DIY Alk Test question

[hawkinsrgk]

I just did the DIY Alk Test and got an answer I wasn't expecting. Wanted to check to see if there appears to be something I did incorrectly.

For the components
New Chemglass CG-1617-100 Glass 100mL +/- 0.10mL Flat Bottom Heavy Duty Wide Mouth Volumetric Flask, with PTFE Stopper, 180mm Height
New bottle Hach 1481253 Hydrochloric Acid Standard Solution, 0.10 N, 1 L
New Pinpoint PH probe and meter calibrated with 4 and 7 solution. The new probe would stay at 4 and 7 for over a min before I determined everything was ok.

Procedure

1. Got 100ml of tank water and put in the volumetric flask. The bottom of the curve of the water was exactly on the 100ml line.
2. Pour that water in a 200ml beaker. Gave the flask a couple of shakes to get the last drop or two out of the flask
3. Used a 2ml syringe with the pink plastic thingy (technical term) that comes with the salifert test kit for the HCl solution. Made sure the end of the black stopper was on the 2ml mark
4. Added the 2ml Hcl solution
5. Drew up 1ml with a 1ml syringe with the pink plastic thingy and started adding that drop by drop. Between each drop I would stir the water with the syringe.
6. When the PH probe read exactly PH 4.3 I stopped.
7. This was exactly 2.7ml of HCl that was added to 100ml of tank water.
8. That would give me an Alk reading of (2.7)(2.8) = 7.56 Dkh.

I normally use Salifert to test Alk. Test it every day so pretty familiar with the way the test works. The expiration date on the test is 2020. Salifert Alk test shows 8.45 Dkh.

Not really sure where my error is, but 7.56 and 8.45 is a larger difference than I was expecting.

Thanks
Randy

 

[Randy Holmes-Farley]

I don't see any issue with the procedure. I'm not sure what to say about the discrepancy. Assuming the syringe was correctly dispensing 2.7 mL, I'd believe the DIY.

 

[hawkinsrgk]

Thank you Randy for taking a look. I didn't think I did anything wrong but just wanted to check with you

 

[JimWelsh]

Looking for all possible sources of error I see, even small or seemingly insignificant ones I have the following comments:

The volumetric flask is very likely marked "TC", as opposed to "TD", which means "To Contain". But, you are using it "To Deliver". Address this issue by quantitatively transferring the contents of the volumetric flask to the titration container by washing it out with a small amount of RO/DI water, rather than simply shaking it out.

I question the purity of the water used to rinse the titration container, and also the VF prior to use.

If I had this challenge, I would use the same 1mL syringe to transfer all of the titrant, rather than using two different syringes. The 1 mL syringe is likely more accurate than the 2 mL syringe is.

Please elaborate the steps you used in the Salifert test. I know what the instructions say. I want to verify the method you are using.

I would NOT stir the titration container's contents with the syringe. I would swirl it.

How long did you let the pH meter sit and stabilize when deciding when to stop at pH 4.3?

The size of the discrepancy is rather large, I agree. Especially for someone trying hard to do a good job.

You might consider trying to create an alkalinity standard, and testing that against both methods.

Good luck with narrowing down where the discrepancy is coming from!

 

[hawkinsrgk]

Hi Jim

Thank you for the help. Will try to address everything that you pointed out.

The volumetric flask is very likely marked "TC", as opposed to "TD", which means "To Contain". But, you are using it "To Deliver". Address this issue by quantitatively transferring the contents of the volumetric flask to the titration container by washing it out with a small amount of RO/DI water, rather than simply shaking it out.

I looked the volumetric flask and did see that it is marked "TC". How much RO/DI water should I add to the flask to wash it out? Trying to make sure I don't cause a greater error by trying to fix the error.

I question the purity of the water used to rinse the titration container, and also the VF prior to use.

What I do is rinse the titration container out 2 or 3 times with tap water. Give it a good shake and then wipe it out with a paper towel.

