Determining Borate Level Using the Taylor K2006 Kit

[Naplespool]

I have a tube of the Insta-Test Borate test strips that I've been using for a year. The levels indicated by color are pretty hard to determine and fairly widely spaced: 0 15, 30, 50 and 80. Borate isn't mission critical so I suppose it's not super important to know it precisely.

This may have been already posted but the other day I realized there was a way to compute Borate using a regular Taylor test kit and Pool Math. I also have a PH meter whose accuracy seems pretty good but more importantly it is repeatable and therefore good for determining PH change over a short PH range--in my case about 8 to 7.7.

My current 17,250 gallon pool chemistry is:

FC 9
PH 7.93 (meter) (between 7.9 and 8.0 with test kit)
TA 80
CH 425
CYA 80
Temp 84
Borate ~50 (on the strips)
Salt 3600

Based on my pool size, each drop of of Taylor Acid Demand Reagent in the 44 mL sample tube requires 16 oz. of muriatic acid. So to calculate Borate Level with Pool Math from my data:

1. Measure 44 mL PH sample from pool: 7.93
2. Add two drops of Acid Demand and remeasure PH: 7.76
3. Enter all the data into Pool Math: For PH now, use 7.93; for PH Target use 7.76
4. Adjust the Borate number until the "Add [ xx oz.] 31.45% Muriatic Acid" reads 32 oz. (the amount that 2 drops of Acid Demand Reagent equates to for a 17,250 gallon pool using the Taylor Test Kit Manual for drops of Acid Demand Reagent)

The Borate number I have to apply to Pool Math, using the above procedure is 61.

When I used Phenol Red to measure PH I got 7.9 and two drops of Acid Demand results in a PH of 7.7. Applying those numbers to Pool Math and adjusting Borate for 32 oz. of MA results in a Borate number of 55.

There's nothing magic about using two drops. You could use 1, 2 or three drops and adjust the MA numbers accordingly. I picked two drops because it created enough PH spread to easily measure but not too wide a PH range for Pooi Math's Borate calculation.

bye bye test strips ...

 

[JoyfulNoise]

Won’t work because the addition of MA is decreasing the carbonate portion of the TA.

See my signature for a discussion of a test procedure using mannitol and bromothymol blue pH indicator.

 

[Naplespool]

I’m having a laugh at myself. I didn’t mean that the MA would actually be added to the pool! And I will have a look at your mannitol procedure.

What my procedure does is use Taylor Acid Demand reagent to infer the amount of Borate using the known Pool Math relationship between the reduction in PH from adding MA and the amount of borate in the sample. The more Borate in the pool, the smaller the reduction in PH when Taylor Acid Demand is added to the 44 mL sample. Give it a try.

This test could be performed in the other direction using Taylor Base Demand and measuring the PH increase.

 

[JoyfulNoise]

Your method won't work for several reasons -

First, the pKa of the boric acid/borate anion reaction is around 9.0 (8.9-9.2 depending on salinity and other factors). Borates only act as strong pH buffer when you get near their pKa and so doing acid/base pH measurements at pool water conditions (pH 7.2-7.8) is really too far from the equivalence point to be noticeable.

Second, the acid demand drops you are using are designed to lower pH and so even if you could accurately measure pH you're going in the wrong direction. You'd want to raise pH, not lower it, to see the effects of borates on buffering. As well, the acid demand drops are too concentrated to be very useful. The R-0005 acid demand reagent has the equivalent concentration such that one drop is equal to adding 9.16 fl oz of 31.45% MA to 10,000 gallons of pool water. So, if you take 7.8 as your starting pH (with 80ppm TA and 50ppm CYA) and reduce the pH to 7.2, it will take 14.1 fl oz of MA which is equivalent to 1.5 drops. If you rerun the same calculations and you use 50ppm B for borate concentration, the volume of MA needed goes up to 29.4 fl oz, or the equivalent of 3.2 drops. The difference there is 1.67 drops. So the reagents you are using are much too concentrated to make any meaningful assessment of borate concentration as exact droplet volume matters a lot when you are talking about a 1 drop difference. Your testing methodology is no better than the strips you are using (in fact, it's worse) as you can barely distinguish between 0ppm borate and 50ppm borate.

