Chromium standard solution

Tris (l-phenyl-l,3-butanediono) chromium (III) NBS No. 1078 ca. 9.6% chromium, for preparation of organo-chromium standard solution. 

With the exception of iron(II) and uranium(IV), the reduced solutions are extremely unstable and readily re-oxidise upon exposure to air. They are best stabilised in a five-fold excess of a solution of 150g of ammonium iron(III) sulphate and 150 mL of concentrated sulphuric acid per litre [approximately 0.3M with respect to iron] contained in the filter flask. The iron(II) formed is then titrated with a standard solution of a suitable oxidising agent. Titanium and chromium are completely oxidised and produce an equivalent amount of iron(II) sulphate molybdenum is re-oxidised to the Mo(V) (red) stage, which is fairly stable in air, and complete oxidation is effected by the permanganate, but the net result is the same, viz. Mo(III)- Mo(VI) vanadium is re-oxidised to the V(IV), condition, which is stable in air, and the final oxidation is completed by slow titration with potassium permanganate solution or with cerium(IV) sulphate solution. 

Aqueous standard solutions are a source of certain difficulties In electrothermal atomic absorption spectrometry of trace metals In biological fluids The viscosities and surface tensions of aqueous standard solutions are substantially less than the viscosities and surface tensions of serum, blood and other proteln-contalnlng fluids These factors Introduce volumetric disparities In pipetting of standard solutions and body fluids, and also cause differences In penetration of these liquids Into porous graphite tubes or rods Preliminary treatment of porous graphite with xylene may help to minimize the differences of liquid penetration (53,67) A more satisfactory solution of this problem Is preparation of standards In aqueous solutions of metal-free dextran (50-60 g/llter), as first proposed by Pekarek et al ( ) for the standardization of serum chromium analyses This practice has been used successfully by the present author for standardization of analyses of serum nickel The standard solutions which are prepared In aqueous dextran resemble serum In regard to viscosity and surface tension Introduction of dextran-contalnlng standard solutions Is an Important contribution to electrothermal atomic absorption analysis of trace metals In body fluids. 

Stein et al. [673] have described a simplified, sensitive, and rapid method for determining low concentrations of cadmium, lead, and chromium in estuarine waters. To minimise matrix interferences, nitric acid and ammonium nitrate are added for cadmium and lead only nitric acid is added for chromium. Then 10,20, or 50 pi of the sample or standard (the amount depending on the sensitivity required) is injected into a heated graphite atomiser, and specific atomic absorbance is measured. Analyte concentrations are calculated from calibration curves for standard solutions in demineralised water for chromium, or an artificial seawater medium for lead and cadmium. 

The same method is given in BS 903 Part A3782 which also contains a national annex giving a second method for assessing the degree of corrosion when the rubber is not in contact with the metal. Zinc is used as the standard metal as this is fairly readily corroded. A strip of zinc and the rubber test piece are both suspended over distilled water in a stoppered container maintained at 50°C. After a period of three weeks, the corrosion products are removed from the zinc by immersion in chromium trioxide solution and the loss in weight used as the measure of degree of corrosion. This is a very sensitive method but even more care has to be taken than in the contact method to avoid contamination and to obtain reproducible results. 

Evaluation of the peaks shown in Fig. 4.18 produced with chromium(III) and chromium(VI) standard solutions enables the concentration of these two species in unknown water samples to be deduced. 

Standard Preparations Transfer 10.0, 30.0,50.0, and 70.0 mL, respectively, of Chromium Stock Solution to separate, 100-mL volumetric flasks, dilute to volume with water, and mix. The Standard Preparations contain, respectively, 1.0, 3.0, 5.0, and 7.0 jxg of chromium per milliliter. 

Another chemical interference encountered in AAS is that with chromium determinations where the chromium absorption in an air—acetylene flame is suppressed by iron, cobalt and nickel. In this instance, the iron interference can be minimised by the addition of 2% (m/v) ammonium chloride solution to the sample and standard solutions. This compound helps volatilise the chromium as chromyl chloride (Cr02Cl). 

Method. To 10 ml of the urine sample add 0.1 ml of nitric acid and allow to stand for 1 hour. Transfer diree 1-ml portions of die solution to separate test-tubes and to two of diem add, respectively, 1 ml of standard solutions containing 0.02 and 0.05 jag/ml of Cr to die third tube add 1 ml of 0.01 M nitric acid. Inhoduce 5 ]al of each solution into die graphite furnace, dry, ash, atomise, and record the absorbance at 357.9 nm. Record the absorbance of each of the diluted standard solutions in a similar manner. Plot die absorbance of each standard solution against die concentration of chromium, read off the concenhation in die three sample tubes, and calculate the concentration in the sample. The calibration curve should be linear at least in die range 0 to 0.03 lag/ml. 

