INDUSTRIAL AND ENGINEERING CHEMISTRY
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thallic, stannic, bismuth, ferric, platinum, and auric-interfere. Ferrous ion (100 mg. or less) may be present. When ferric ion is present the addition of about 2 mI. of sirupy phosphoric acid and 2 grams of disodium phosphate will eliminate interference UP to 200 mg. of iron, but results are not ideal with
All common organic solvents interfere either by preventing precipitation or by obscuring the end point.
Summary
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TABLEVI. EFFECTOF CERTAIN CATIONS ON TITRATION 15 ml. of (CsHs)rAsC1in 100 ml. or solution saturated with sodium ,chloride. 5 ml. of (C0Hs)tAsCl are equlvalent t o 7.17 ml. of iodine solut~on] Iodine Substances Present besides Sodium Chloride Required
M1. 7.18 7.17 7.17 7.23 7.17 7.10 7.20 7.28 7.33 7.16 7.22
MnClt.2Hz0, 1 gram CrCls.BHz0, 1 gram Sodium citrate, 8.8 grams Sodium citrate, 17.5 grams C u t + , 400 mg. Sodium citrate, 4 grama Cd + + 88 mg. Sodiub citrate, 4.5 gram8 Z n + + , 10 mg. Sodium citrate, 4.5 grams R i + + + . 33 me. sodium &race, 4.5 grama S n + + + +16mg. Sodium kitrate, 4.5 grams Fe+++, 200 mg. Disodium phosphate, 2 grams FeS04.7Ha0, 500 mg.
amounts beyond 100 mg. Citrate and tartrate are not effective in this respect, but citrate prevents interference by rather large amounts of cupric ion and by very small amounts (a very few milligrams) of tin, bismuth, zinc, and cadmium. A few grams of sodium citrate and enough citric acid to make the solution acid to methyl red are added before titrating. Table VI shows the effect of various cations on the titration.
VOL. 11, NO. 4
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Tetraphenylarsonium chloride is useful as a reagent for determining mercuric, stannic, cadmium, zinc, perrhenate, periodate. Derchlorate, and other ions. Peribiate, perchlorate, permanganate, perrhenate, fluoride, bromide, iodide, thiocyanate, molybdate, chromate, tungstate, and large amounts of nitrate interfere by forming insoluble salts wiih the reagent. Mercury, tin, cadmium, zinc, platinum, gold, bismuth, and iron, the complex halide ions of which form insoluble compounds with the reagent, and all ions that can oxidize iodide or reduce iodine interfere. Interference by copper, iron, cadmium, ~ i n cbismuth, , and tin may be eliminated to some extent. The reagent may be standardized potentiometrically with standard iodine, the reaction producing a rusty-orange precipitate of tetraphenylarsonium periodide. The total volume to be titrated should be about 100 ml. of neutral or slightly acid solution saturated with sodium chloride just before the end point is reached.
Literature Cited (1) Blicke and Maroano, J. Am. Chem. SOC.,55, 3056 (1933). (2) Blicke and Monroe, Ibid., 57, 720 (1935). thesis, University of Michigan, 1935. (3) Lamprey, H., FROMa thesis presented by G. M. Smith t o the Graduate Soh001 of the University of Michigan in partial fulfillment of the requirements for the degree of doctor of philosophy.
Duplicating Pipets FREDERIC E. HOLMES Children’s Hospital Research Foundation, Cincinnati, Ohio
P
IPETS similar to Ostwald-Van Slyke “between marks” pipets, but having two bulbs, offer many advantages. 1. The amount of solution required to fill the uncalibrated’ part of the pipet below the lower mark is half as great for the two samples as for samples taken in two fillings of a single-bulb pipet. This small saving of thk specimen is often important where a number of different analyses are required on a limited sample of blood or serum or where capillary blood samples are used. Pipet A , Figure 1, has been used where economy of sample is important-for instance, routinely in taking samples of serum or plasma for cholesterol determinations by the author’s method. 2. When duplicate samples of a supernatant solution are to be drawn off from above a precipitate which is easily disturbed, the use of the duplicating pipet avoids stirring up this precipitate between samples. Pipet B has been used for taking samples of supernatant fluid for ascorbic acid determinations after precipitation of proteins. Its greater length of tip is convenient in removing samples from or delivering to narrow tubes, etc.
A
B
FIGURE1. DUPLICATING PIPETS
3. Whenever it is undesirable to‘ use the same single bulb pipet for a second s a m p l e f o r example, in avoiding exposure of a thin film of sample to, air on the walls of the pipet or the trapping of persistent bubbles in a viscous solution-the use of a single duplicating pipet in place of two single pipets effects a saving in cost of pipets, and in time consumed in washing the pipets. 4. Even though a single pipet could be used, the duplicating pipet saves the analyst’s time, and, what is more important, speeds up the handling of unstable solutions. 5. The average of duplicates involves errors a t only two calibration marks instead of four. Specifications are purposely omitted from Figure 1, since these are type forms which can be modified to adapt them to particular uses. FROMThe Children’s Hospital Research Foundation end the Department of Pediatrics, College of Medicine, University of Cincinnati.