If I had this challenge, I would use the same 1mL syringe to transfer all of the titrant, rather than using two different syringes. The 1 mL syringe is likely more accurate than the 2 mL syringe is.

Thanks. I will try using the same 1ml syringe for all of the titrant. The reason I switched from the 2ml to the 1ml was to try to get it more accurate.

Please elaborate the steps you used in the Salifert test. I know what the instructions say. I want to verify the method you are using.

1. Get the 5ml syringe and add 4ml of water to it.
What I do here is draw 5ml of tank water out of the sump 3 times and immediate release it back into the sump. I then draw about 4.5 ml of water into the syringe.
Then I goto my workbench that has a light on it and push the syringe until the bottom of the plastic plunger is even with the line on the 4ml mark on the syringe.
Then I give the syringe a quick flick. This gets the water out that is in the end of the tube on the syringe. I take a look to verify the start of the water is on the 0 mark on the syringe
2. I wipe out the plastic piece with a paper towel and then add 4 ml of water to it.
3. Add 4 drops to the plastic piece.
What I do here is give the dropper a quick shake 2 or 3 times.
Turn the dropper upside down until the drop begins to form. Once the drop begins to form I add the 4 drops. I do this to try to get uniform drops.
4. Add the pink plastic piece to the 1ml syringe and draw 1ml of the titrant. My stop point is where the bottom of the plunger is even with the 1ml mark on the syringe.
5. Push the plunger into the plastic piece until it is around the .6 mark on the syringe. I know my alk is higher than that mark.
6. Add one drop at a time until it turns red.
Hopefully I can explain this. With my left hand I use my index finger and thumb to hold the plastic piece and use my middle finger to tap the plastic piece 8 times between each drop. I use my right hand to hold the syringe to add drops.

I would NOT stir the titration container's contents with the syringe. I would swirl it.

It almost sounds like I should consider getting a magnetic swirl bar

How long did you let the pH meter sit and stabilize when deciding when to stop at pH 4.3?

Maybe 20-30 seconds

The size of the discrepancy is rather large, I agree. Especially for someone trying hard to do a good job.

You might consider trying to create an alkalinity standard, and testing that against both methods.

Will do some searching to see what I can come up with. Hopefully it is not too advanced.

Thank you again Jim for your help.

 

[hawkinsrgk]

This is where I am at so far with the Alk standard with 9 dkh and Soda Ash. Hopefully I am not too far off.

(9dkh)/(2.8dkh)(meg/L) = 3.21428571
molar mass of Soda Ash is 105.98844 g/mol
Therefore 1 millimole is .10598844 grams by weight
So I would need (.10598844)(3.21428571) = .34067713 grams of Soda Ash in 1L of water to make 9dkh

I am not able to weigh that accurately. My scale only does tenths of a gram.

Could I do 34.1 grams of Soda Ash in 1 Liter of water and then take 1ml of that solution and add 99ml of RO/DI to get 9dkh?

Thanks
Randy

 

[JimWelsh]

This is a good start, Randy, but I see a few things that need clarification.

I think I understand that you intend to create a 9 dKH standard, which would be a 3.21 meq/L standard. Please lose the unnecessary and quite unrealistic excess significant figures, such as "3.21428571". That's 3.21 in my book, because I can rarely justify more precision than that in the real world. While it is appropriate to retain a large number of significant figures while doing the calculations, the resulting final number should be rounded to a justifiable number of significant figures, which is rarely more than three.

Please remember that Na2CO3 has two negative charges per carbonate ion, so while it is true that 1 millimole of Na2CO3 per liter would weigh 0.106 g, it would have twice the alkalinity than what you are calculating for. In other words, you only need 0.17 g of Na2CO3 per liter to make a solution with 9 dKH = 3.21 meq/L, and not the 0.341 that you have calculated above.

When I have to do this kind of thing, I usually do first create a more concentrated stock solution, and then do a serial dilution of that, exactly as you suggest. A better way of describing what you are attempting might be to first create a solution of 17.0 g of Na2CO3 in 1000 mL of water, and then to deliver 1 mL of the resulting solution into a 100 mL volumetric flask, and then fill the flask to the mark with pure water.

