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Total dissolved solids (TDS) are the sum of non-volatile substances obtained after water is filtered and dried at a certain temperature. High concentrations of TDS can give water an unpleasant taste and damage water distribution pipes and equipment, making it an important indicator of water mineralization.
Currently, two methods are used for detecting TDS in water: drying at 105℃ and drying at 180℃. These are national standard methods, but they are cumbersome, susceptible to interference, difficult to achieve constant weight, and time-consuming, requiring more than a day for a single test.
A new method, the electrode method, has been developed for determining TDS in water. This method is simple to operate, has a short testing time (a water sample can be tested in just a few minutes), and has yielded satisfactory results with excellent precision and accuracy.
Tests of the same water sample using the electrode method, the 105℃ drying method, and the 180℃ drying method showed that the difference between the electrode method and the 105℃ drying method was <10%, and the difference between the electrode method and the 180℃ drying method was <5%.
1. Instruments and Reagents
Sension™ 156 Portable Multi-Parameter Analyzer (Hach Instruments, USA)
Constant Temperature Electric Hot Water Bath (Shanghai Yuejin Medical Instrument Factory)
101-2-BS Electric Heating Drying Oven (Shanghai Yuejin Medical Instrument Factory)
Millipure Water System
10g/L Sodium Carbonate Solution
Total Soluble Solids Standard (National Standardization Institute)
2. Method Comparison
When determining the water sample using the electrode method, the instrument was calibrated with a prepared 200mg/L standard before the water sample was measured. The water sample did not need to be filtered and was measured directly. There were no specific requirements for the sampling volume, as long as the electrode probe could be fully inserted into the water. When determining the water sample using the drying method, the water sample needed to be filtered through a 0.45um pore size filter membrane before measurement, and the sampling volume was accurately 100ml. The same water sample was measured using both the electrode method and the drying method. Result:
(Unit: mg/L)
Sample number | Electrode method measurement value | Measured value at 105℃ | 180℃ measurement value | The difference from the result measured at 105℃ is %. | The difference from the result measured at 180℃ is %. |
1 | 272 | 284 | 273 | 4.2 | 0.4 |
2 | 273 | 281 | 266 | 2.8 | 2.3 |
3 | 139.9 | 152 | 138 | 8.0 | 1.4 |
4 | 146.2 | 158 | 144 | 7.5 | 1.5 |
5 | 142.2 | 149 | 146 | 4.6 | 2.6 |
6 | 142.5 | 152 | 148 | 6.4 | 3.7 |
The table shows that: when measuring water samples using the electrode method, the measured values are closer to those measured at 180℃ (drying temperature), with a difference of <5%; when measuring water samples using the electrode method, the measured values are more different from those measured at 105℃ (drying temperature), with some results showing a difference >5%, but <10%. There is no significant difference between the electrode method and the drying method results.
3. Standard Measurement
Preparation standard value | Measured values | RSD% | relative error % |
400 | 402 402 402 402 402 | 0 | 0.5 |
100 | 102.4 102.4 102.3 102.4 102.3 102.4 | 0.1 | 2.4 |
Using a standard calibration instrument with a concentration of 200 mg/L, the prepared standards of 400 mg/L and 100 mg/L were measured. Results: (unit: mg/L)
As can be seen from the table, the results of the electrode method for determining TDS (total dissolved solids) are accurate and the precision is very good.
4. Spike Recovery Determination
A certain amount of standard was added to the water sample using a 200 mg/L standard calibration analyzer, and then the test was performed. The results are as follows:
(Unit: mg/L)
water sample baseline | scalar | Measured values | RSD% | Recovery rate % |
273 | 100 | 375 375 375 375 375 375 | 0 | 102 |
139.9 | 100 | 242 242 242 242 242 | 0 | 102.1 |
142.5 | 100 | 243 243 243 243 243 243 | 0 | 100.5 |
The table shows that the recovery rate for TDS (total dissolved solids) determination using the electrode method ranges from 100.5% to 102.1%.
The electrode method for determining TDS (total dissolved solids) in water shows no significant difference in results compared to the drying method, indicating that the results are accurate and reliable. The electrode method for determining TDS in water is simple to operate, has a short detection cycle, and can be widely used.
