An Alternative to TDS and TS Analysis

An Alternative to Total Dissolved Solids (TDS) and Total Solids (TS)
by Joe Boyd

An alternative to preparing, weighing, cooling and cleaning dishes and crucibles for total dissolved solids (TDS) and total solids (TS) analysis uses disposable prepared vessels, eliminating hours of manual labor and increasing efficiencies.

The basics of TDS testing

TS are the materials left in a vessel after a sample is evaporated at a defined temperature. TDS are the portion of the total solids that pass through a 2.0-μm (or smaller) nominal-pore-size filter. For TS, the liquid sample is evaporated at a temperature of 104 °C; for TDS, the filtrate is first evaporated and the evaporating dish is then transferred to a 180 °C oven for at least one hour. In both analyses, the evaporating dish is cooled in a desiccator to balance temperature and is then weighed. These heating, cooling and weighing steps are repeated until a constant weight of ±0.5 mg is achieved. The difference between the post- and preweight is the amount of TS or TDS in milligrams for the sample.

The key to successful TS and TDS Analysis is the container from which the filtrate is evaporated. Approved analytical methods such as Standard Methods 2540B and 2540C have specific instructions regarding preparation of the container to achieve the most accurate results possible; the evaporation dishes must hold at least 100 mL and be constructed of porcelain, platinum or high-silica glass. The dish must be washed with soapy water to remove residue, rinsed with deionized water and then dried for one hour at 104 °C for TS or 180 °C for TDS. It should then be stored in a desiccator until ready for use (the dish should be weighed immediately prior to use).1,2

Vessels for TDS

StableWeigh™ vessels from Environmental Express are an alternative to traditional dishes or crucibles. They are constructed of a proprietary blend of high-temperature plastics and arrive at the laboratory as prepared vessels, ready to use for TS or TDS Analysis. Each vessel meets the preparation requirements of Standard Methods and is preweighed, with the weight and ID printed on the vessel. This saves several hours that would be spent manually cleaning and weighing dishes. The vessels are also disposable.

StableWeigh vessel advantages

Traditional dishes used for TS and TDS analysis weigh around 80 g, and the sample residue range per Standard Methods is 10 to 200 mg. This creates sensitivity issues in the analysis that can lead to erroneous results and difficulties establishing weight stability. StableWeigh vessels weigh between 3 and 4 g, giving much better resolution on lower TS/TDS ranges and helping to achieve weight stability faster.

One of the most widely used evaporating dishes is the porcelain crucible. These are shipped to the laboratory with a glaze that helps decrease moisture absorption from the atmosphere, which is important when establishing dish preweights and weight stability at the end of analysis. Over time, the glaze wears off from washing the crucibles to remove residue from previous analyses. Porcelain is a porous material that draws moisture from the atmosphere, affecting weights. This leads to data uncertainty and adversely affects results. StableWeigh vessels are constructed of a proprietary blend of high-temperature plastics that will not absorb moisture from the atmosphere. Because the vessels are disposable, there is also no risk of altering their composition by washing.

Unique properties of vessel material and design greatly reduce cooling times. While dishes or crucibles constructed of porcelain or glass typically require about one hour in a desiccator to cool down after a process conducted in a 104 °C or 180 °C oven, StableWeigh vessels require only 10 to 15 minutes.

Method acceptance

Although StableWeigh vessels are not constructed of one of the materials stated in Standard Methods, they are covered under the method flexibility allowed in U.S. EPA Rule 40 CFR Part 136.6, which sets out the requirements for a modified analytical method to be considered equivalent to a promulgated analytical method:3.

The March 12th Methods Update Rule promulgated 136.6 which allows the regulated community more flexibility that includes:

1. If the underlying chemistry and determinative technique in a modified method are essentially the same.
2. The modified method must be sufficiently sensitive and meet or exceed performance of the approved method.

Data generated by Environmental Express and independent laboratories has proven that StableWeigh vessels meet or exceed the performance of Standard Methods for both TS and TDS (see Tables 1 to 5). Standard Methods is also actively working to update Methods 2540B and 2540C to include StableWeigh as an acceptable evaporating dish.

References

1. Rice, E.W.; Baird R.B. et al., Eds. Standard Methods for the Examination of Water and Wastewater; vol. 22, 2012; APHA, AWWA, WEF: Washington, D.C.
2. Baird, R.B.; Eaton A.D. et al. Standard Methods for the Examination of Water and Wastewater; online edition, 2016; APHA, AWWA, WEF.
3. https://www.epa.gov/sites/production/files/2015-08/documents/cwa-method-flexibility_memo_11-20-2007.pdf

Table 1 – Method blank data for StableWeigh shows results within method acceptance criteria*
*Laboratory Reagent Blank (LRB)

Table 2 –LFB, inorganic material for StableWeigh shows results within ±4% of the true value and obtaining weight stability in 10 out of 10 samples on first try*
*LFB, laboratory fortified blank.

Table 3 –LFB, inorganic material for porcelain evaporating dishes shows results within ±6% of the true value and obtaining weight stability in 6 out of 10 samples on first try

Table 4 – Biosolids (high organic content) sample results using StableWeigh shows weight stability in 10 out of 10 samples on first try

Table 5 – Biosolids (high organic content) sample results using porcelain evaporating dishes shows weight stability in 0 out of 10 samples and weight stability never being achieved in 6 out of 10 samples after two tries