Reef Chemistry Question of the Day #190 Evaporation and TDS

Randy Holmes-Farley

Reef Chemist
View Badges
Joined
Sep 5, 2014
Messages
66,553
Reaction score
62,858
Location
Arlington, Massachusetts, United States
Rating - 0%
0   0   0
Reef Chemistry Question of the Day [HASHTAG]#190[/HASHTAG]

You are working on your RO/DI one day and you collect a tap water sample to measure its TDS. You measure it to be 93 ppm TDS.

You get interrupted by your dogs barking excitedly upstairs, and by the time you get back from dealing with the salesperson at the door, you have forgotten the container with the tap water.

A week later you spot it, but it clearly has evaporated significantly. In fact, while there is no apparent precipitate in it, the water volume looks to be 1/4 of what it was before.

Curious, you remeasure its TDS.

Which of the following values is most likely?

A. 23 ppm TDS
B. 47 ppm TDS
C. 93 ppm TDS
D. 186 ppm TDS
E. 372 ppm TDS

Good luck!




































.
 

Salt Creep

Active Member
View Badges
Joined
May 6, 2014
Messages
365
Reaction score
354
Location
Minnesota
Rating - 0%
0   0   0
I'm going with E. Gravimetric method is another way to measure TDS (most, if not all of us use conductivity). This takes a set amount of water and after evaporation measures the mass of what is left. Being that the gravimetric method is considered more accurate this tells me the TDS do not evaporate with the water, but stay in the container of water.
 

Rybren

Valuable Member
View Badges
Joined
Jun 13, 2011
Messages
1,492
Reaction score
1,895
Location
Ottawa, ON
Rating - 0%
0   0   0
My guess would be E as well. This is based on the assumption that none of the elements causing the TDS evaporated along with the water.
 

Toadfish

Community Member
View Badges
Joined
Dec 23, 2015
Messages
49
Reaction score
21
Rating - 0%
0   0   0
E for me. It's like distillation. The solid get left behind.
 

Cory

More than 25 years reefing
View Badges
Joined
Oct 30, 2014
Messages
6,882
Reaction score
3,129
Location
Canada
Rating - 0%
0   0   0
I think its D
 

JimWelsh

Valuable Member
View Badges
Joined
Nov 5, 2011
Messages
1,547
Reaction score
1,679
Location
Angwin, CA
Rating - 0%
0   0   0
C1V1 = C2V2. C1 = 93. Let V1 = 1.00, then V2 becomes 0.25. Thus, C2 = C1V1 / V2 = 93 * 1.00 / 0.25 = 372. Answer is E.
 
OP
OP
Randy Holmes-Farley

Randy Holmes-Farley

Reef Chemist
View Badges
Joined
Sep 5, 2014
Messages
66,553
Reaction score
62,858
Location
Arlington, Massachusetts, United States
Rating - 0%
0   0   0
And the answer is...E. 372 ppm TDS

Nearly everything that contributes to TDS in tap water (calcium, sodium, chloride, bicarbonate, potassium, phosphate, silicate, etc.) will not be able to evaporate. When the water evaporates to the point where there is 1/4 as much, you have the same number of conductive ions in 4x less volume, so the TDS rises by a factor of 4, to 372 ppm. :)

Some things that contribute a bit to TDS and might evaporate would be ammonia and carbon dioxide, but their contribution to TDS will be minor in drinking water.
 

Mastering the art of locking and unlocking water pathways: What type of valves do you have on your aquarium plumbing?

  • Ball valves.

    Votes: 53 48.6%
  • Gate valves.

    Votes: 60 55.0%
  • Check valves.

    Votes: 25 22.9%
  • None.

    Votes: 28 25.7%
  • Other.

    Votes: 9 8.3%
Back
Top