Finally, Pool Math is not as accurate as you think it is. I can do some of these calculations using chem geek's pool equations spreadsheet which iteratively solves for various parameters. Pool Math, while based on some of those calculations, does not take into account that TA changes with the addition of acid and so the pH calculations are only good for very small changes in pH, typically less than 0.2 units.

At the end of the day, what you are trying to do in a theoretical sense is an acid-base titration where you are using a strong acid to titrate a weak base. That doesn't ever work. This is the reason why the mannitol complexation reaction is used - when mannitol reacts with boron it causes a strongly acidic reaction to happen. At that point you then do a standard strong base (NaOH) titration of a strong acid (H+ ions) and look for the equivalence point using a known acid/base indicator (bromothymol blue). One can do the same strong base/strong acid titration using a pH meter as well, but it's somewhat unnecessary.

Your options for more accurate boron testing is to either use the mannitol procedure outlined in my signature OR Hanna sells a boron test kit for about $100. However, the Hanna kit is only designed to test concentration from 0-5ppm, so you'd have to do some accurate dilution of your pool water sample (1:9 dilution with distilled water) to bring the concentration in range of the testing reagents they sell.

 

[Naplespool]

Matt,
Although Borates do buffer the addition of acid (which can be seen in the amount of acid that Pool Math predicts for a given desired pH change versus actual level of Borates in the sample), I do understand that the buffering effect is stronger in the upward direction. I also appreciate that you are knowledgeable on this topic. (I have ordered the Mannitol test chemicals.)

If you are willing, I’d like to go over this again. You suggest that Pool Math is only useful for pH changes of about 0.2 or less due to TA changes from the drops (understood). That is in the range I am making.

You also do an example with fractional drops. That’s not the method I’m suggesting. I’ve only proposed using 1, 2 or 3 whole drops of Acid Demand OR Base Demand.

Today I made the test using Base Demand, whole drops and a small pH change to satisfy your comments. And I’ve checked my pH meter against Taylor pH Indicator solution. Starting Chemistry in my 17,250 gallon pool (MA added a day ago):

FC 10
PH 7.66
TA 90
CH 400
CYA 80
Borate 50 (from strip test)
NaCl 3750

After adding 3 drops Base Demand to 25 ml sample:
PH 7.85
TA 110

Based on the above, Pool Math implies a Borate Level of 40 to reach the Taylor manual level of 27 oz. of required Soda Ash (using an average TA of 100).

Your comments please.
Roger

 

[JoyfulNoise]

Your numbers are a bit off. Again, Pool Math is just not that accurate. The base demand drops have the following equivalence - 1 drop of R-0006 is equivalent to adding 5.13 oz (by weight) of soda ash to 10,000 gallons of water. Here's what I get assuming I use your numbers -

pH 7.66 ---> 7.85 (with a TA of 100, CYA of 80, salt 3700 and CH of 400)

ppm ....... Soda Ash (oz) ....... R-0006 equiv. (drops)

0 ................. 6.43 ...................... 1.25
10 ............... 8.65 ...................... 1.67
20 ............... 10.86 ..................... 2.11
40 ............... 15.27 ..................... 2.97
50 ............... 17.47 ..................... 3.40

What you see is that you can certainly "infer" that your water has borates in it based on the reaction to adding acid or base. The problem is one of accurately measuring what you're seeing. The drops are just too concentrated to be all that useful when you're adding so few drops. It's very easy for the droplet volume to vary. So yes, you can use your pH probe and add acid/base demand drops and see the differences, but I'm not confident in the process such that you could tell the difference between 10ppm and 40ppm . So in that case, how do you know how much boric acid or borax to add to your pool? This is why the mannitol test is useful - there is a strong enough acid/base reaction that the titrant can give the user an accurate reading to within a few ppm .

Now with that said, as long as you don't have any small pets or children drinking pool water exclusively, overshooting and having a borate level above 50ppm is not a problem per se. It just means you'll need more acid on hand to correct pH when it does need to be changed.

