Does TDS equal Density 4

[NMLS]

I have read that TDS (total dissolved solids) can be expressed in mg/L or alternativly in ppm. Since density= mass/volume, does that make TDS = Density?
I have also read that TDS can be found using density,
TDS = 1.1 106 (? - 1)(1)
where
TDS = Total Dissolved Solids (ppm)
? = relative density of the boiler water at 15.5oC
This is a manual method and requires measuring a cooled down sample with a very sensitive hydrometer.
Does TDS = Density?
Is there a way to convert TDS into density?
What is the Hydrometer graduated in (g/cm3)?
THANK YOU FOR YOUR TIME!

 

[IRstuff]

TDS represents the density of the total dissolved solids:

http://www.swbic.org/education/env-engr/chem/tds.html

http://en.wikipedia.org/wiki/Total_dissolved_solids

TTFN

 

[kenvlach]

There are different scales for hydrometers, but the ones commonly used for aqueous solutions read SG (specific gravity) relative pure water = 1.000 at a specific temperature like 60 ^oF (15.5^oC) (you should cool samples to this T).
To a good first approximation, the dissolved solids are what raise the measured SG above 1.
So, if SG = 1.1106, then [TDS] = 0.1106 g/cm³ = 110,600 ppm. Sounds like a concentrated solution of something, more like a plating or etching solution.

 

[waross]

I doubt that you can directly convert a TDS value into SG.
You would have to know the SG of the individual disolved solids and the proportions where there is more than one solid involved. The volume of the water containing disolved solids may not equal the sum of the volume of the water and the solids seperately. This will also have an effect on the correlation between TDS and SG.
However, repetitive testing of samples containing similar solids can probably be done quickly and fairly accurately with a hydrometer once a conversion table is developed.
respectfully

 

[orenda1168]

NMLS:

Further to Kenvlach's recommendation, I would be more inclined to gravimetrically determine the specific gravity as hydrometers are notoriously difficult to accurately read to more than two or three decimal places.

Using a volume-calibrated flask, weigh to four decimal places and determine the SG for pure water at a given temperature (not necessary to do this at 60F as the temperature effect on SG is cancelled out by the weight difference below), do the same for the elevated TDS sample at the same temperature, with the difference of the two weighings being the TDS expressible as g/cm3 or ppm as you desire.

I disagree with Waross in that the individual components of the TDS are immaterial....the total TDS is a mass calculation per above, not dependent on the specific composition.

Orenda

 

[waross]

I believe that you have to reconcile the amount of water displaced by the disolved solids. A denser solid will displace less water.
A hypothetical solid with the same density as water would not be detected by weighing a sample.
respectfully

 

[kenvlach]

wwaross, that is not accurate at all -- maybe coincidentally true in some cases, such as slurries (sand + water), but not dissolved substances (salt + water).
It ignores the thermodynamics of solution. Whether material is polar or non-polar, dissolves as ions or molecules, etc., is more important.

Instead of a hypothetical substance, consider 2 real examples:
Example 1. Create a 50 wt% solution of NaOH in water. Very simple,
1000 g H₂O, volume = 1000 cc
+ 1000 g NaOH, volume = 469.5 cc (from density 2.13 g/cc)
= 2000 g of solution.
But is the SG = 2000g/1469.5 cc = 1.36 g/cc?
No, the true SG by measurement is 1.52, because there is strong interaction of the Na⁺ & OH^- ions with water (gets hot, you have to cool before measuring SG).

Example 2. Create a 50 v/v% conc. sulfuric acid-water solution.
1000 mls H₂O, wt = 1000 g
+ 1000 mls sulfuric, wt = 1830 g

But is the SG = 2830 g/2000 mls = 1.415?
No, true SG = 1.555, because strong interaction, known as exothermic heat of solution, so volume is only 1820 mls (after cooling).

orenda1168, it is true that you can accurately determine SG as described, maybe you also did a pyncnometer experiment in physical chemistry lab?
But, easier to use accurate hydrometers:

https://www1.fishersci.com/Coupon?cid=1328&gid=190941&details=Y

 

[waross]

Hi kenvlatch;
Thanks for the information.
My point was that TDS does not equal SG. I tried to use a simple illustartion.
I think you have shown that although you may determine the DTS of a sample with an SG test, you must know the composition of the sample under test.
ASlthough there is a correlation between TDS and SG, they are not equal and the ratio depends on the nature of the disolved solids.
If you are testing boiler water fed from a water source with a consistent TDS composition, you can quickly and accurately determine DTS from an SG test, but TDS and SG are not equal.
respectfully

 

[kenvlach]

Actually, SG measurement isn't used for boiler feedwater -- not nearly sensitive enough for TDS:
Since TDS, ppm = (SG - 1) x 10⁶ and an SG hydrometer may have only be readable to 0.0005, the smallest increment of TDS is 500 ppm. The error from not being at the temperature of calibration may be greater.

Commonly used are conductivity meters with built-in temperature compensation. These can give TDS directly, although based on typical impurities in water.
Cheers

 

[NMLS]

To Kenvlach,
We use hydrometer on board our steam ships to obtain an approximate measurement of the dissolved solids in the boiler water. Since the relative density of water is related to its dissolved solids content, each increase of 0.0001 relative density at 15.6°C is approximately equal to 110 ppm. Of course, the sample must be cooled down to 15.6°C or a correction chart used(supplied with the hydrometer) for proper analysis. The hydrometer that is used is specifically for our boiler water taking into consideration the types of chemicals used for water treatment. I my-self have not carried out the tests but have been explained breifly, so if you have any more to add please do so.

 

[kenvlach]

Your understanding is correct.
TDS limits in low pressure boilers, typically used for heating, can be quite high, so SG measurements can be useful -- although not for the feedwater, but in the boiler itself, where dissolved solids become concentrated. Maybe where the cost of producing water in a once-through system is high, less pure feedwater is tolerable -- weighed against the increased maintainence cost of the system being fed the steam.

TDS limits decrease for higher pressure boilers, as typically used for power. Some info on TDS limits:

http://www.engineeringtoolbox.com/feedwater-chemistry-limits-d_1064.html

TDS measurement by conductivity is much easier, and the instruments I've used had automatic temperature compensation (ATC), so not necessary to completely cool samples. A very brief description:

http://www.engineeringtoolbox.com/total-dissolved-solids-d_1062.html

For more detailed boiler systems info, see

http://www.gc3.com/techdb/manual/boilfs.htm

Especially, note 'Table: ASME Guidelines for Water Quality.' Gives limits for both feedwater & boiler water for various steam pressures.