These tests were done in my kitchen, accuracy is not guaranteed. I am not a chemist and could be way off base with all of this data. I am, however, quite anal about my water chemistry, and I do believe they are fairly accurate and provide some useful information when testing for borate using a drop test. Sorry if I made this write-up too long, but I want to give you all of the details of my experiment. These are my interpretations and I welcome criticisms and comments.
I am providing five titration curves. One using a Phenolphthalein indicator, two using a Bromothymol Blue indicator, and three using a Ph Meter and beginning the test at different pH levels, namely: 7.4, 8.3 and 8.9. It will be noted that the starting pH level has a effect on the curve shape and the endpoint interpretation.
The tests were done using a borate solution of approximately 50 ppm. All tests used the same solution. The results of the tests tend to show the value to be around 45 to 48 ppm which I accept as being a correct.
All tests involved lowering the pH to neutralize any carbonates as spelled out in the TFP procedure. Then, in the BTB and PHPH tests, the pH was raised to the indicator's color point with R-0010. In the pH Meter's test, the solution's pH was then raised to the desired pH, i.e. 7.4, 8.3, or 8.9 with R-0010. Mannitol was add and the solution was titrated using R-0010 until the proper color or the starting pH was reached. In the case of the pH Meter tests, this usually resulted in an overshoot of one drop. The drops were multiplied by four to find the final ppm value.
Examining the three pH Meter curves you will note that the 7.4 pH curve is somewhat flat and results in a reasonable ppm between 40 and 44. The 7.4 pH endpoint is lower on the curve than it should be and reflects the possibility of an inaccurate value. Lower pH starting points can be deemed to result in more and more inaccuracy.
The 8.9 pH curve, on the other hand, shows a value under 32 ppm and is not accurate at all.
The 8.3 pH curve looks good. It has a value between 44 and 48 ppm and an endpoint about where it should be.
Now for the 7.6 pH BTB curve. Determining the point where the solution first became blue was subjective for me. The graph was based on the point where I saw pale blue and the solution had a pH of 7.6. This BTB curve is similar to the previous 7.4 pH curve.
Repeating the test, I added an extra drop of R-0010 when I reached the first blue point resulting in a 8.4 pH. This BTB curve has a good shape and appears quite accurate with a ppm of 48.
The PHPH test was started with a pH of 8.6 which is just shy of the 8.7 pH faint pink color endpoint. The curve is well defined and returns a value of 44 ppm. The 8.7 endpoint is where it should be on the curve. Note how this curve and the previous 8.3 pH curve are quite similar. One problem with using PHPH is that it cannot be of any use when lowering the pH initially to neutralize carbonates since it remains colorless.
In summary I think all three methods can give acceptable results, but extra precautions should be used when testing with BTB or pH Meter. These are the caveats are below.
I don't know at which pH point the carbonates are eliminated. Using the pH Meter, I lowered the pH to 6.0 or below. When going by the BTB color, I could get a yellow reading as high as 6.9. It would take 8 more drops of R-0009 to reach 6.0. Is this an issue? I don't know, comments needed.
The pH starting point of the test is critical. With BTB, I noticed a blue hue with a pH as low as 7.6 and you can see the results are questionable. In addition, the reaction between the R-0010 and the test solution's color change is very slow, even with constant stirring. If you don't wait for the color change to stabilize before adding the Mannitol, the results will be incorrect. I feel that a starting pH of 8.3 to 8.7 provides the best results. I also suggest when the first stable blue color is noticed add one more drop of R-0010. If the color remains the same, you are good to go. If the color changes add one more drop.
In my test solution, one drop of R-0010, added to a pH of 7.6, raised it to 8.2, another drop to 8.5 and another to 8.7. A fairly low increase per drop. I assume, however, when testing borate solutions with a lower ppm, the increase per drop would be greater. Personally, when using the BTB test, I will always add at least one more drop.
OK, questions, comments, pitchforks and torches anyone?