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According to the Apollo Experience Report: Crew Provisions and Equipment Subsystem, NASA Tech Note D-6737, p. 29, the survival kit in the Apollo command module had a means to remove the salt from seawater:

The desalter kit is a standard off-the-shelf Department of Defense (DOD) item. The kit has been modified slightly to meet the NASA requirements. The kit consists of two processing bags, eight chemical packets, and mending tape. Each chemical packet is designed to produce 1 pint of potable water. The water is processed by mixing sea water and a chemical packet for a certain period of time. The mixture is then filtered to produce the drinking water.

How did the desalter work? I'm asking for the chemical reactions involved, not the instructions on how to use it.

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From Legislative History: Saline Water Conversion Act

During World War II, the castaway problem reached such proportions that the military initiated several projects to find ways for providing emergency supplies of water. Once solution to the problem was the desalinating kit using de-ionizing chemicals. These kits produced by the Permuti Co. utilized a briquet composed of silver zeolite and silver oxide; when added to sea water in a plastic bag, the crushed briquet removed sodium, calcium, magnesium, and chloride ions as an insoluble precipitate.

This paper DESALTING SEA WATER.. . . . A Practical Chemical Method has extensive details and shows this (poorly reproduced) image of how the system is used.

It is captioned

The picture at the top shows a briquet of desalting chemical. Next the briquet is dropped into the plastic desalting bag filled with sea water to the marked line. In the third photo the hard briquet disrupts spontaneously, and water and chemical are thoroughly mixed by kneading. After about an hour clean water is sucked through the filter at the bottom of the bag.

enter image description here

The complete instructions are given as:

The operation of the kit is best indicated by the directions printed on on the desalting bag:

  1. Screw plug firmly into outlet tube of this bag.

  2. Fill bag to filling line with sea water.

  3. Pull off one package of chemical from tape. Follow directions on wrapper. (The directions on the wrapper of the desalting briquets read as follows: Use only one at a time. To remove foil, cut end of wrapper on edge of can. Tear off foil, then cut end of inner cellophane wrapper on edge of can and pour entire contents into bag of sea water.)

  4. Then fold top of bag down tightly and roll toward buckle. Strap securely.

  5. Chemical breaks up in a few minutes. Further pulverize it by kneading gently for 10 minutes.

  6. Shake bag gently for an hour. Water is then ready to drink when filtered.

  7. To filter water, first unscrew plug from outlet tube. Then gently squeeze bag or suck on tube. Spit out first few drops only if they are too salty.

  8. Replace screw plug. Water may be left in bag with chemicals until wanted.

  9. When all water is drawn from bag, rinse out chemical with sea water.

The paper also contains extensive discussions of the chemistry involved. Chemistry is not my forte but if I am interpreting the paper correctly (a stretch, admittedly), these are the reaction equations involved.

enter image description here enter image description here enter image description here

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  • 1
    $\begingroup$ Thanks for the edit! $\endgroup$ – Organic Marble Aug 28 at 0:02
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    $\begingroup$ There is no Sodium within these chemical reactions. But desalting of ocean water requires removement of sodium chloride. Removing chlorine only would leave sodium in water which is harmful to drink. $\endgroup$ – Uwe Aug 28 at 17:20
  • $\begingroup$ @Uwe my chemistry knowledge is too poor to address your point. I think it is a valid one though. $\endgroup$ – Organic Marble Aug 28 at 17:42
  • $\begingroup$ I found some sentences about sodium removement in the paper. $\endgroup$ – Uwe Aug 28 at 18:07

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