 

[JoyfulNoise]

Let me put it to you another way - you’re just doing what your test strips are doing. The test pads have a pH indicator dye and a prescribed amount of dry acid and buffering chemicals in them. In a zero borate water sample, the indicator dye on the test pad will achieve a certain color. If borates are present, the buffer/indicator reaction is slightly changed as is the ultimate pH reached. This the color change. The problem with the test strips is that the reaction is not that strong and color change is hard to detect. So that is why you see such a large delta between colors. Your pH probe could improve on that slightly but the changes you see are more dependent on the base demand reagent concentration than the precision of your pH meter.

Standard acid/base titration is the way to go with boron measurements and that’s what the mannitol complexation gives you.

 

[Naplespool]

Thanks for continuing the dialog with me.

The issue of per drop volume is one that I've wondered about in all the Taylor tests. But I don't think my numbers are off, I believe that you're using 10,000 gallons and I'm using 17,250. It works out to the same borate in either case but requires a different target Soda Ash, based on pool volume.

Remember that my method is test the sample pH, add two drops of reagent, and remeasure the pH. Then iterate Borate in pool math until the answer is 27 oz. of Soda Ash. (I could use a different number of drops but I'd then have a different Soda Ash target and would expect a different pH movement.)

At 40 ppm, which is about the Borate in my pool, our numbers are in complete agreement. You've used 10,000 gallons and I've used 17,250 gallons. Hence, you get 15.27 oz Soda Ash and I get about 27 oz.

If my actual Borate level was 20 ppm, I still would have added two drops to the sample, but my actual PH reading after adding the two drops to the sample (which initially measure 7.66) would have been 7.93.

Amazon is shipping me all the mannitol stuff, but the glass beaker arrived just now, crushed.

 

[JoyfulNoise]

The Taylor titrating bottle fitments are supposed to be designed to yield 40 uL/drop (25 drops = 1mL). Fluid viscosity, finger pressure, back-pressure inside the bottle, static electricity, etc, can all affect the droplet volume. Typically you want to achieve a slow drop rate of about 1 droplet per sec where the droplet forms on the tip and falls off under the forces of gravity (not you wiggling it). If that is the case, then dispensing something on the order of 10 to 20 drops or so should have the effect of cancelling out drop-to-drop volume variations. More drops than that and you risk having to suck air back into the bottle and messing up the pressure. Excessive droplets counts can lead to long term systematic errors in volume. It's complicated in the details but that about the best you can achieve with a low-tech dropper bottle.

One can always purchase a glassware stand and some precision graduated burettes, stop-cocks and Erlenmeyer flasks to measure exact volumes dispensed but that's a bit overkill for pool chemistry


If you have a pH probe and a good way to setup the experiment, you can do the mannitol-base titration with the pH probe and graph pH versus volume dispensed of R-0010. When you do that, you'll clearly see the standard strong acid / base titration S-curve. If one is careful right around the equivalence point, you can get very exact curves.....

...or you can just buy the bromothymol blue indicator dye and get a "good enough for pool care" measurement...

 

[Naplespool]

Thanks very much. As I said, I've got all the chemicals on order for a better test. It sounds like with any titration test, you want enough drops to avoid drop to drop variation and not so many drops that you add other errors. One thing I could do in that regard is to use 6 drops in a 75 ml sample.

The full table for the exact pool numbers (above) that we're discussing, with a starting pH of 7.66 would be:

Borate pH after 2 drops
10 8.03 (using 1 drop would be better)
20 7.93
40 7.85
50 7.82

Looking at the table, there are big implied differences in Borate for very small changes in pH, which I guess is what you've been saying all along.

Enjoy Black Friday!

 

[AUSpool]

I'm not sure what pH meter your using but I've always thought the accuracy of the second decimal place was a bit questionable. I looked up the specs of a medium quality meter (~$300) which has a resolution of 0.01 pH units and a published accuracy from the manufacturer of +/- 0.02 pH units @ 25degC.

This infers, I think, that at 40ppm borate, pH 7.85 from above, the pH could be anywhere between 7.83 - 7.87 pH units. This I think represents a borate level of 35ppm - 47ppm from a pH reading of 7.85.

