Iodate, determination

Silver diethyldithiocarbamate [1470-61-7] is a reagent commonly used for the spectrophotometric measurement of arsenic in aqueous samples (51) and for the analysis of antimony (52). Silver iodate is used in the determination of chloride in biological samples such as blood (53). 

Analytical Methods. A classical and stiU widely employed analytical method is iodimetric titration. This is suitable for determination of sodium sulfite, for example, in boiler water. Standard potassium iodate—potassium iodide solution is commonly used as the titrant with a starch or starch-substitute indicator. Sodium bisulfite occurring as an impurity in sodium sulfite can be determined by addition of hydrogen peroxide to oxidize the bisulfite to bisulfate, followed by titration with standard sodium hydroxide (279). 

The determination of tin in metals containing over 75 wt % tin (eg, ingot tin) requites a special procedure (17). A 5-g sample is dissolved in hydrochloric acid, reduced with nickel, and cooled in CO2. A calculated weight of pure potassium iodate (dried at 100°C) and an excess of potassium iodide (1 3) are dissolved in water and added to the reduced solution to oxidize 96—98 wt % of the stannous chloride present. The reaction is completed by titration with 0.1 Af KIO —KI solution to a blue color using starch as the indicator. 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

Sample decomposition is the critical operation in determination of total iodine in complex organic matrix. Iodine in simple form (I ) is highly volatile, so it should be transformed into nonvolatile analytical fomi (iodide or iodate) to prevent loses during the decomposition. 

Copper(II) compounds. Many other metallic ions which are capable of undergoing oxidation by potassium iodate can also be determined. Thus, for example, copper(II) compounds can be analysed by precipitation of copper)I) thiocyanate which is titrated with potassium iodate ... 

The liberated iodine and the excess of iodide is determined by titration with standard potassium iodate solution the hydrochloric acid concentration must not be allowed to fall below 7JVf in order to prevent re-oxidation of the vanadium compound by iodine chloride. 

Soluble sulphides. Hydrogen sulphide and soluble sulphides can also be determined by oxidation with potassium iodate in an alkaline medium. Mix 10.0 mL of the sulphide solution containing about 2.5 mg sulphide with 15.0 mL 0.025M potassium iodate (Section 10.126) and 10 mL of 10M sodium hydroxide. Boil gently for 10 minutes, cool, add 5 mL of 5 per cent potassium iodide solution and 20 mL of 4M sulphuric acid. Titrate the liberated iodine, which is equivalent... 

Determination of cerium as cerium(IV) iodate and subsequent ignition to cerium(IV) oxide Discussion. Cerium may be determined as cerium(IV) iodate, Ce(I03)4, which is ignited to and weighed as the oxide, Ce02. Thorium (also titanium and zirconium) must, however, be first removed (see Section 11.44) the method is then applicable in the presence of relatively large quantities of lanthanides. Titrimetric methods (see Section 10.104 to Section 10.109) are generally preferred. 

Discussion. These anions are both determined as silver bromide, AgBr, by precipitation with silver nitrate solution in the presence of dilute nitric acid. With the bromate, initial reduction to the bromide is achieved by the procedures described for the chlorate (Section 11.56) and the iodate (Section 11.63). Silver bromide is less soluble in water than is the chloride. The solubility of the former is 0.11 mg L 1 at 21 °C as compared with 1.54 mg L 1 for the latter hence the procedure for the determination of bromide is practically the same as that for chloride. Protection from light is even more essential with the bromide than with the chloride because of its greater sensitivity (see Section 11.57). 

Determination of iodate as silver iodide Discussion. Iodates are readily reduced by sulphurous acid to iodides the latter are determined by precipitation with silver nitrate solution as silver iodide, Agl. Iodates cannot be converted quantitatively into iodides by ignition, for the decomposition takes place at a temperature at which the iodide is appreciably volatile. 

Note. For practice in this determination, potassium iodate may be employed. 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

Bancroft and Gesser [870] conclude that kinetic factors are predominant in determining whether decomposition of a metal bromate yields residual bromide or oxide. The thermal stabilities of the lanthanide bromates [877] and iodates [877,878] decrease with increase in cationic charge density, presumably as a consequence of increased anionic polarization. Other reports in the literature concern the reactions of bromates of Ag, Ni and Zn [870] and iodates of Cd, Co, Mn, Hg, Zn [871], Co and Ni [872], Ag [864], Cu [867], Fe [879], Pb [880] andTl [874]. 

