Worksheet ρ
s
Physical and selected chemical methods of soil analysis,
IHLW, SS2018
Particle density
Particle density is the density of solid soil material (without pores) –
Lab method using pycnometer
Sample preparation
•prepare air dry, disturbed soil sample, grain size < 2mm (ca 20g needed),
2 replicates per soil sample
Measurement
•weigh empty, dry pycnometer + plug, note mass (
mP
) and pycnometer nr.
•fill pycnometer with soil up to ca 1/3 of its volume, note mass (
mP+s+w)
,
use ca another 5 g of the same soil sample to measure the gravimetric
water content
w
of the air dry sample (see sheet for water content)
•fill pycnometer up to 1/2 of its volume with de-aerated water
•boil samples using a Bunsen brenner to remove gases, be careful not to
let spread out soil-water-suspension, keep it boiling until no bubbles arise
•fill pycnometer with de-aerated water up to 2 mm above bottleneck
•place pycnometer in the desiccator, close it and establish suction; watch
sample carefully while suction rises – stop vacuum pump when sample
starts boiling again
•wait at least 24 h until samples are completely de-gassed and cooled
down
•take pycnometer out of the desiccator, if needed, fill up with de-aerated
water until 2 mm above bottleneck, close it with the glass plug, avoid air
bubbles below the plug, dry the pycnometer carefully
•weigh pycnometer and note mass (
mP+s+w+W
), then remove plug and record
sample temperature
Results
•calculate values using Eq. (1), comprehend derivation in the lecture notes,
Vp
= inner volume of the pycnometer, given at list of pycnometers,
density of water ρ
w
depends on temperature and has to be calculated
using Eq.(2)
ρs=ms
Vs
=mP+s+w−mp
VP(1+w)−mP+s+w+W+w(1+w)−mP+s+w−w mP
ρw(t)
(1)
ρ
w=a5
[
1−(t+a1)2(t+a2)
a3(t+a4)
]
(2)
Equipment
•pycnometer, 50 mL,
with corresponding glass plug
•scale, precision ±1mg
•Bunsen burner
•desiccator
•vacuum pump
•thermometer, precision ±0.1°C
Why do we measure?
Particle density is mostly considered as
supporting information and used in
calculation procedures for other soil
physical characteristics.
Some background information
In practise, ρ
s
is often estimated via the
density of quartz, ρ
s
= 2.65 g cm-3. For
common mineral soils this is a valid
approximation. Considerable amounts of
ores or organic soils are exceptions.
In theory, ρ
d
could be measured using
saturated soil samples. In practice,
measurements at full saturation are hardly
feasible.
ai
are empirical coefficients:
a1
/°C = -3.983035
a2
/°C = 301.7997
a3
/°C = 522528.9
a4
/°C = 69.34881
a5
/(g cm-3) = 0.99997495
