Fixed Solids

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Definition - What does Fixed Solids mean?

Fixed solids are solid substances that do not easily evaporate at temperatures less than 550°C (1022°F) and is the residue that is left in the vessel after a sample of solution has been ignited and heated to dryness at 550°C or greater. In other words, total fixed solids are the portion that remains after the solution has gone through heating.

Fixed solids determine the corrosion rate of a metallic substance when exposed to an environment that is conducive for corrosion to occur.

Corrosionpedia explains Fixed Solids

Fixed solids refers to an approximation of the mineral matter that is present after the heating has occurred. The remains of total dissolved solids (TDS) and that of total suspended solids (TSS) after being heated to dryness for a specified period of time and at an assigned temperature is what is referred to as fixed solids. Fixed solids is the residue of total dissolved solids and total solids or total suspended solids after being ignited at 500ºC ± 50ºC.

When fictionalizations resulting from ignition and filtration are combined, there are a total of 9 separate categories, which are known as:

  1. Total solids (TS)
  2. Volatile solids
  3. Fixed dissolved solids
  4. Total suspended solids (TSS)
  5. Volatile dissolved solids
  6. Fixed suspended solids
  7. Total dissolved solids (TDS)
  8. Volatile suspended solids (VSS), and
  9. Fixed solids

Only four of these (VSS, TDS, TSS, and TS) are commonly used. One would expect positive bias from incomplete oxidation of organic matter, and negative bias from decomposition of certain inorganic matter when comparing fixed solids with inorganic content.

Ammonium salts are lost during low temperature drying and also upon ignition, while others are stable under the conditions necessary for volatile solids determination with the exception of magnesium carbonate. A laboratory study found an average standard deviation of 11 mg/L on a sample of 170 mg/L volatile solids. Volatile solids may also be affected by these as well as loss of water of crystallization that are recalcitrant, and previous losses of organic matter to volatilization during low-temperature drying.

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