Salt Water Test Standard

[avn]

I've been following you guys before my pool was even built. First time poster (I think). Last year was 100% trouble free (as far as pool water goes). Thank you all.

I've read in detail here about testing the salt. And I get it - just make sure the cell is happy.

Well, I'm not comfortable with that. Sorry, I'm a dork. I want to test my testers (strips and Taylor kit).

I have a suspicion that my Taylor kit is off because it expires in 45 days. My test strips are from last year but are darn close to what the pool stores measure. I haven't started my salt cell yet - so I don't even know what it thinks right now. My pool temp is really low.

So, my goal is to create my own Salt Standard. Similar to the CYA, TA, CH, and pH standards that you can buy here. Simple enough - how difficult could it possible be? We've all done it in grade school.

I googled around with different sources and came up with a 1/4 tsp of table salt in 500 ml of water should be about 2600 ppm (roughly). I'm really just trying to hit it in the in-field of the ballpark.

• source
• Distilled water=500ml
• Target Salinity=2.6 ppt (2600 ppm)
• Temp=77 (yes, i even heated it up)
• Water Impurity=0
• Calculate - > Use 1.3 grams salt (about 1/4 tsp).

1/4 tsp - I can measure that!!! And I have a flask from my beer making days which accurately measures 500ml.

Ohhh - I'm so smart

Well I did it. The test strips registered at 1600 ppm? And my Taylor salt test came up with 4000 ppm? So this was a huge failure .

Does my sample need to be more balanced? I didn't measure pH or anything. The water was from grocery store - distilled. Help me out please...

For reference,

• Two pool stores measured my pool water at about 2200 ppm.
• My Taylor kit measures my pool water at 3200 ppm. If the reagent gets weaker with age, then this makes sense.
• The AquaChec test strip measures my pool water at about 2200 ppm if you don't measure to the TIP of the white cone - instead measure 2 tenths less where the white is aprox 95% of the color change. The instructions are subjective because the picture on the bottle does not show a cone shape. Instead it shows a really, really small but definitely rectangular shape - which is not what really happens. There's enough subjectivity in these tests. Why do they create more...

Thank you in advance.

 

[Patrick_B]

I just wanted to say welcome to TFP, and good to have you aboard.

If your kit has been stored in something near indoor conditions, your readings aren't off with the expire date being 45 days away. Taylor suggests one year on most reagents, but most can and do last much longer.

 

[avn]

Thank you. I would agree with you, but....

I mentioned in my original post that everything has been trouble free. I didn't mention that my pool builders are the most inept, unprofessional, inconsiderate, untrustworthy, conniving, (need i go on)...did I say they lie through their teeth?

My pool build project was an absolute nightmare. All because of them. I've dealt with contractors my entire adult life with no issues - although not a project quite this large.

I apologize for digressing....On opening day (May) they left a bottle of 7-in-1 strips and Aquachec salt strips in my pool house. I was surprised and had to review my contract which clearly stated it included water test kit (not cheap test strips). I questioned the contract (not the first time) and they said "that was the kit". I wasn't happy with the strips and they reluctantly brought me a real kit later in September.

So it wouldn't surprise me that this kit was sitting in their 150 degree truck all summer.

I've lost all respect for those guys. They sweet talk you up and down. But once that hole is dug, everything changes. My family is beyond all that. We can sip & relax this year in our backyard oasis and make some wonderful family memories.

I enjoy geeking out on the water balancing. It's right up my alley. But from reading so much in this forum, I'm careful not to get too caught up in details. This simple salt calibration sample should be trivial to do. I just need some direction.

 

[Patrick_B]

No problem, and if you suspect that about your kit, replace all the reagents pronto, so you have no questions. The value good testing provides outweighs the small cost of reagents by the thousands of dollars.

 

[JoyfulNoise]

The reagents in the salt kit won't go bad easily. The potassium chromate indicator is very stable and the silver nitrate drops would be affected by light more than heat. You would know if the silver nitrate went bad because the drops would look yellowish-brown, not clear.

The problem with your experiment is that you measured salt by volume and not weight. All salt is ground differently and some (most) have anti-caking additives in them like potassium ferrocyanide and/or potassium iodide. You need to measure by weight, not volume and you really need a pure, reagent grade of salt.

