Not too long ago, parameters of dry mass and dissolving substances were characteristics of drinking water quality, surface water, gutter or soil. The importance of such parameters has decreased of late and now only a handful of legal acts specify norms for dry mass or dissolved substances.
As of 20th August 2008, maximal permissible values for dissolving substances in flowing water class I – 500 mg/l and class II – 800 mg/l are now determined by the Environment Ministry regulation on means of classification of uniform surface water status. Dry mass is now outlined by the Regulation of Health Ministry on natural mineral water, natural spring water, and determination of their mineral components and used as a basis for classification of mineral water:
- Water with low mineral content < 500 mg/l
- Water with average mineral content 500 – 1000 mg/l
- Water with high mineral content >1500 mg/l
The concentration value of dissolving substances in water extracts is now one of the parameters for the acceptance of water for storage. Obtained by standard wash-out tests, the maximal permissible values are very wide and accept values between 2500 mg/kg and 100,000 mg/kg of wastes.
It is, however, evident that there is still a need for determination of dry mass and dissolved substance within laboratories. Such tests are carried out by laboratories in accordance with standard norm PN-78 C-04541 or with their procedures.
Procedures for evaporating water and obtaining dry mass in such a volume that its remaining substances are within 10-250 mg are specified by Polish norms. The remainder should be dried until the dry mass at a constant temperature of 105 ºC.
If the Ph of gutter is below 4.3 or more alkaline than 10, then its factor should be corrected before tests are started. The process is such that an analytical sample should be gradually inserted into an evaporating dish. Renowned organizations, for example, US EPA or Standards Methods, provide similar conditions for matching dry mass, except for higher drying temperatures (180 ºC) for determining dissolved substances. Drying temperatures are set at 105 ºC for dry mass determination. The standard drying method is applied in long-lasting tests with relatively low requirements for measurement accuracy and precision. This has led to moisture analyzers becoming more accessible on the market.
Moisture analyzers are laboratory measuring instruments used to determine the dry mass, and moisture content in multiple substances. A measuring process of a sample in heating time is performed to determine these values. Start mass of a sample is determined by a moisture analyzer and provides continuous measurement during an intensive heating process with a halogen lamp.
The drying process can be performed in one of the few available drying profiles dependent on the sample structure, its size, and water permeability. For these reasons, it seemed appropriate to use a moisture analyzer to determine dry mass from water. First, a vessel was dried before water was poured into it. The water sample was cooled down in an exsiccator and weighed. A moisture analyzer conducted evaporation of water from a sample, and maintained a set temperature until a stable dry mass readout of sediment was obtained. The moisture analyzer provided a corresponding indication on its display throughout the process.
Tests were carried out with a moisture analyzer MAC 50/1 manufactured by RADWAG. This moisture analyzer has increased maximal capacity from 50 g to 75 g, for the application of standard laboratory glass (Petry’s pan). Samples were evaporated with maximal capacity of 30 ml and a standard temperature profile was used.
Dry mass determination was carried out for six types of market-accessible mineral water (Cisowianka, Carrefour, Kryniczanka, Muszynianka, Wielka Pieniawa and Źródła Muszyny) as well as municipal water and a reference solution of sodium chloride c(NaCl) = 2000 mg/l.
Hydrogencarbonate anion (500 mg/l – 14000 mg/l) was the main component of all the waters. Sulfate concentration varied from a couple to 30 mg/l, and chloride concentration was usually 2 – 3 times lower. Ca2+ concentration in Wielka Pieniawa water was even 10 times lower. Among univalent cations, the Na+ remained at a concentration level of dozens of mg/l and concentration of K+ was at a couple of mg/l.
Dry mass matching process was carried out in accordance with norm PN-78 C-04541. Chart no.1. displays all the results.
Chart 1. Dry mass determination process for mineral water and municipal water samples. Source: Radwag Balances & Scales
||Dry mass [mg/L]
||MAC – moisture analyzer
||445,3 (SD = 4,6)
||486 (SD = 46)
||1215 (SD = 22)
||1208 (SD = 21)
||1521 (SD = 39)
||1549 (SD = 16)
||1148 (SD = 14)
||1189 (SD = 36)
||904 (SD = 4)
||911 (SD = 65)
||1196 (SD = 24)
||1225 (SD = 73)
||579 (SD = 12)
||537 (SD = 21)
|Solution c(NaCl)=2000 mg/L
||2007 (SD = 15)
||2020 (SD = 26)
There was no real difference between results obtained through application of a moisture analyzer and those obtained from the reference method. Matching precision reached 10%, but results obtained with a moisture analyzer are usually less precise.
An average recovery from NaCl reference solution was 101% (100.4% in case of procedure no. PN-78 C-04541). The procedure time was shortened from six hours (traditional method) to three hours by using the moisture analyzer drying method. The process was also less labor-intensive, as there was no need to dry a sample for 2 or 3 times in an exsiccator before weighing it.
The limiting factor with a moisture analyzer is that it is only possible to dose a water sample once. This eliminates the possibility of match substances in low mineralized water (<300 mg/l). Operators believe that it would be of advantage to add a vessel for testing 50 ml water samples to moisture analyzer equipment. This would form a very useful combination for routine testing of dry mass with an acceptable level of accuracy and precision, and a matching limit that enables the testing of the vast majority of water types on the market.
In response to moisture analyzer operators from Radom Technical University, RADWAG now offers aluminum weighing pans that can hold around 50 ml of water. These pans are higher than standard ones, but they mean that there is now no need to increase the moisture analyzers' maximal capacity. The tests can be carried out on standard devices with no need for any modification for specific applications. As the volume of the water sample increases during the testing procedure, the accuracy of dry mass determination also increases, resulting in better repeatability of tests.
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