Colorimetric Methods.—The intensity of the yellow colour of a solution of an alkali chromate is proportional to the amount of chromate in the solution. If, therefore, a given quantity of the solution to be tested has the same tint as an equal depth of a standard solution, it is assumed that there is the same concentration of alkali chromate in both solutions. The sensitiveness is found to be greatest at concentrations between 0-004N and 0-008N with respect to the gram-atom of chromium. ... 

Table 1 Absorbance data measured from standard solutions of chromium and nickel by AAS (a). Calculation of the best-fit, least-squares line for the nickel data, (b)...

Figure 1 Calibration plots of chromium (a) and nickel (b) standard solutions, from data in Table 1. For chromium, a good jit can be dravm by eye. For nickel, however, a regression model should be derived. Table 1(b)...

H.OJ and 5 ml of 10% potassium hydroxide (or sodium hydroxide). Boil the solution gently for 1 h to oxidize Cr to C.f + and to get rid of excess peroxide. Add 10 ml of 6 N H,SO to acidify the solution. After standing in the dark for 5 min, add 20 ml of 10% (w/w) potassium iodide. The solution turns to a dark reddish brown owing to the formation of iodine. Titrate the iodine in the solution with 0.1 N standard solution of sodium thiosulphate until the colour of the solution turns to yellow. Add 1-2 ml of starch solution (1% w/w) as an indicator. The colour of the solution turns to dark blue. Continue titration until the colour just disappears. Record the volume of the standard solution of sodium thiosulphate used (V ). The concentration C (g 1 ) of initial chromium sulphate solution can be calculated by the following equation ... 

Prepare the standard chromium solutions for calibration. Pipette respectively, for example, 0.1 ml, 0.2 ml, 0.4 ml, 0.5 ml and 0.7 ml of a concentrated standard chromium solution (such as 1000 ppm, commercially available) into 100 ml volumetric flasks. Dilute them to the mark with deionized water. The concentrations of the diluted standard solutions are 1 ppm, 2 ppm, 4 ppm, 5 ppm and 7 ppm, respectively. 

If there is no available salt of constant composition, such as copper(II) sulphate or iron(III), aluminium or chromium alums, it is advisable to prepare first a stock solution of an approximate concentration, slightly higher than required and to determine the concentration by a gravimetric or volumetric method. After suitable calculations the solution is diluted with pure solvent to obtain a solution containing exactly, e.g., 1 mg/ml of the given element. In some cases, standard solutions are obtained by dissolving a precisely weighed amount of the element in its pure form. 

Neutralize the chromium(VI) solution, and then add 5 ml of H2SO4 (1+3). Make the solution up to the mark with water in a 25-ml standard flask, and measure the absorbance at 400 nm against water. 

Notes. 1. Chromium(VI) standard solution is used to construct the calibration curve. The procedure is thereby simplified, the oxidation of chromium being unnecessary. 

Standard chromium(III) solution 1 mg/ml. Dissolve 9.1970 g of chrome alum CrNH4(S04)2.12H20 in water containing 2 ml of cone. H2SO4, and accurately dilute the solution with water to 1 litre. Working solutions are obtained by suitable dilution of the standard solution with 0.01 M H2SO4. 

Spectrophotometric determination of the extractable content of chromium, lead, nickel and zinc in sewage sludges. A similar strategy to the above-mentioned [355] was used but different standard solution streams were involved. Matrix effects were successfully overcome [356],... 

To obtain the working curve, the following conditions and procedures were used. Standard solutions containing 2, 4, 8, 10, and 20 ppm germanium were prepared, all containing chromium at the same concentration, 50 ppm. The chromium serves as the internal standard. 

A solution containing 0.319 g of C1CI3 6H2O was passed through a cation-exchange resin in the acid form, and the acid liberated was titrated with a standard solution of NaOH. This required 28.5 ml of 0.125 M NaOH. Determine the correct formula of the chromium(m) complex. 

Standard addition sample preparation Using the standard solutions specimens are made up with 0, 2, 4, and 6 xg/liter of added chromium in terms of the urine volume assayed. A reagent blank is also assayed in parallel, the urine being replaced by 0.05 M nitric acid. 

---