Total, Volatile, and Fixed Solids

Discussion

The principle is to dry off water to determine the solids, and burn of volatiles to determine the volatile solids and fixed solids. This method is commonly used for characterizing sludges from wastewater treatment.

Materials and Equipment

·        Porcelain evaporation dish/crucibles

·        Drying Oven – e.g. Fisher Scientific Isotemp Oven

·        Muffle Furnace – e.g. Lindberg Blue Box Furnace

·        Gloves for handling hot samples in oven

·        Desiccator (with color indicator desiccant)

·        Mass Scale (use the same scale throughout)

·        25 ml serological pipette

·        Motorized pipette

·        Meker-Fisher burner with ceramic triangle and spark liter. 

Reagents

None

Procedure

1.  Set Muffle Furnace to between 550 ˚C and 580°C
2. Prepare at least three samples for each volatile solids measurement. Having 5 or more samples helps ensures that three meet all the requirements (sometimes crucibles break or a sample does not agree within 5% of the average and must be discarded).
3. Set Drying Oven to between 45 and 97 ˚C (Splashing due to boiling should be avoided and if witnessed, then the temperature should be lowered). Use of higher temperatures is OK if there is no free water in the sample.
4. Place cleaned evaporating dishes in Muffle Furnace for 1 hour or for Total Solids only, place in Oven at 105°C (these may already be in oven)
5. Cool evaporating dishes in desiccator
6. Homogenize the sample to be measured (mix thoroughly so that there is even solids distribution when sampled).
7. Record the dry weight of each evaporating dish as value “B_b” and return to the desiccator until ready to use. “b” corresponds to the label of the dish.
8. Fill the dish approximately 30 ml full with a measured amount of sludge.  Record the value of sludge added in ml as “C_b”
9. Place in the drying oven at 68°C until sample appears to be moisture free.
10. Once the sample appears to be moisture free, increase the temperature of the drying oven to 104 ˚C and dry for an additional hour.
11. Cool sample in desiccator until it reaches the same temperature as the scale being used. Record mass of the dried as “A_b1”.
12. Repeat drying for 1hr in oven, and cooling in the desiccator until the weight change is less than 4% or 0.5mg, whichever is less. Record results as “A_b2, A_b3, ect”. Duplicate determinations should agree within 5% of their average. Discard any samples which cannot meet this requirement by the fourth drying cycle. Use the last “A_b” value recorded as the final measurement for the equation below. IF ONLY ACQUIRING TOTAL SOLIDS MEASUREMENT then stop here, wash crucibles and place back in Oven at 105°C for future use.
13. Burn off initial volatiles over the Meker-fisher burner for approximately 5 minutes; Tilt the crucible so that the flame comes across the top opening. The contents should gassify under oxygen free conditions and then ignite in the flame upon exiting the crucible. Take care not to have the solid contents burning uncontrollably as this can result in the escape of non-volatile solids and lead to error.
14. Transfer crucible and contents to the Muffle Furnace (set at ~575°C) for 15 minutes (wait until the furnace is at required temperature to start time if it is not already and store the sample in the desiccator while waiting).
15.  Remove crucible from muffle furnace and cool in open air partially, then a desiccator until it is the temperature of the scale and weigh the dish with contents. Record results as “D_b1”.
16. Repeat igniting in the muffle furnace for 10 to 15 minutes, cooling outside then inside the desiccator and weighing until the weight change is less than 4% or 0.5 mg, whichever is less. Record results as “D_b2, D_b3, ect”. Use the last “D_b” value recorded as the final measurement for the equation below. Duplicate measurements should agree within 5% of their average. 
17. Calculate the Total Solids , Volatile Solids , and Fixed Solids ( in g/L for each of the three samples that were selected in the previous step (Equations below).

Total Solids (g/L) = ((A_b-B_b) x 1000)/C_b (ml)

Volatile Solids (g/L) = ((A_b-D_b) x 1000)/C_b (ml)

Fixed Solids (g/L) = ((D_b-B_b) x 1000)/C_b (ml)

Average the results for all samples acquired.

Common Mistakes

Mistake	Evidence of the Mistake	Cause	Effect on Results
Opening Dessicator too rapidly when retrieving crucibles	Material gets blown out of the crucible	Vacuum develops in the dessicator while samples are cooling and results in an in rush of air	Lower values
Inadequate burn off of volatile solids prior to placing in the muffle furnace	Seeing flakes of burnt material outside the crucible	Solids caught fire while in the muffle furnace	Lower values

Precision and Interference

Calcium, magnesium, chloride and sulfate ions may bind water and prevent it from evaporating in the oven. High amounts of carbonate or bicarbonate in the water can form a crust which prevents additional water from evaporating.