Interference with ammonia determination

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

Ammonia Gas Sensor. The determination of ammonia is important in process analysis. An ammonia gas electrode is usually used for this purpose. However, volatile compounds such as amines often interfere with the determination of ammonia. Therefore, a sensor based on amperometry is desirable for the determination of ammonia. 

The primary use of hydrofluoric acid is for the decomposition of silicate rocks and minerals in the determination of species other than silica. In this treatment, silicon is evolved as the tetrafluoride. After decomposition is complete, the excess hydrofluoric acid is driven off by evaporation with sulfuric acid or perchloric acid. Complete removal is often essential to the success of an analysis because fluoride ion reacts with several cations to form extraordinarily stable complexes that interfere with the determination of the cations. For example, precipitation of aluminum (as AI2O3 XH2O) with ammonia is incomplete if fluoride is present even in small amounts. Frequently, it is so difficult and time-consuming to remove the last traces of fluoride ion from a sample that the attractive features of hydrofluoric acid as a solvent are negated. 

There are some disagreements in the literature about which ions interfere with the determination. Some state that potassium, thallium, and rubidium give precipitates similar to the one seen with ammonia, and the reaction is, as mentioned, also used in the test in 3.27. Potassium. One reference lists cesium, barium, zirconium, lead, and mercury as ions that interfere with the test but without stating the nature of interference. However, since none of these cations are capable of traveling from test tube A to test tube B, they should be viewed strictly as possible causes of interference if they unintentionally are present in test tube B and not as candidates of false positive reactions. If the test solution by mistake or contamination is alkaline, the reagent will be destroyed by precipitation of black cobalt(lll) hydroxide. The test is also mentioned to be sensitive toward reducing substances. ... 

Seasalt also may cause errors in the spectrophotometric signal. These salt effects are either a suppression of the analyte absorbance (e.g., in the determination of silicate and phosphate) by the ions of seawater or an effect of the buffer capacity of seawater (e.g., shifts in the reaction pH interfere with the determination of ammonia). 

Sulfide, cyanide, and ammonia interfere with the determination by reacting directly with the measuring membrane of the electrode. After acidification with sulfuric acid the hydrogen cyanide and hydrogen sulfide can be carefully purged out (imder hood), eliminating their interfering effect. 

High concentrations of bromide, iodide, cyanide, or sulfide ions interfere with the determination. They can damage the electrode. Free ammonia would also damage the measuring surface of the electrode dissolving the silver chloride crystals. 

An additional cleanup step of the final extract using SPE on CI8 cartridges provided better removal of possible interference from meat in FZD, NFT, and NFZ assay. The extract after homogenization and n-hexane washing steps was loaded on a preconditioned cartridge, and NFs were eluted with ammonia in MeOH (141). The combination of alumina and C18 SPE cartridges was used for the determination of FZD in shrimp tissue (142). Recoveries ranging from 77% to 85% were obtained with sufficient LOD. 

As an example, a sample that contains a mixture of copper(II) and nickel(II) salts can be analyzed by first electrolyzing the sample solution under acidic conditions with platinum electrodes such that the copper is plated onto a platinum gauze electrode. Because the solution is acidic, hydronium ion is reduced before nickel ion and there is no interference. After the electrolysis for copper is completed, the electrolysis solution can be neutralized and made basic with ammonia. Having determined the copper and removed it from the platinum electrode, one can electrolyze the remaining basic electrolysis solution to plate nickel on the platinum electrode. 

The interferences in the determination of copper, owing to noble metals, are most conveniently eliminated by extracting these metals at the start with dithizone from a 1 M mineral acid solution. Noble metals (except Pd) form yellow-orange dithizonates, and their rates of extraction are much higher than that for copper. The solution is shaken with small portions of dithizone in CCI4 until the organic layer no longer rapidly becomes yellow. The free dithizone should be stripped from the extract with a very dilute ammonia solution. 

Kopito and Shwachman (K7) have described a method for the analysis of lead in either freshly voided or partially decomposed urine. The lead in 25 or 50 ml of urine is coprecipitated on bismuth hydroxide by adding bismuth nitrate and ammonia. After centrifuging, the precipitate is dissolved in acid to a final volume of 5 ml, and this solution is aspirated. With a 25-ml sample, 0.05-0.2 ppm of lead can be determined. The bismuth does not interfere with the lead absorption, and it suppresses interferences from sodium, potassium, calcium, magnesium, and phosphates. This procedure appears to offer sufficient sensitivity and the advantages of freedom from interference and simplicity in operation. Control of pH is not critical. Kopito and Shwachman claim that in... 

The determination of chloride involves the precipitation of chloride with silver nitrate, dissolution of the precipitate in ammonia, and determination of silver in the resulting solution by FAAS using air-acetylene or air-hydrogen flames. Standard silver nitrate solutions in ammonia are used for calibration since bromide and iodide interfere in the determination of chloride. 

Determination of alkali metals, alkaline earth metals, and ammonia is the most often used application of ion chromatography in the range of cation analysis. The key advantage of this method is the ability to determine ammonia in complex samples that contain both inorganic cations and organic amines, as the latter compounds can interfere with the conventional colorimetric or ion selective electrode methods used for ammonia analysis. 

Probably the most comprehensive published assay of DU used in armor pen-etrators was reported on the basis of analysis of an unfired CHARM-3 penetrator (Trueman et al. 2004). A sample from the penetrator was dissolved in 9 M HCl, spiked with U as a yield monitor, and the uranium was separated from impurities on an ion-exchange resin. The isotopic composition of uranium was determined by mass spectrometric techniques. Actinides ( - Am and Np) were determined in the uranium-free solution by gamma spectrometry and 239+24opy and Pu were measured by alpha spectrometry and their presence was confirmed by ICPMS. Technetium-99 was determined by ICPMS when rhenium was used as a carrier and interferences from iron were eliminated by precipitating with ammonia while ruthenium and molybdenum were removed by separation on a chromatographic resin. The content of these radioactive nuclides is summarized in Table 2.7. 

Amines interfere with this assay aliphatic amines yield fluorescent products with these compounds, while ammonia, although not giving a fluorescent product, will combine with (and hence use up) the reagent. Unlike the other methods for protein determination, these reagents will give responses with peptides in addition to proteins. Whether or not this is advantageous depends on the requirements of the investigator. 

Ammonia interferes in the determination. However, its equivalent signal is a mere 5 % of the glycine response over the entire linear range of the method. If necessary, a correction may be applied after parallel determinations of the ammonia content (see Chapter 10). Samples with primary amine concentrations exceeding the linear range, e.g., from anoxic pore... 

The electrode in Figure 23-13a is a glass electrode that responds to the ammonium ion formed by the reaction shown in the upper part of Equation 23-23. The electrode in Figure 23-13b is an ammonia gas probe that responds to the molecular ammonia in equilibrium with the ammonium ion. Unfortunately, both electrodes have limitations. The glass electrode responds to all monovalent cations, and its selectivity coefficients for NHi over Na and are such that interference occurs in most biological media (such as blood). The ammonia gas probe has a different problem — the pH of the probe is incompatible with the enzyme. The enzyme requires a pH of about 7 for maximum catalytic activity, but the sensor s maximum response occurs at a pH that is greater than 8 to 9 (where essentially all of the NH4 has been converted to NH3). Thus, the sensitivity of the electrode is limited. Both limitations are overcome by use of a fixed-bed enzyme system where the sample at a pH of about 7 is pumped over the enzyme. The resulting solution is then made alkaline and the liberated ammonia determined with an ammonia gas probe. Automated instruments (see Chapter 33) based on this technique have been on the market for several years. 

---