Methods Manual for Salt Lake Studies/Salinity/measuring brine density

Authors: PSJ Coleman,

Overview of measuring brine density
The density, as specific gravity, of brine may be measured with a hydrometer or with a density meter such as those designed by Anton-Paar. The resulting measure (usually specific gravity, but some hydrometers measure baume) will be directly related to the mass of total dissolved salts in the water. The method may be tripped up where very high levels of extremely fine suspended particles are present in water – they will be included in the measure of density. In the field this may be avoided by only measuring clearwater samples, or by placing samples in tall tubes to settle, or by filtering the sample. While this method provides information on the mass of dissolved salts, it provides no information on the composition of the salts or their conductivity.

Compared to most other methods, the use of hydrometers has the advantage of cheapness. Hydrometers are calibrated by the manufacturer, and because they are simply weighted glass, they reveal any lack of calibration clearly (they break). The breakability of hydrometers may be considered a disadvantage for field studies, however they are reasonably robust if they are carried in a hydrometer box.

Hydrometers are available in several ranges, and the precision of the results depends on the range of the hydrometer. For salt lake studies, a set of three hydrometers will conveniently cover the entire range of specific gravities from freshwater (SG 1.000) through the crysatallisation point of common salt (SG 1.218) up to the approximate precipitation range of epsom salts (SG 1.300).

Hydrometer readings need to be corrected for temperature because warmer water expands and becomes less dense.

Densimeters are expensive but are available in versions that measure an extremely fine resolution. The fine glass capillary U-tube used in these machines may break if it is exposed to excessive heat. This can happen when carrying these items in cars in desert conditions.

Densimeter method for estimating brine salinity
[needs someone to write this method] ?

Hydrometer method for estimating brine salinity as specific gravity
The use of a hydrometer is the same, regardless of whether it is reading Baume, Brix, SG or some other measure. The instructions here are for a specific gravity (SG) hydrometer. If you use other types of hydrometers, a table is available to convert the readings into SG or TDS (g/L or g/kg).

Equipment required

 * Measuring cylinder
 * hydrometer
 * thermometer
 * temperature correction chart

To determine brine specific gravity

 * Fill a measuring cylinder with brine sample to the point of overflowing
 * Float a hydrometer in brine sample
 * Read and record density level on shaft of hydrometer to level of water
 * Insert thermometer in brine sample
 * Read and record temperature of sample
 * Add or subtract degrees of density according to the temperature of the sample (see Temperature Correction, below)
 * Record corrected reading in laboratory daybook

Things to keep in mind

 * If the hydrometer either sinks to the bottom, or floats too high for a reading to be made, you are using an incorrect hydrometer. Select one with a different range.
 * All hydrometer readings must be corrected from the temperature they were read at, to the temperature the hydrometer was designed for. Most hydrometers are calibrated at 15.6 oC (60oF). Some saltfields have hydrometers calibrated at 20oC. The calibration temperature is noted on the shaft of the hydrometer. Check any hydrometer prior to use!
 * Always record the uncorrected reading and the temperature, as well as the corrected reading. In the event of a miscalculation, there is no way of correcting a "corrected" reading unless the raw data is available!

Temperature correction
The following advice for temperature correction is based on the CRC Handbook of Physics and Chemistry and on Bonython's research for ICI (1948),

1. For brines between SG 1.000 and SG 1.100 The difference each degree of temperature makes to the SG is not constant, however very approximate corrections are:

For every 5 degrees the sample is above the hydrometer's calibrated temperature, add 0.001 For every 5 degrees the sample is below the hydrometer's calibrated temperature, take off 0.001 This is sufficiently accurate for temperatures between 0oC and 40oC, but should not be used outside this temperature range. A more accurate formula is:

Correction (to add on to raw hydrometer reading) = Raw SG + (0.00000359 x temp2 + 0.00006971 x temp – 0.00151687)

2. For brines between SG 1.100 and SG 1.200 The difference each degree of temperature makes to the SG is not constant, however very approximate corrections are:

For every 3 degrees the sample is above the hydrometer's calibrated temperature, add 0.001 For every 3 degrees the sample is below the hydrometer's calibrated temperature, take off 0.001 This is sufficiently accurate for temperatures between 0oC and 40oC, but should not be used outside this temperature range. A more accurate formula is:

Correction (to add on to raw hydrometer reading) = Raw SG + (0.000012 x temp2 + 0.000016 x temp – 0.00288)

3. For brines above SG 1.200 The difference each degree of temperature makes to the SG is not constant, however very approximate corrections are:

For every 2 degrees the sample is above the hydrometer's calibrated temperature, add 0.001 For every 2 degrees the sample is below the hydrometer's calibrated temperature, take off 0.001 This is sufficiently accurate for temperatures between 0oC and 40oC, but should not be used outside this temperature range. A more accurate formula is:

Correction (to add on to raw hydrometer reading) = Raw SG + (0.000009 x temp2 +0.000235 x temp – 0.005475)

Conversion from specific gravity to TDS (g/L)
If you wish to convert temperature corrected SG to total dissolved solids (TDS g/L), a curve has been derived from Baseggio's paper on the composition of seawater derived hyperaline brines (Baseggio, 1974)

TDS (g/L) = -91897 SG4 + 403869 SG3 – 663919 SG2 + 485355 SG – 133408

This curve is most accurate in the range of brines that have densities between SG 1.000 and SG 1.250. For specific gravity readings above this, the curve produces conversions that are not as accurate.

If you wish to convert directly from uncorrected SG and temperature to TDS (g/L), the table below provides a useful field conversion for the range from freshwater to SG 1.050, for temperatures between 5oC and 33oC. Table above: obtaining TDS (g/L) from uncorrected SG (LH column) and temperature (top row)