g/mole) is 74.25 millimoles

With you here.

6687 grams of ro/di water at 25 deg C (density =0.997 g/mL) occupies a volume of 6707 mL.

Yes, with you here as well. I neglected conversion because of insignificant difference.

The volume won't change much on addition of NaOH, but we are not making that correction.

Again, intentional neglected calculation because of insignificant difference.

Thus, the NaOH solution is 74.25 millimoles/0.6707 L = 110.7 mM or 0.1107 N = 110.7 meq/L

lost me here, it was 6.707L
74.25/6.707 = 11.07 meq/l = 30.997dKH

After I made this solution, I further diluted the titration sample 1:2. One part 30.997 dKH solution to two parts rodi water. 166.66 grams dKH solution to 333.34 grams rodi. Understood that the solution density will differ slightly.

Thus the titration sample should have had a dKH of 10.33. When I tested the sample with a red sea pro test kit I received a value of 10.4dKH.

 

[Randy Holmes-Farley]

Ok, I see my stupid mistake.m (decimal point in wrong place).

Here’s the corrected version:


Thus, the NaOH solution is 74.25 millimoles/6.707 L = 11.07 mM or 0.1107 N = 11.07 meq/L

That NaOH solution of 11.1 meq/l = 31dKH.

 

[Randy Holmes-Farley]

It is not 31.5 dKH due to the assumption that 1 g equals 1 mL. It does not, and is temp dependent.

However, that makes your alk standard more dilute than expected, not more concentrated, so you should have needed less acid, not more. I’m not sure what the specific issue is.

 

[ss88]

Yes I agree 31dKH. I further diluted this to 1:2.

So the titration sample was 10.33dKH @ 489.8g roughly.
Should have taken (10.33/280)*489.8=18.07g of 0.1m hydrochloride acid.

Titration 4.5pH test ended with 16.1 grams 0.1m hydrochloride acid. Thats a fairly large difference. 1dKH.

The red sea reefer pro dKH test was gave 10.4dKH when I tested the 1:2 diluted solution. So I’m fairly sure the solution was 10.33dKH

Is there a margin of error with these test outside of equipment calibration? Can you explain why I ended up using roughly 2g less acid then expected.

 

[ss88]

Any ideas on this?

 

[Randy Holmes-Farley]

Sorry, but can you summarize exactly what you did step wise?

As a separate question, did you dry the sodium hydroxide before weighing it?

 

[ss88]

I did not dry the sodium hydroxide.
Can I do this in an convection oven? Is there any risk of noxious gas? 350F for an hr?

 

[Randy Holmes-Farley]

I did not dry the sodium hydroxide.
Can I do this in an convection oven? Is there any risk of noxious gas? 350F for an hr?

That should be fine.