Order of chemical additions when removing metal stains

[stev32k]

I was reading this: ascorbic-treatment-to-rid-pool-of-metal-stains-t2298.html series of posts on treating pools with ascorbic acid to remove metal stains. The recommendation was to add the AA before the sequestering agent. Richard brought up the question of why the sequestrant was not added first. That seems logical to me because the sequestrant would be there to lock up the metal ions as soon as they were put into solution. But maybe there is something about adding the sequestrant first that interferes with the AA.

I don't believe the question was ever answered. Anyone have any ideas or experience with order of addition?

 

[257WbyMag]

...bump...

To really know, I would guess that someone would just have to try it both ways. Because sequestrants bind metals, any metals, then what the post by waterbear states would seem to make sense in the fact that as soon as it were added, it would immediately start gathering the calcium and the magnesium up. This leaves less sequestrant available for the other metals that you intend the AA to release into solution. I would think that the for the short amount of time that the offending metal is suspended in solution before you add the sequestrant to bind it is not enough time for much redeposition to occur. Just be sure that you have enough sequestrant on hand to do the job up front and keep some on hand at all times to add as needed.

 

[stev32k]

...bump...

To really know, I would guess that someone would just have to try it both ways. Because sequestrants bind metals, any metals, then what the post by waterbear states would seem to make sense in the fact that as soon as it were added, it would immediately start gathering the calcium and the magnesium up. This leaves less sequestrant available for the other metals that you intend the AA to release into solution. I would think that the for the short amount of time that the offending metal is suspended in solution before you add the sequestrant to bind it is not enough time for much redeposition to occur. Just be sure that you have enough sequestrant on hand to do the job up front and keep some on hand at all times to add as needed.

I would think that the sequestrant would tie-up the Ca and Mg regardless of when it was added - before or after the acid. As you say the real key is making sure you have enough in the water to bind everything that will bind. That is another problem - how can one tell when enough is enough? Is there a "free sequestrant" test? I wonder if the calcium hardness test would work by testing the HA then add the sequestrant until the HA was zero?

 

[JasonLion]

Experience suggests that adding sequestrant afterwards requirers less total sequestrant. However, nothing bad happens if you add sequestrant first. In fact, occasionally sequestrant alone can remove the stains.

Jack's Magic sells a Sequest Test Kit, which can measure the sequestrant level. It is fairly expensive however, $70+.

A more affordable, and more complex, approach is to pay a lot of attention for the first signs of staining and add more sequestrant the moment that starts to happen. If you maintain all of your levels correctly and catch the stains just as they are starting, simply adding more sequestrant, and lowering the PH if required, will usually remove them. Once you gain some experience at monitoring for stains just starting it gets much easier and this procedure becomes routine.

 

[stev32k]

Experience suggests that adding sequestrant afterwards requirers less total sequestrant. However, nothing bad happens if you add sequestrant first. In fact, occasionally sequestrant alone can remove the stains.

Jack's Magic sells a Sequest Test Kit, which can measure the sequestrant level. It is fairly expensive however, $70+.

A more affordable, and more complex, approach is to pay a lot of attention for the first signs of staining and add more sequestrant the moment that starts to happen. If you maintain all of your levels correctly and catch the stains just as they are starting, simply adding more sequestrant, and lowering the PH if required, will usually remove them. Once you gain some experience at monitoring for stains just starting it gets much easier and this procedure becomes routine.

Thanks for the response. What sequestrant would you recommend? I looked over Jack's site and there are several to choose from, but I don't have any experience with any of them.

 

[JasonLion]

Look for sequestrants based on HEDP, phosphonic acid, or phosphonic acid derivatives. Some of the top choices are ProTeam's Metal Magic and Jack's Magic the Pink Stuff (regular), the Blue Stuff (fresh plaster), and the Purple Stuff (SWG). I recommend starting with one of the top brands and then when things settle down consider switching to an off brand to save money. The idea is to get a sense of what sequestrant can do and how much you need to use with a known good brand, and then try out less expensive brands to see which ones work just as well for you.