1ppm = 1 mg/L

2600ppm = 2.6grams in one liter

 

[Patrick_B]

Thanks Matt!

 

[JoyfulNoise]

There are also commercially available salt standards (in the reef chemistry world) but they tend to be much higher salinity.

 

[avn]

Thank you!

I agree with all of your comments above. I have ordered more reagents (from here of course!). I will follow up with my experiment results shortly...

 

[AUSpool]

Hi avn, Ive been down your salt standard solution path and in the end used a commercial grade standard and the Taylor K-1766 potassium chromate silver nitrate test.

Reading your first post my thoughts was to recommend throwing out the strips, stay out of the pool shop and ignore their result, run your Taylor salt test again and go with that but don't do anything. Once your SWG is up and running if the 'add more salt' indicator is on add some salt till it goes off.

I too thought that making a standard using a volumetric salt measure is just not accurate enough. Normal kitchen scales don't have the range and given the salt options even if you could measure accurately to 0.1g or 0.01g you'll probably still end up with a home made standard that has an accuracy of +/- 100ppm or worse.

 

[avn]

I agree. When it comes to salt, cya and borates - I use caution cause I don't want to have to dump water.

I received the new silver nitrate and a new bottle of strips the weekend. Drum role....

Nothing changed. Taylor test still at 3200 ppm and strips are around 2300 ppm. Is there anything that would cause one test to be that far off - and very consistent...

My pool water is balanced pretty well:
FC: 4
CC: 0.5
TA: 70
pH: 7.7
CYA: 40
Borates: 30-40?
Salt: ???
Temp=70F
clarity: sparkling

Because my salt is so low, I'm wondering how low my borates really are. I use a drop test for that which is a little subjective.

I attached a picture of my strip - what would you read that at? 5.8, 6, or 6.2?

 

[JoyfulNoise]

Your strips are 6.2 which is 2700ppm which 500ppm less than the true value (as measured by your K-1766) which is 3200ppm. Being off by 500ppm for strips is entirely within reason....because they are junk!

Dump the strips! They are not useful at all!

The K-1766 is pure chemistry, no inference, no guessing, no assumptions. The silver nitrate reacts DIRECTLY with chloride (Cl-) to form insoluble silver chloride. There are ZERO interferences to that test (unless you've added cyanide to your pool lately or have 100's of ppm's worth of bromine in your water) and so it is dead-on accurate. If you want the test to be MORE accurate, use a 40mL water sample and your precision will be 50ppm/drop of titrant.

If you want an accurate drop based test for borates, look at my signature where I have a link for the mannitol borate testing.

 

[AUSpool]

I read that as the same, at 2700ppm, ignoring the extra 10ppm. Reporting down to 10ppm gives the user a false sense of accuracy. Aquacheck's product litriture doesn't give a tolerance or accuracy. I think you've calculated aquachecks salt strip accuracy avn, +/-500ppm. :grin:

 

[avn]

I turned on my salt cell this morning. It read 2800 and then turned off indicating salt level was too low. It was used for one season and has not been inspected/cleaned. I'll do that this weekend and if it still reads low, I'll add a single 40# bag. Does this sound like the right way to go?

 

[avn]

This weekend I removed the salt cell. It was clean as a whistle - no deposits. I then added a 40# bag of salt and later turned on the cell. Now the cell is reading 3200 and producing chlorine. Interesting to point out that the Taylor test is still at 3200 ppm.

I don't have time to monkey around with this much longer. My main goal was to be sure not add too much salt and I believe I achieved that. Now that I have confidence in the Taylor kit, I will save a sample of this 3200 ppm pool water and use it next year to make sure the Taylor kit is accurate.

When I find some copious amount of free time, I'll return back to making a diy salinity solution.

Thanks guys!

 

[txnole]

As a suggestion, move to a binary testing standard: either your salinity is (A) within sufficient range that your cell is operating per specification, or (B) your salinity is out of range for standard operation. If (A) then move to another chem test. If (B) then validate salinity using strip test. This will enhance peace of mind and chill level to "10"!!