Factors affecting the determination of total phosphorus by ammonium molybdate spectrophotometry

- Jul 29, 2020-

Factors affecting the determination of total phosphorus by ammonium molybdate spectrophotometry


1. Washing of sampling bottles and sample storage


Since total phosphorus is easily adsorbed, the sampling bottle should be cleaned before sampling. When cleaning, you can wash with chromic acid lotion first to wash away the foreign matter adhering to the inner wall of the sampling bottle, and then rinse with tap water and distilled water to control the dryness. Do not use phosphorous detergent for scrubbing. The water sample of total phosphorus is unstable and should be analyzed immediately after sampling to minimize the change in the determination result. If the analysis cannot be performed immediately after sampling, add hydrochloric acid or sulfuric acid to the sample to pH ≤ 2, and determine as soon as possible within 24 hours.


2. The influence of potassium persulfate on the determination result


In the daily determination of total phosphorus, potassium persulfate has a great influence on the determination results. The quality of commercially available potassium persulfate is uneven and the impurity content is high, resulting in high blank absorbance, which affects the experimental results. Therefore, use imported potassium persulfate as much as possible. Secondly, in the preparation process of potassium persulfate, because it is not easy to dissolve, it can be dissolved in a water bath on a water bath, but the temperature should not exceed 50℃ to prevent the temperature from being too high and potassium persulfate from decomposition and failure.


3. Influence of chroma and turbidity

total phosphorus

The wastewater we routinely test often encounters color or turbidity in the water sample, which requires us to use turbidity-color compensation to eliminate its influence on the final measurement result. In the measurement process, after we digest the sample at high temperature, we will find that the water sample becomes clear. When the total phosphorus content in the sample is low and the sample is turbid, this method is more appropriate and accurate.


4. Dissolve the influence of time


Under neutral conditions, the sample is digested with potassium persulfate, and all the phosphorus contained is oxidized to orthophosphate. In an acidic medium, orthophosphate reacts with ammonium molybdate to generate phosphomolybdenum heteropoly acid in the presence of antimony salt, which is immediately reduced by ascorbic acid to form a blue complex whose color is directly proportional to the concentration. Determine its content by spectrophotometry. This shows that the change of the digestion time directly affects the conversion rate of orthophosphate. In theory, if the conversion rate of orthophosphate becomes lower, the total phosphorus content of the water sample will also be directly affected and become smaller. Once the conversion of non-orthophosphate in the water sample is complete, the subsequent digestion is unnecessary. Therefore, the best digestion time is 30 minutes (refer to the instrument manual for details).


5. The influence of color temperature and color time


For high-concentration samples, the color reaction is faster, and the color can be completely developed in 7-8 minutes. The color development time of low-concentration samples is longer; when the temperature is high, the molecules move faster, and the color development time is shortened. In winter, the room temperature is often very low, which requires increasing the environmental temperature of the color reaction of the sample. The color reaction can be carried out in a water bath at 20-30°C to ensure a complete color development time of 15 minutes. It should be measured as soon as possible after the color development is complete, to prevent fading from affecting the absorbance.

total phosphorus test

6. Adsorption of the cuvette


The final colored phosphomolybdenum blue has a strong adsorption on the cuvette, and the adsorption is especially obvious at high concentrations, which leads to higher results. For accurate determination, for high-concentration samples, you can take one sample for each measurement, rinse the cuvette 3 times with distilled water, and then measure the next sample, and try to determine the concentration from low to high to reduce the error caused by adsorption. After the measurement is completed, you can often see obvious blue adsorption on the wall of the cuvette. The cuvette should be soaked in chromic acid lotion or dilute nitric acid for a while, and then rinsed for next use