Hi Irene,

if you mix SDS and any potassium salt in solution you end up with potassium dodecyl sulphate (KDS), which has very low solubility in water. Consider using sodium phosphate instead of potassium phosphate.

Hope this helps,

Arne

Hi Luciano,

Are you sure that the temperature overnight isn't lower than 21ºC?

I usually have a 10% SDS stock solution in my lab and work with a 1% SDS, 0.2M NaOH. When the temperature drops drastically during the night (and I mean 10ºC or lower) the SDS, NaOH solution does precipitate, but that rarely happens with the 10% SDS stock solution.

If you have correctly prepared this solution there is no reason for it to precipitate at 21ºC.

I have the same solutions in the lab as Mario (I guess they are pretty standard eh?) and they only precipitate when they get cold. I accidentally put on in the fridge overnight once! Check the quality of your chemicals & your 'recipe' again!

try to prepare this solution from SDS solution not from powdered

@ Mario: I have monitored the temperature of the solution overnight and it never went below 21 °C.

@ Ghaleb: I have tried both: from solid SDS in NaOH 0.5M, diluting 1:1 two concentrated stock solutions (1 M NaOH and 0.2% SDS) and even to dissolve solid SDS and solid NaOH (pellets) in water.

In your opinion is the order of addition important? I mean is it different if I add SDS to the NaOH solution or if I add the NaOH solution to the solid SDS?

Thanks a lot to everyone

Hello Luciano,

In my experience, I have the same trouble as you a few years ago. It was due to the bad quality of the SDS batch. So as mentionned by Valeire, check the quality of your chemicals or try to use another SDS.

Finally, Prepare your solution wtih SDS first , then add NaOH.

Good luck

First dissolve SDS in water and then add NaOH. Check the quality of SDS. The best quality SDS may solve your problem. Eg. Merck, Good luck.

The SDS is not pure you must recrystalize it from ethanol.....

so I think the problem in quality of your SDS

it is right that quality of SDS is important... but precipitation depends on temperature also.... luke warm the solution just before use nd go ahead.... try to store the solution above 30 degree celsius...

Update: apparently the unsolubility does not depend upon the quality of my SDS as I've tried two different quality and the result doesn't change; all ranges of concentrations between 0.1% and 0.5% do not dissolve in NaOH unless heated > 24°C and precipitate back overnight when temperature reaches around 21°C.

I've found that the only soluble solution is 0.1% SDS in WRO: in this case no matter if the water is at 21°C or 24°C and the solution stays clear for several days.

Unfortunately for what was my scope (Ultrafiltration membrane cleaning) the SDS 0.1% in WRO is not as effective as the SDS 0.5% in NaOH 0.5M that I've tested at the beginning.

Thanks again for the help

weird..commercial 20% SDS solution is soluble at room temperature..20-22°C...

it is wise to buy the liquid:-)

SDS solution will precipitate at temperatures below 15 °C. If there is any precipitate, re-dissolve by warming the solution (68°C for 10 min).

SDS in water gets hydrolyzed and solubilized. In NaOH solution its solubility is afftected because of the presence of Na+ that reduces SDS' hydrolysis as per basic solubility equilibrium principle.

Hi every body

Please help me!

I have the same precipitation problem. I dissolve SDS in water (1.5 wt%) and then I add NaOH solution to the SDS solution to have finally 1 Molar NaOH + 1.5 wt% SDS solution. Immediately after addition of NaOH, the solution becomes opaque, Besides, when I add phenolphthalein the color of solution becomes pink but after one minute it's color changes to approximately white!!!

It's very important that the concentration of NaOH in solution dose not change.

Does anyone know what's wrong with this solution?

Please!!

I experienced the same issue of precipitation after my 1% SDS solution stood overnight on my bench. I tried to vortex, but as you might know you get more foam than anything else. After reading some of the comments, I decided to heat up the SDS (55 degrees Celsius) for about 3 - 5minutes. There were no more precipitates.

I'm trying to prepare 1 g SDS solution in 100 ml phosphate potassium buffer 0.1M (pH 6.8). The solution appears to be insoluble unless heated to >60°C.

We want to develop a  tecnique at 37ºC and at this temperature the solution becomes opaque. Can anyone help? 

Hi Irene,

if you mix SDS and any potassium salt in solution you end up with potassium dodecyl sulphate (KDS), which has very low solubility in water. Consider using sodium phosphate instead of potassium phosphate.

Hope this helps,

Arne

If you gradually add SDS to water, then first DDS dissociates into ions, then it forms dimers, spherical micelles, cylindrical micelles, plate micelles (liquid crystals). In micelles, SDS molecules “hide” from water. Then a precipitate falls. All of these aggregates are in equilibrium with the sediment and are caused (built) by water. When you add 0.5 M NaOH, these ions bind water and it is not enough to create the listed structures. Researchers say that NaOH salts out SDS and the solubility of surfactants decreases.