I am new in this area and working on the isolation of VLDL and LDL. While making the density gradient, the solubility is an issue, It could not be freely soluble in water at 1gm in 1ml. So, how could we make or adjust density of Kbr at 1.006g/cm3 and 1.063g/cm3 and 1.21g/cm3. for ultracentrifugation.
Hi Vipn,
It's important to keep in mind here the difference between concentration and density - what you describe above is an attempt to make the solutions by concentration, not by density. In this case, you are wanting to add only enough KBr to water such that the density of the water increases from about 1 g/cm3 to 1.006 g/cm3 (and so on for the other desired densities).
Density can be assessed with an analytical balance, though applications like this typically use a refractometer to measure the solution refractive index which corresponds to a given density. You can search for equations to calculate the interconversion of density and refractive index, and the CRC Handbook of Chemistry and Physics includes tables of this information, as well.
Hi Vipn,
It's important to keep in mind here the difference between concentration and density - what you describe above is an attempt to make the solutions by concentration, not by density. In this case, you are wanting to add only enough KBr to water such that the density of the water increases from about 1 g/cm3 to 1.006 g/cm3 (and so on for the other desired densities).
Density can be assessed with an analytical balance, though applications like this typically use a refractometer to measure the solution refractive index which corresponds to a given density. You can search for equations to calculate the interconversion of density and refractive index, and the CRC Handbook of Chemistry and Physics includes tables of this information, as well.
I. Lange's (*) gives 1.380 for the relative density of saturated KBr aq. sol. at 25 ºC (relative to H2O at 4 ºC).
i) In absence of further data, other than that for pure water and for the saturated solution, it is possible to predict intermediate densities with good accuracy based on the empirical exponential correlation: ρ = A·exp(B·c), with ρ in g/cm3 and c in wt%, i.e., 100·(msalt/msol.)% (alternative concentration units could be considered). A and B are temperature-dependent correlation parameters. Considering just pure water and saturated solution as data points: A = ρH2O (at the temperature of solution); B = {ln (ρsat/ρH2O)} / csat. I have checked this correlation against a set of tabulated NaCl aq. sol. density data elsewhere at this forum (**); the maximum correlation (relative) error did not exceeded ±0.1%. It should perform well enough for another alkali halide salt (KBr).
ii) Alternatively; densities can be predicted with reasonable accuracy from the same data by accepting volume additivity to hold while mixing pure water to the saturated solution (besides mass additivity). It is straightforward enough to derive the adequate mixture rule from mass and volume balances for the conceptual mixing operation that would yield solution at a given concentration. The former proposed correlation (i) is not restrained by the volume additivity assumption.
(*) J.G. Speight (Ed.), "Lange's Handbook of Chemistry", 16th ed., 2005, McGraw-Hill; Table 1.73.
(**) https://www.researchgate.net/post/How_can_I_dissolve_6_molar_sodium_chloride
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