Yellow pH sample slowly turns purple

[QPSUtah]

Chlorine is 8 ppm
TA is 30

Base Demand requires 9 drops to raise pH to 7.4.

Is it normal for a yellow pH test (less than 7) to slowly turn purple?

I’m not sure how the pH got so low. Perhaps incorrectly calibrated pH probe.

 

[JamesW]

Typically due to chlorine interference.

High CYA slows the process.

Most likely the CYA is well over 100.

Can you post a full set of readings including CC?

Was FC tested with FAS-DPD?

Post current readings and also prior readings.

Post all chemicals added since prior readings.

Is the pool on Trichlor or liquid chlorine or salt etc?

 

[JamesW]

Most likely, the "Adjusted" or Carbonate TA is zero.

What is the CYA?

Borates and Adjusted Alkalinity​

Adjusted TA = TA – (CYA X CYA C.F) – (Borate x Borate CF)

Borate C.F (correction factor) based on pH.

pH.......CF

7.2.....0.051

7.4.......0.0786

7.6......0.1248

7.8......0.1989

Cyanuric Acid correction factor based on pH.

pH........CF

7.0.......0.22

7.1.......0.24

7.2.......0.26

7.3.......0.28

7.4.......0.30

7.5.......0.32

7.6.......0.33

7.7.......0.34

7.8.......0.35

7.9.......0.36

For example, if the pH = 7.6, TA = 90, Borate = 50 and CYA = 70, the adjusted alkalinity is 90 - (70 x 0.33) – (50 x 0.1248) = 60.66.

 

[JamesW]

Base Demand requires 9 drops to raise pH to 7.4.

9 drops of base demand increases the TA by 33 ppm.

The TA starts at 30, so most or all TA is probably CYA.

Address CYA and TA and the pH will fall in line.

 

[JamesW]

Phenol red gets converted into Chlorophenol red with high FC.

Chlorophenol red is a pH indicator dye that changes color from yellow to violet within the pH range of 4.8 to 6.7.

The pH is likely to be about 6.4.

Did you ever get a pH meter?

 

[JamesW]

Chlorine is 8 ppm

Usually, it takes over 10 ppm, but the issue is probably chlorine interference.

Maybe the FC is higher than 10 and you have test error?

 

[QPSUtah]

Typically due to chlorine interference.

High CYA slows the process.

Most likely the CYA is well over 100.

Can you post a full set of readings including CC?

Was FC tested with FAS-DPD?

Post current readings and also prior readings.

Post all chemicals added since prior readings.

Is the pool on Trichlor or liquid chlorine or salt etc?

It’s a commercial pool. Uses liquid chlorine. Just opened for the season. Chlorine feeder is off due to ruptured tubing, so added two gallons manually to raise chlorine.

At the time I took the readings yesterday, the acid feeder had been running and reading 7.2, so was not properly calibrated and had fed too much acid in.

It’s just odd to see the color change like that.

 

[QPSUtah]

Phenol red gets converted into Chlorophenol red with high FC.

Chlorophenol red is a pH indicator dye that changes color from yellow to violet within the pH range of 4.8 to 6.7.

The pH is likely to be about 6.4.

Did you ever get a pH meter?

View attachment 644648View attachment 644649

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View attachment 644642

I’m did get a pH meter. I keep on leaving it at the office, LOL.

 

[JamesW]

Typical with FC over 10 and high CYA.

Verify FC and CYA levels.

 

[QPSUtah]

Free chlorine was 8 and CYA was 0 (semi public pool (apartment) with liquid chlorine system).

I ended up dosing with Soda Ash to raise the pH and it's now doing fine, but just weird that I was seeing it change like that. Maybe 8FC was enough to do that.

 

[JamesW]

0 CYA makes the process happen pretty fast, so maybe more noticeable.

If you had CYA, the process is slow enough that you have already taken the reading and dumped the sample before the reading changes enough to be noticeable.

 

[JamesW]

At the time I took the readings yesterday, the acid feeder had been running and reading 7.2, so was not properly calibrated and had fed too much acid in.

Normally, the problem is caused by trichlor use, which is why I was thinking high CYA.

An overactive acid feeder is a less common cause of readings like this.

Definitely need better maintenance, training and oversight on the automatic feeders.

There have been some serious accidents caused by feeders going berserk and dumping acid and/or chlorine into a commercial pool causing many injuries.

 

[JamesW]

Free chlorine was 8 and CYA was 0 (semi public pool (apartment) with liquid chlorine system).

Is 8 ppm acceptable for that commercial pool?

8 ppm with 0 CYA is pretty harsh on bathers.

Is the liquid chlorine feeder also automated?

Is it on ORP control?

What was the ORP setting and reading?

Is that feeder also overfeeding?