Re: [ccp4bb] Equilibrium Relative Humidity matching

[Boaz Shaanan]

Boaz Shaanan, Ph.D.                                         Dept. of Life Sciences                                      Ben-Gurion University of the Negev                          Beer-Sheva 84105                                            Israel                                                                                                                  E-mail: bshaa...@bgu.ac.il Phone: 972-8-647-2220  Skype: boaz.shaanan                  Fax:   972-8-647-2992 or 972-8-646-1710                          >This leads to a counter-intuitive observation - it is only the number of molecules in solution that affect the RH and not the type >of molecule/ion - therefore one molecule of glycerol has the same contribution as a chloride ion or anything else.  This means >that there is no effect for charge etc.  What does matter is how many species the salt dissociates into - this means that a given >concentration of sodium malonate (3 species) will have a lower RH than ammonium sulphate (2 species (NH4+ and >(NH4SO4)-) and not 3 as might be expected).  This means that relatvie humidity is a colligative property:   http://en.wikipedia.org/wiki/Colligative_properties  Doesn't it? So it should not be too surprising.           Boaz On 22/11/2012 18:32, Patrick Shaw Stewart wrote: Matt My old "Rubber Book" (Handbook of Physics and Chemistry, 1976) has a table (attached) showing the "lowering of vapor pressure by salts in aqueous solution", taken from the Smithsonian Tables. I've never been able to make any sense of it in terms of cations, anions, valency, or charge density (position in Hofmeister series), whether concentrations are expressed as M or N solutions. For example, of the salts mentioned on your website, MgSO4 seems to be anomalous. My spreadsheet also has a little converter that my colleague wrote many years ago to convert vapor diffusion conditions to batch.  This might be of interest to people who have to work in batch, e.g. people making crystals for X-FEL data collection.  You can download it as a program from http://www.douglas.co.uk/vdtomb/vdtomb.htm - it seems to work pretty well. Best wishes Patrick On 22 November 2012 12:11, Matthew Bowler <mbow...@embl.fr> wrote: Dear All,     after a few requests, I have now added equations to the online calculator that uses Raoult's law to calculate the relative humidity equilibria for precipitant solutions (see http://go.esrf.eu/RH). The new equations (4 and 5) allow the calculation of salt concentrations that will be in equilibrium with a certain PEG or other molecule solution - this will allow slow and controlled dehydration experiments to be designed in vapour diffusion plates, by slowly increasing the salt concentration in the reservoir above the equilibrium and thereby reducing the amount of water in the crystallisation drop by a controlled amount.   Hope it is useful, cheers, Matt. -- Matthew Bowler Synchrotron Science Group European Molecular Biology Laboratory BP 181, 6 rue Jules Horowitz 38042 Grenoble Cedex 9 France =================================================== Tel: +33 (0) 4.76.20.76.37 Fax: +33 (0) 4.76.88.29.04 http://www.embl.fr/ =================================================== --  patr...@douglas.co.uk    Douglas Instruments Ltd.  Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK  Directors: Peter Baldock, Patrick Shaw Stewart  http://www.douglas.co.uk  Tel: 44 (0) 148-864-9090    US toll-free 1-877-225-2034  Regd. England 2177994, VAT Reg. GB 480 7371 36 -- Matthew Bowler Synchrotron Science Group European Molecular Biology Laboratory BP 181, 6 rue Jules Horowitz 38042 Grenoble Cedex 9 France =================================================== Tel: +33 (0) 4.76.20.76.37 Fax: +33 (0) 4.76.88.29.04 http://www.embl.fr/ ===================================================