On the basis of these facts, it was speculated that plutonium in its highest oxidation state is similar to uranium (VI) and in a lower state is similar to thorium (IV) and uranium (IV). It was reasoned that if plutonium existed normally as a stable plutonium (IV) ion, it would probably form insoluble compounds or stable complex ions analogous to those of similar ions, and that it would be desirable (as soon as sufficient plutonium became available) to determine the solubilities of such compounds as the fluoride, oxalate, phosphate, iodate, and peroxide. Such data were needed to confirm deductions based on the tracer experiments. 

Iodine was determined by an iodometric titration adapted from White and Secor.(3) Instead of the normal Carius combustion, iodide was separated from the samples either by slurrying in 6M NaOH, or by stirring the sample with liquid sodium-potassium (NaK) alloy, followed by dissolving excess NaK in ethanol. Precipitated plutonium hydroxides were filtered. Iodine was determined in the filtrate by bromine oxidation to iodate in an acetate buffer solution, destruction of the excess bromine with formic acid, acidifying with SO, addition of excess KI solution, and titrating the liberated iodine with standard sodium thiosulfate. The precision of the iodine determination is estimated to be about 5% of the measured value, principally due to incomplete extraction of iodine from the sample. 

Cyanide and thiocyanate anions in aqueous solution can be determined as cyanogen bromide after reaction with bromine [686]. The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample, which converts the cyanide ion to the unreactive cyanohydrin. The detection limits for the cyanide and thiocyanate anions were less than 0.01 ppm with an electron-capture detector. Iodine in acid solution reacts with acetone to form monoiodoacetone, which can be detected at high sensitivity with an electron-capture detector [687]. The reaction is specific for iodine, iodide being determined after oxidation with iodate. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [688,689]. The detection limit for the nitrate anion was less than 0.1 ppm. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. Nitrite can be determined directly by alkylation with an alkaline solution of pentafluorobenzyl bromide [690]. The yield of derivative was about 80t.with a detection limit of 0.46 ng in 0.1 ml of aqueous sample. Pentafluorobenzyl p-toluenesulfonate has been used to derivatize carboxylate and phenolate anions and to simultaneously derivatize bromide, iodide, cyanide, thiocyanate, nitrite, nitrate and sulfide in a two-phase system using tetrapentylammonium cWoride as a phase transfer catalyst [691]. Detection limits wer Hi the ppm range. 

Impact sensitivities of mixtures of red phosphorus with various oxidants were determined in a direct drop-ball method, which indicated higher sensitivities than those determined with an indirect striker mechanism. Mixtures with silver chlorate were most sensitive, those with bromates, chlorates and chlorites were extremely sensitive, and mixtures with sodium peroxide and potassium superoxide were more sensitive than those with barium, calcium, magnesium, strontium or zinc peroxides. Mixtures with perchlorates or iodates had sensitivities comparable to those of unmixed explosives, such as lead azide, 3,5-dinitrobenzenediazonium-2-oxide etc. 

Calculations derived from the measurement of final periodate consumption indicate the number of reactive groups and can often be interpreted to reveal the extent of overoxidation. Chemically, this determination involves the use of one of two general reactions. These are (a) the reduction of periodate and iodate to free iodine in acid solution, and (b) the reduction of periodate to iodate in neutral solution. 

Hartzfeld PW, Forkner R, Hunter MD and Hagerman AE. 2002. Determination of hydrolyzable tannins (gal-lotannins and ellagitannins) after reaction with potassium iodate. J Agric Food Chem 50(7) 1785—1790. 

Truesdale [77] has described autoanalyser procedures for the determination of iodate and total iodine in seawater. The total iodine content of seawater (approximately 50-60 ig/l) is believed to be composed of iodate (30-60 xg/l of I) and iodine-iodine (0-20 xg/l) with perhaps a few ig/l of organically... 

Iodate in the buffered sample is reacted with sulfamic acid (to destroy nitrite) and potassium iodide to produce the iodonium ion I3, which is determined spectrophotometrically at 350 nm. 

This determination will test for the presence of naturally occurring reducing agents in seawater which by their action on iodonium ions could lead to an underestimate in iodate concentration. (The use of the method on anoxic waters containing sulfide is a prime example of when this precaution should be taken.)... 

Determinations of iodate without pre-oxidation in Pacific seawater by the previous method gave a mean result of 583 ig/l with a standard deviation of 0.23 xg/l. For samples containing between 40 and 60 xg/l, standard deviations of 0.19 xg/l (iodate method with pre-oxidation), 0.12 xg/l (iodate method without pre-oxidation), and 0.43 ig/l (total iodine method) were obtained. 

