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i've made sodium hydroxide solution adding 100g of hydroxide to water 100g. i've been adding it slowly and constantly stirring. few days later on top of solution was thick layer of white powder. it stayed sealed in glass. i tried to filtrate using membrane, it didn't change a thing. i used tap water and osmotic water (did solution few times each time failed) and glass flask. can i make it pure by decantation or should i leave it for few weeks in HDPE bottle?

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    $\begingroup$ What is the source of sodium hydroxide you are trying to dissolve? Could you please add a photo of the floating powder? What membrane did you use for filtration? What are TOP and PEP? $\endgroup$
    – andselisk
    Commented Jul 10 at 5:38
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    $\begingroup$ i used chemically pure NaOH. it have been stored for 14 years, but seemed to be OK. membrane pores diameter 15 micrometer. by TOP i meant that sodium carbonate floats on top of solution. PEP is missclik. i used HDPE bottle for storing $\endgroup$ Commented Jul 10 at 10:41
  • $\begingroup$ Here is a recipe. They mention the carbonate "haze" and filter it. They also remove most of the carbon dioxide from the water first (by boiling). $\endgroup$
    – Karsten
    Commented Jul 10 at 13:59
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    $\begingroup$ After 14 years, "chemically pure" is just a label. $\endgroup$
    – Poutnik
    Commented Jul 10 at 16:23

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It should be sodium carbonate. Sodium hydroxide, solid or dissolved, is the trap for aerial carbon dioxide.

$$\ce{2 NaOH(s,l) + CO2(g) -> Na2CO3(s,aq) + H2O(l)}$$

The carbonate presence in hydroxide solution complicates determination of equivalence during titration.

There is the trick of old school chemists involved in volumetric analysis to prepare a hydroxide solution relatively free of carbonate:

  • Prepare a saturated sodimum hydroxide solution
  • Filtrate out undissolved solids.
  • This eliminates excessive carbonate, which has just slight solubility in saturated solution of sodium carbonate.
  • Ensure there is as little exposure to air as possible during manipulation.
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  • $\begingroup$ how should i filtrate saturated sodimum hydroxide solution? i've been using membrane filter with 15 micrometer pores, it didn't help much. is there a way to separate sodium carbonate which floats on top of solution? $\endgroup$ Commented Jul 10 at 10:32
  • $\begingroup$ Water (including tap water; but equally deionized, and distilled water) if left exposed can absorb $\ce{CO2}$ from air without change noticeable to the eye (like color, viscosity, etc). If this one is used to prepare the lye, soluted $\ce{NaOH}$ then forms $\ce{Na2CO3}$ and its hydrates in the solution, around the rubber stopper, the mantle of the bottle, the shelf. During storage, pellets of NaOH can react to yield a coating of $\ce{Na2CO3}$ because NaOH is hygroscopic ($\ce{ -> NaOH * n H2O}$, the eye doesn't discern either one); naturally, fine granules/beads are even more prone to this. $\endgroup$
    – Buttonwood
    Commented Jul 10 at 14:18
  • $\begingroup$ If you can afford it (terms of scale of the operation) and the layers separate well, use a separating funnel. Draw the lower/infranatant phase via the stem, discharge the upper/supernatant phase via the upper part. More importantly than performing «just an extraction», wear protection for eyes, skin; tissue/clothes. $\endgroup$
    – Buttonwood
    Commented Jul 10 at 14:21
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Citing the entry in Perrin about solid anhydrous sodium hydroxide, p. 349:

Common impurities are water and sodium carbonate. Sodium hydroxide can be purified by dissolving 100g in 1L of pure EtOH, filtering the soln under vac through a fine sintered-glass disc to remove insoluble carbonates and halides. (This and subsequent operations should be performed in a dry, $\ce{CO2}$-free box.) The soln is concentrated under vac, using mild heating, to give a thick slurry of the mono-alcoholate which is transferred to a coarse sintered-glass disc and pumped free of mother liquor. After washing the crystals several times with purified alcohol to remove traces of water, they are vac dried, with mild heating, for about 30hr to decompose the alcoholate, leaving a fine whit crystalline powder [Kelly and Snyder JACS 73 4114 1951].

The authors of the primary reference report 1.1% of $\ce{H2O}$ and 1.2% of $\ce{Na2CO3}$ in the starting material used. Their characterization of the purified product (already on the first page of the publication, i.e. available for free) reads:

Analysis of the product by the usual methods showed that the maximum limits of $\ce{Na2CO3}$, $\ce{SiO2}$, and $\ce{Cl^-}$ were 0.02, 0.02 and 0.005%, respectively. The minimum assay of NaOH was 99.84%. The low silica content indicated negligible pick-up from the glass vessels used in this preparation. The final product, as placed in the calorimeter, appeared as a fine, white, crystalline powder.

Assess in advance if the intended use is worth to invest in this route.

Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: Oxford ; New York, 1988.

Kelly, J. C. R.; Snyder, P. E. Low Temperature Thermodynamic Properties of Sodium Hydroxide$^1$. J. Am. Chem. Soc. 1951, 73, 4114–4115. https://doi.org/10.1021/ja01153a019.

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