Phosphates and TDS

[gregt]

If I put fresh RO-DI (0 or near 0 TDS) water in a plastic container wait overnight and measure TDS, will the TDS go up if there are phosphates leaching from the container?

[yardboy]

yes indeedy.
Just tried out that very experiment. As you may know, I work at a power plant and one of our tasks is ownership of the potable water system. We dose a posphate solution called "Aqua-Mag" to reduce corrosion and scaling of the system. Those barrels (nice white plastic) are just too tempting. Potable water runs ~1.0ppm PO4. Flushed the barrel with water to remove traces of chemical, tested at 0.5ppm. Flushed with RO/DI for 30 minutes, tested 0.00ppm. Let sit overnight -0.0ppm, 1 week later - 0.0 ppm. Aquamag is an organic phosphate solution so we use a Total phosphate test (Hach) Evidently not much was uptaken by the plastic. If it had leached Iwas gonna try rinsing and soaking with HCL for a while to maybe solubilize the phosphate, but no need.
We maintain a level of 2.5-10ppm phosphate in our boilers, and monitor with specific conductivity. (same as a TDS meter, eh?)
Have you ever seen problems with phosphate leaching from recycled plastic? I've used Rubbermaid trash can since I began reefing and never have. I wonder if the acid wash would have worked if you had problems?

[yardboy]

Oh yeah, ions leachingn will increase TDS, but organics leaching will not (or not likely)

[Habib]

Greg:

TDS measurement is actually nothing more than a conductivity measurement.

Acid (protons) in low concentration will give a relatively high conductivity.

The acid can simply be created by uptake of CO2 from the air giving carbonic acid.


Are you seeing an increase in TDS reading?
I might then be able to do some calculations to see what is likely or not.

[gregt]

Habib, this is more theoretical than practical. I'm curious on a test procedure that I can use to determine if a new container would be safe to hold RO water in.

It appears that you are saying that an elevated TDS is NOT necessarily an indication of leaching. Am I interpreting that correctly?

[tims]

that would depend on the meter you are using.. we have sevearal at work... researchers hae one shared monster RO system.. in each lab is TDS meter that gives high reading s as a good thing.. in my lab my meter is a low reader so the lowest is the best... container wise stainless steel is a very bad thing.. polyproplene is good to use as long as you wash it first and polystirene is a very bad thing for long term storage...

[Habib]

Yes, you're interpreting it correctly.

To give an example. water purification a step further then the regular RO/DI produces very pure water and the conductivity(TDS) is measured in-line. If it is measured not in-line but after it left the faucet will show a tremendous rise in TDS value just because of the CO2.



So depending on the values you are getting it might be CO2 or CO2 + something else (e.g. phosphates).

[Habib]

Here is a clipping re CO2 and conductivity:

CARBON DIOXIDE AND CONDUCTIVITY

The proposed USP PW monograph will call for on-line (or immediate off-line) conductivity at or below 1.3 µS/cm [when the temperature is at or above 25°C (77°F)] as stage 1 testing. The second stage testing calls for off-line analysis showing a conductivity of 2.4 m S/cm (at 25° ± 1°C). This off-line conductivity requirement is higher than the on-line requirement allowing for the increase in conductivity due to the contribution of dissolved CO2 gas present in the water. A key to producing water that meets the on-line requirement is the removal of CO2 from the water.


This is from:
http://www.gewater.com/library/tp/825_Two-Pass_RO.jsp

[Spanky]

Greg

Fill the bucket up to the top, test it (base line), cover it tight, then test it the next day.

[yardboy]

Or why not just test for phosphate after soaking it in the container for a while? It'd be difficult to separate the contribution of CO2 to the conductivity from any other ions present, unless it leached so bad that your conductivity went even higher (than the 2.3uS proposed in the GE standard.)
On the other hand, anything but ions leaching into the water wouldn't register on a conductivity meter anyway. You'd end up doing a TOC leachate on a garbage can. Not so cheap.