1.6: Particle Size Analysis: The Hydrometer Method

Purpose:

The purpose of this exercise is to introduce you to one of the most common laboratory techniques for determining soil particle size distribution and soil textural class.

Learning Outcomes:

Upon completion of this exercise you should be able to:

• determine the percent sand, silt, and clay of a soil sample using the hydrometer method

• classify soil textural class

Background:

The hydrometer method is one commonly used method to accurately determine particle size distribution in a soil sample. As the name implies, a hydrometer is used; a hydrometer is an instrument used to measure the specific gravity of a fluid. The basis for this test is Stoke’s Law for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the grain diameter and the densities of the grains in suspension and of the fluid. The grain diameter thus can be calculated from knowledge of the distance and time of fall. The hydrometer also determines the specific gravity (or density) of the suspension, and this enables the percentage of particles of a certain equivalent particle diameter to be calculated.

The hydrometer method is useful only for measuring particles with a grain diameter of 2 mm or less (sands, silts, and clays). Based on Stoke’s Law, it is known that sand size particles (0.05 mm to 2 mm) fall from suspension rapidly. Smaller silt sized particles (0.002 mm to 0.05 mm) remain in suspension longer, but eventually fall from suspension. Clay sized particles (less than 0.002 mm) are small enough to remain in suspension indefinitely. Therefore, two hydrometer readings are necessary to determine particle size distribution. The first reading gives a measure of the percent of silt and clay in suspension. The second reading gives a measure of the percent of clay in suspension. By subtracting the second reading from the first, percent silt can quickly be determined. Also, by knowing that the sample must add to 100%, the percent sand can also quickly be determined. Calculations for this method are provided below.

Once percent sand, silt, and clay are known for a sample, the soil can be classified by textural class using the textural triangle.

Equipment required:

• Soil spatula
• 100 mL graduated cylinder
• Top load balance
• R-O water bottle
• Sharpie marker
• Weighing paper
• Cylinder plunger
• Calculator
• 250 mL Erlenmeyer flask (3)
• 1000 mL graduated cylinders
• Fahrenheit thermometer
• Buoyoucose Hydrometer calibrated to 68 F
• Soil samples
• Electronic mixer and mixer cups
• 5% Calgon solution- (Sodium hexametaphosphate- Na₆(PO₃)₆) is created by mixing 50 grams of Calgon powder into 1 liter (1000 mL) distilled water. I’ll try to always restock- but now you have the recipe just in case!

Exercise:

1. Place 50 grams of your dried, ground, and sieved soil sample in a 250 mL Erlenmeyer flask. Add 100 mL of 5% Calgon solution to the sample, cap flask, and swirl until solution and soil are well mixed (several minutes). Let the mixture sit over night (a minimum of 12 hours) to allow the solution to effectively disperse the soil separates (sand, silt, clay).
2. Transfer soil-Calgon mixture from flask to electric mixer cup. Use a water bottle to completely rinse all material from the flask into the mixing cup. Fill the mixing cup with water to about 3 inches from the top. Attach cup to mixer and stir for 3 minutes.
3. Slowly remove and lower the mixing cup so that the mixer propeller is just above water level. Use a water bottle to rinse all of the soil mixture remaining on the mixing rod and propeller into the cup.
4. Empty mixing cup of soil, Calgon, and water into 1000 mL graduated cylinder. Completely wash remaining residue out of the mixing cup with a water bottle into the graduated cylinder and continue filling graduated cylinder to 1000 mL mark. All soil material should be below the 1000 mL mark 
5. Insert the plunger into the graduated cylinder and gently mix the soil until a uniform suspension is obtained (at least 30 seconds). Make sure that a clock with a second hand is readily visible and that a clean hydrometer is on hand. 
6. As soon as you remove the plunger, check the exact time, record/remember it, quickly rinse the plunger into the graduated cylinder using as little water as possible, and gently insert the hydrometer into the suspension. After 40 seconds has elapsed from the time the plunger was removed, read and record the 40-second hydrometer reading.
7. Remove the hydrometer, rinse it clean, wipe dry, and put it back in its protective case. 
8. To correct for temperature effects and density of the dispersion agent, mix 100 mL of 5% Calgon and 880 mL of distilled water in a clean 1000 mL graduated cylinder and allow it to sit for two hours. (NOTE: 100 mL + 880 mL = 980 mL… the missing 20 mL accounts for the approximate volume occupied by 50 grams of soil).
9. After 2 hours have elapsed, take another hydrometer reading from soil solution and record the 2-hour hydrometer reading. Remove and clean hydrometer.
10. Place clean hydrometer into water-Calgon solution and record blank hydrometer reading. This allows for hydrometer calibration to account for temperature differences.
11. Place thermometer into water-Calgon solution and read temperature. If temperature is above 68◦ F, add 0.2 units to the blank hydrometer reading for EACH degree above 68◦ . If the temperature is below 68 ◦F, subtract 0.2 units from the blank hydrometer reading for EACH degree below 68 ◦F. Record this as the corrected hydrometer reading. 
12. Subtract corrected blank hydrometer reading from 40-second and 2-hour hydrometer readings to calculate calibrated 40-second and 2-hour readings.
13. Calculate the percentages of sand, silt and clay in soil sample using the following equations: % Clay = (calibrated 2-hour reading) x (100/sample weight) % Silt = (calibrated 40-second reading) x (100/sample weight)-(%clay) % Sand = 100 – (%silt + % clay)

Example:

40 second reading = 37 g

2 hour reading = 16 g

Blank hydrometer reading = 4 g

Temperature = 74 o F

Corrected hydrometer reading = 5.2 g = (4 g + 1.2 g temperature correction)

Calibrated 40 second reading = 31.8 g = (37 g – 5.2 g)

Calibrated 2 hour reading = 10.8 g = (16 g – 5.2 g)

% clay: 10.8 g x (100/50g) = 21.6 % clay

% silt: (31.8g x 100/50g) -21.6 = 42 % silt

% sand: 100 – (42 + 21.6) = 36.4 % sand

Textural class = loam

	Sample 1	Sample 2	Sample 3
40 second reading:
2 hour reading:
Blank reading:
Temperature:
Corrected Hydrometer:
Calibrated 40-sec:
Calibrated 2-hour:
% clay:
% silt:
% sand:
Textural class:

** You must turn in a sheet that shows all the work for your calculations. **