Clarification needed on CH test

[Charlie_R]

Was reading this thread:http://www.troublefreepool.com/calcium-hardness-and-metal-ions-t68511.html and need a bit of clarification not really answered in pool school extended-test-kit-directions-t25081.html

When I do the CH test, when I reach the point of first color change, the indicator appears to "clump" into miniscule flakes, which appear violet or purple. It then takes about the same number of drops to turn them to a obvious blue. I am assuming that this is the "metals interference" indication. Looks like I will have to order the appropriate test modules from Dave to find out how much is in the fill and pool water, and possibly start using a sequestrant.

Side note for chem geek: According to a conversation with a local water company employee, they keep our domestic water at a relatively high pH -- above 8.4 -- with TA target around 100 ppm (his words) "to help protect the cast iron mains". Does this sound reasonable?

They also use what he described as mono- and tri- chloramines in puck form as a sanitizer. However, some of this seems to differ from their published water report, seen here: http://www.amwater.com/ccr/mx.pdf The actual report starts at page 4 of the PDF.

 

[duraleigh]

Does this sound reasonable?

Probably. High pH does inhibit the aggressive tendencies towards metals. I'm not sure it has to be up that high but apparently the water department does.

 

[chem geek]

You are right that their water quality report indicates using chlorine and not monochloramine. For whatever reason they indicate that it is 0.5 ppm when leaving the treatment facility but is at 1.9 measured in the distribution system. This doesn't make sense so perhaps the 0.5 meant the level after treatment but before more was added to last in the distribution system. 1.9 pm FC with no CYA (if at pH 7.5) is over 20 times higher in active chlorine level than in our pools. You said they told you they have cast iron piping and iron is subject to corrosion. So there is little wonder that they have the pH be higher to try and reduce the active chlorine level. At a pH of 8.4, the active chlorine level is reduced to the same amount as in 0.4 ppm FC at pH 7.5. That's still over 5 times higher than the level at the minimum FC in our pools.

My tap water has the following characteristics:

FC 0.0 ppm
CC 1.2 ppm
pH 7.7
TA 80 ppm
CH 55 ppm
TH 72 ppm (total hardness; includes magnesium)
CYA 0 ppm
Phosphates 400 ppb
Temp 67ºF

Our water district switched from chlorine to monochloramine some years back, hence the CC level instead of FC. There is 300-500 ppb phosphates added to the water for corrosion control.

So having a higher pH is not uncommon to reduce the rate of metal corrosion but the level of active chlorine is much higher than we are using in our pools and our pools do not usually have iron. They usually have stainless steel, copper (or cupro-nickel or titanium), and sometimes aluminum (in header bars for vanishing electric safety covers).