A set of Pacific open-ocean samples were analysed for iodate-iodine using both the procedure which incorporates pre-oxidation with iodine water and that which does not. Also, in a similar exercise total iodine was determined using both the method that incorporates pre-oxidation with bromine water and the catalytic method using the reaction between Ce(IV) and As(III) [81]. Variance tests showed that differences between either replicates or methods was not significant. 

Truesdale and Smith [80] also carried out a comparative study of the determination of iodate in open ocean, inshore Irish seawaters and waters from the Menai Straits, using the spectrophotometric method (with and without pre-oxidation using iodine water) and also by a polarographic method [82]. 

Schnepfe [83] has described yet another procedure for the determination of iodate and total iodine in seawater. To determine total iodine 1 ml of 1% aqueous sulfamic acid is added to 10 ml seawater which, if necessary, is filtered and then adjusted to a pH of less than 2.0. After 15 min, 1 ml sodium hydroxide (0.1 M) and 0.5 ml potassium permanganate (0.1M) are added and the mixture heated on a steam bath for one hour. The cooled solution is filtered and the residue washed. The filtrate and washings are diluted to 16 ml and 1ml of a phosphate solution (0.25 M) added (containing 0.3 xg iodine as iodate per ml) at 0 °C. Then 0.7 ml ferrous chloride (0.1 M) in 0.2% v/v sulfuric acid, 5 ml aqueous sulfuric acid (10%) - phosphoric acid (1 1) are added at 0 °C followed by 2 ml starch-cadmium iodide reagent. The solution is diluted to 25 ml and after 10-15 min the extinction of the starch-iodine complex is measured in a -5 cm cell. To determine iodate the same procedure is followed as is described previously except that the oxidation stage with sodium hydroxide - potassium permanganate is omitted and only 0.2 ml ferrous chloride solution is added. A potassium iodate standard was used in both methods. 

The total iodine procedure is claimed to be relatively free from interference by foreign ions. The iodate procedure is subject to interference by bromate and sulfite ions. This method is claimed to be capable of determining down to 0.1 ig iodine in the presence of 500 mg chloride ion and 5 mg of bromide ion. 

Matthews and Riley [99] preconcentrated iodide by co-precipitation with chloride ions. This is achieved by adding 0.23 g silver nitrate per 500 ml of seawater sample. Treatment of the precipitate with aqueous bromine and ultrasonic agitation promote recovery of iodide as iodate which is caused to react with excess iodide under acid conditions, yielding I3. This is determined either spectrophotometrically or by photometric titration with sodium thiosulfate. Photometric titration gave a recovery of 99.0 0.4% and a coefficient of variation of 0.4% compared with 98.5 0.6% and 0.8%, respectively, for the spectrophotometric procedure. 

Shizuo [100] allowed the silver halide precipitate obtained in the co-precipitation process to stand in contact with the solution for more than 20 h to ensure quantitative collection of iodide on the precipitate. He then evaporated the oxidised iodate solution to 5-10 ml and again allowed the solution to stand for more than 12 h before the colorimetric determination. There was no interference from bromine compounds. The errors were then within 3%. 

Wong [8] found that the determination of residual chlorine in seawater by the amperometic titrimetic method, potassium iodide must be added to the sample before the addition of the pH 4 buffer, and the addition of these two reagents should not be more than a minute apart. Serious analytical error may arise if the order of addition of the reagents is reversed. There is no evidence suggesting the formation of iodate by the reaction between hypobromite and iodide. Concentrations of residual chlorine below 1 mg/1 iodate, which occurs naturally in seawater, causes serious analytical uncertainties. 

In the sodium borate solution containing bromide, when the pH 4 buffer is added before the potassium iodate solution, titrations give low total residual chlorine concentrations. This loss increases with the amount of stirring time between the addition of the reagents. Even for a stirring time of 10 seconds, there is a loss of about 17% of the total residual chlorine. If the solution were stirred for 30 min, 85% of the chlorine would have disappeared. The concentration of total residual chlorine determined by the reference methods does not change throughout the experiment. This implies that this loss of chlorine does not occur in the reaction vessel, but in the titration cell as a result of the analytical procedure. 

Wong [8] reported that the losses of chlorine are not related to the formation of iodate by the oxidation of iodide by hypobromite. The presence of iodate in seawater may cause significant uncertainty in the determination of small quantities of residual chlorine in water. Determinations of residual chlorine at the 0.01 mg/1 level are of questionable significance. 

---