I am having a very difficult time with my samples recently and is suspected of causing our Ion Chromatograph (IC) to always have clogging problem due to possible precipitate formation caused by the samples.
My samples consist of 3 different anions (sodium nitrate, sodium sulfate and sodium selenate) in different combinations and all in concentration of 0.5mM. I use lactate as the carbon source. My mineral media is composed of the following:
- KH2PO4 250 mg/L
- Na2HPO4•2H2O 250 mg/L
- CaCl2•2H2O 15 mg/L
- KCl 250 mg/L
- NH4Cl 300 mg/L
- MgCl2•6H2O 120 mg/L
- plus trace acid and base trace elements added 0.1ml/L
After my samples were stored for 2 weeks in the fridge and left out in room temperature for 2 days, they tested it with mixing the IC eluant 5x (1.8mM carbonate and 1.7mM bicarbonate) and said that it formed precipitate after 24 hours.
However, when I left all my fresh samples 1 set in room temperature (15-20 oC) and another in the fridge (4 oC) for 3 days, it was only the sulfate and sulfate+selenate containing solution left in room temperature that formed a precipitate (getting around 67.9 NTU and 18.1 NTU respectively).
However, I can't figure out the reason behind this. I am currently still conducting other test but does anyone know the possible reason for this? Or does my mineral media somehow causes the ions to precipitate? Or does the eluant for the IC reacts with it?
Ah, I do know that calcium would cause some precipitate however, I don't see that in all of my samples. Is there a reason why no precipitates are formed with those solutions without sulfate and those keep in the fridge? As I mentioned before, I have different combinations of ions and the mineral media (which has the calcium and phosphate ions) stays constant in all solutions. To break it down, the solutions I have are the following:
1 - Mineral Media + Lactate + Nitrate
2 - Mineral Media + Lactate + Sulfate
3 - Mineral Media + Lactate + Selenate
4 - Mineral Media + Lactate + Nitrate + Selenate
5 - Mineral Media + Lactate + Sulfate + Selenate
6 - Mineral Media + Lactate + Nitrate + Sulfate + Selenate
7- Mineral Media
So far, the ones which have visible and high turbidity are in solutions 3, 5, and 6 in decreasing turbidity values (NTU). And only the ones left in room temperature. All samples in the fridge do not show any visible precipitates or register high NTU values (all are less then 1 NTU).
Additionally, when I mixed the eluant (which contains the carbonate) with my samples, no precipitates formed within 24 hours.
The concentrations that are being added to water are moderately high and can easily cause precipitation, most likely involving calcium, phosphate, and carbonate. These are more likely to precipitate at basic pH. Unexpected precipitate are a common issue in laboratory solutions.
My suggestions are:
1) measure pH. If possible, make more acidic to keep ions in solution.
2) If possible, dilute your solutions until precipitates do not form.
2) Collect precipitates and analyze to determine their composition. This can be done by surface analysis (XPS), or, alternatively, dissolve in your laboratory and measure composition of anions by IC and cations by AA or ICP-MS. Once the precipitating ions are identified, try to change your recipe to avoid them.
sodium sulfates precipitates
below 8 degrees centigrade. don't keep samples in fridge which is cold enough to precipitate sodium sulfates.
For a RG discussion on the pH of pure sodium sulfate (Na2SO4) aq. sol.: https://www.researchgate.net/post/What_do_you_call_the_electrolyte_of_sodium_sulfate_a_base_an_acid_or_neutral
I have proposed a combined reverse osmosis ― refrigeration system, while addressing a somewhat related problem elsewhere at this forum: https://www.researchgate.net/post/Is_it_possible_to_re-use_the_water
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