Not to derail the thought process here, but another thing to consider while doing this is the ionic strength of the solutions when trying to compare a standard to a real-life saltwater sample. If we just use a little Na2CO3 and pure water to create our 9 dKH / 3.21 meq/L standard, then we have a solution with an "ionic strength" (google it) near zero. But, in fact, seawater has an ionic strength of close to 0.7, which is relatively large. In order to create a solution that has both the alkalinity you desire and also the ionic strength of seawater, then you might consider adding 40.9 g of sufficiently pure NaCl to each liter of 9 dKH standard solution you create. A detailed discussion of why this might be important is more than I'm prepared to go into right now, but I humbly submit that my suggestion is worth considering, especially when using pH to determine an alkalinity titration curve endpoint in a seawater matrix. ;)

 

[hawkinsrgk]

Thank you Jim for your help and detailed explanations. I have created the 17g of Soda Ash in 1L of water solution. Unfortunately I do not have sufficiently pure NaCl, so going to have to wait until I get it. Its too bad I can't run up the street to get these things. I have morton table salt, but thats about it.

Again, I really appreciate your help especially with it being so late when you sent your response.

 

[hawkinsrgk]

Maybe I can find it at a grocery store. Going to give that a shot.

 

[hawkinsrgk]

This is what I picked up. Wanted to see if this would work

I read that natural sea salt is 85% sodium chloride. If this would work could I add 46.8 g of sea salt to the 1 liter of stock solution

 

[JimWelsh]

Just use kosher salt, like Diamond Crystal. It is nearly reagent grade.

 

[hawkinsrgk]

Ok will do. Thank you again

 

[Brew12]

Not to derail the thought process here, but another thing to consider while doing this is the ionic strength of the solutions when trying to compare a standard to a real-life saltwater sample. If we just use a little Na2CO3 and pure water to create our 9 dKH / 3.21 meq/L standard, then we have a solution with an "ionic strength" (google it) near zero. But, in fact, seawater has an ionic strength of close to 0.7, which is relatively large. In order to create a solution that has both the alkalinity you desire and also the ionic strength of seawater, then you might consider adding 40.9 g of sufficiently pure NaCl to each liter of 9 dKH standard solution you create. A detailed discussion of why this might be important is more than I'm prepared to go into right now, but I humbly submit that my suggestion is worth considering, especially when using pH to determine an alkalinity titration curve endpoint in a seawater matrix. ;)

Jim, I hope you don't mind my bringing back an old thread. I'm looking at making my own Alk standard. I have a question about the NaCl addition.
You recommended doing this to create a stock solution.

When I have to do this kind of thing, I usually do first create a more concentrated stock solution, and then do a serial dilution of that, exactly as you suggest. A better way of describing what you are attempting might be to first create a solution of 17.0 g of Na2CO3 in 1000 mL of water, and then to deliver 1 mL of the resulting solution into a 100 mL volumetric flask, and then fill the flask to the mark with pure water.

Is your recommendation to add 40.9g of NaCl to this stock solution or would it be added to the pure water used to dilute the stock solution?

 

[JimWelsh]

Is your recommendation to add 40.9g of NaCl to this stock solution or would it be added to the pure water used to dilute the stock solution?

It is to add 40.9 g NaCl to each liter of "pure water" used to dilute the stock solution. In other words, you could take 1 mL of the stock solution above, put it in a 100 mL volumetric flask, then put 4.09 g NaCl into that flask, and top it off to the mark with pure water.

 

[Brew12]

It is to add 40.9 g NaCl to each liter of "pure water" used to dilute the stock solution. In other words, you could take 1 mL of the stock solution above, put it in a 100 mL volumetric flask, then put 4.09 g NaCl into that flask, and top it off to the mark with pure water.

Perfect, that is what I thought but didn't want to make the assumption!