 

[Naplespool]

You’re right about the accuracy of my inexpensive, handheld pH meter but its repeatability (short term stability) seems okay. By quickly making two measurements, close in value (about 0.2 apart) my results of the difference between the two readings seem to be stable to +/- 0.1 or better. In other words if I measure before and after over & over, I get the same results and the same difference. And it’s mainly the difference between the two readings that matters. By selecting whether I use 1, 2 or 3 drops I can control the size of the difference—1 drop gives the smallest delta.

All that said, my Bromthymol Blue has arrived and I’m waiting for the other elements to make Matt’s measurement.

 

[Naplespool]

I finally received the BTB, and Mannitol. The 50 ml beaker is another matter, it has been shipped twice by Amazon and arrived crushed both times. They're shipping another but I had a glass container about the same size and got a 50 ml sample by using the Taylor test kit and measuring 50 ml twice.

The test is straightforward and much easier than the write-up made it seem. Although there are a bunch of steps, I followed the instructions, step by step. My measured Borate number is 48 (12 drops to light blue, the 13th drop made it dark blue).

Matt, Thank you for this well designed procedure, it seems to me that it should be a documented procedure in the TFP front page summary.

So that I can do an even better job, which steps are the sensitive ones. For instance I know that counting the final drops is critical, so what other steps must I perform accurately (mannitol measurement, BTB measurement, etc.)?

Also, how would the math work out if I used a 44 ml sample, the other easy measurement in the Taylor kit?

 

[JoyfulNoise]

If you read through the entire thread, I think Richard outlines other volumes you can use. I just use 50mL because the math is easy.

The amount of BTB is not super critical as it is just an indicator. You want to use enough BTB to make the color easy to see. BTB is not easily made into a water solution so the concentration of the indicator dye is fairly weak and means you have to use more of it than most people are used to with their standard test kit reagents that have concentrated indicators.

Mannitol has to be added in excess and the 1/4 teaspoon is about right to cover the amount of boron in solution. I think I did the calculations once where I assumed one would create a saturated solution of mannitol in distilled water and use that as the reagent rather than the powder. Based on the total solubility of mannitol in water, it would a fairly significant volume of solution to replicate 1/4 tsp of the powder and so wasn't really worth it. As long as you add excess mannitol, the test should work fine.

The hardest part of the test is getting the initial and final shades of blue to be the same. As you can see, when you add the first drops of R-0010 after the BTB is added, it only takes about 2 drops to get to the light blue color. Then, after adding the mannitol, you can see it takes a lot more drops of R-0010 to get back to that blue color. So trying to remember the shade of blue adds a bit of confusion to the test but it's not so bad as to make the test results unusable. It would be nice of the R-0010 was slightly lower molarity of NaOH, but it is what it is.

In the Hanna test kit, they actually give you a digital pH probe to use instead of the BTB indicator. They then provide a chart that equates the starting and ending pH values with the drop count of the reagent to get the correct concentration. However, that kit is not designed to work with levels greater than 10ppm and so one would have to use dilution to get it to work with pool water (and, since it's a soil based test, they assume no chlorine is present). Taylor also has a test reagent that works with their colorimeter unit (the colorimeter costs about $1,600) but that is limited to 2ppm .

Unfortunately, the use of borates in swimming pools is considered an adjunct chemistry that doesn't have a lot of market demand. So the availability of test kits for it is quite limited.

 

[Naplespool]

Thanks for all the added explanation. I also went back and read several more pages in the test instruction thread.

In order to facilitate getting back to the same color blue, is there any reason I couldn’t start out with 75 ml through step 3 and then save 25 ml as a reference color?

 

[JoyfulNoise]

The color will tend to drift over time so it may not be that helpful. You could always give it a try and see what you come up with.

 

[Naplespool]

If the color shifts over the 2-3 minutes it takes for the final steps then it wouldn't be of much value. I wondered if it would be better than trying to remember the color.

 

[JoyfulNoise]

If the color shifts over the 2-3 minutes it takes for the final steps then it wouldn't be of much value. I wondered if it would be better than trying to remember the color.

Give it a try and see what you get. I don’t find remembering the colors too difficult but I’m used to doing the test.