Lead chromate preparation

Lead chromates are prepared by precipitation techniques from soluble salts ia aqueous media. The raw material Hst iacludes a number of different lead compounds, eg, Htharge, lead nitrate, basic lead acetate, basic lead carbonate, as well as acids, alkahes, sodium bichromate, and sodium chromate. The typical reaction can be represented by the foUowiag equation ... 

Before lead in paint was discontinued, lead chromate was a common pigment in yellow paint. A 1.0-L solution is prepared by mixing 0.50 mg of lead nitrate with 0.020 mg of potassium chromate. Will a precipitate form What should [Pb2+] be to just start precipitation ... 

Preparations containing >0,5% lead chromate are required to be labelled according to EU Guideline 67/548/EEC (Annex I) as toxic (Symbol skull and crossbones) and beside others with the Risk phrases R 40 (limited evidence of carcinogenic effect), R 61 (may cause harm to the unborn child) and R 62 (possible risk of impaired fertility). 

Prepare the copper oxide-lead chromate as follows Spread a thin layer of coarse copper oxide over a small iron plate, heat from above with a blow pipe until the glow is as bright as possible and sprinkle with a thin layer of finely powdered lead chromate. The chromate melts at once and spreads over the copper oxide forming a firmly adherent layer and causing the strips of copper oxide to stick together a little. Now turn the cake over and treat the under side in the same way. When the mass has cooled break it up gently in a mortar and remove powder and unduly large pieces with a sieve. 

Lead chromate is found naturally in minerals crocoite and phoenicochroite. It also is readily prepared by adding a soluble chromate such as sodium or potassium chromate to a solution of lead nitrate, lead acetate or other soluble lead(II) salt in neutral or slightly acidic solution ... 

Prepare about 30 g. of normal lead chromate, PbCr04 (Exercise 23), by the double decomposition of lead acetate and potassium dichromate and wash the precipitate until it is free from soluble lead salts. Suspend the yellow chromate in a dilute solution of sodium hydroxide (1 20) and boil until the color has changed to a bright brick red. This material is a basic chromate known as chrome red, having the composition represented by the formula PbCr04-PbO ... 

Lead chromates, lead molybdates, chrome greens, and fast chrome greens are supplied as pigment powders, low-dust or dust-free preparations, or as pastes. For standard specifications, see Table 1. 

It is often true that a basic salt is less soluble than either the base itself or the salt, and in such cases the basic salt is especially easy to prepare. In general, basic salts do not have definite composition but are indefinite mixtures of base and salt. This is true of basic lead chromate in which the ratio of PbO to PbCrCh varies with the method of preparation. Lead chromate, PbCrCh, is used as a yellow pigment for paints. Basic lead chromate is bright red and is also used as a pigment. 

Lead chromate occurs in nature as crocoite, an orange-red mineral. Synthetically prepared lead chromate and its solid solutions with lead sulfate and lead molybdate represent a hue range from primrose yellow to red. The various hues of chrome yellow, chrome orange, and molybdate orange depend not only on composition but also on crystal structure and particle size. 

Acetone-Pis an unstable, white powder or crystalline mass with a melting point of 90 to 93 Celsius. The solid is insoluble in water, but soluble in ether and tetrahydrofuran. It is quite unstable and is rarely used in military or commercial explosives. However it can be utilized as a primary explosive in blasting caps or detonators when desensitized with appropriate materials. To do so, it should be mixed with gum Arabic, carbon black, tri sodium phosphate, chalk, or silicon dioxide powder, and then mixed with a small amount of paraffin s or saturated oils prior to use. Acetone-P can also be slurried with 10% water and 5% hexane for use in blasting caps or detonators. Pure acetone-P should not be used by itself, as it will decay over time potentially leading to explosions. Acetone-P is rather volatile, and a small sample left out in the open will completely evaporate after several days—partly due to decomposition. Acetone-P can also be used in initiating compositions when mixed with sulfur nitride or other primary explosives, and then added to a small amount of a saturated oil. The sulfur nitride and other primary explosives can be replaced by bari urn chromate, copper perchlorate, or lead chromate. Even when acetone-P has been successfully desensitized, it should be used withi n 2 weeks of preparation. ... 

In accordance with the 14th amendment of Council Directive 76/769/EEC [3.146] and revised ChemVerbV [3.147], lead chromate based pigments and preparations are no longer permitted to be used by private consumers and have to be labeled with the phrase Only for industrial purposes . 

For road transport in accordance with GGVSE [3.150] and ADR 2003 [3.151], lead chromate pigments and lead chromate containing preparations are assigned to class 6.1 (T5), III (UN-No. 2291) and have to be labeled with hazard symbol 6.1 (Toxic), if the lead content soluble in hydrochloric acid ]c(HGI) = 0.07 mol L ] exceeds 5%. 

For maritime transport in accordance with IMDG code [3.152] such lead chromate pigments and lead chromate-containing preparations have to be labeled additionally as marine pollutant. 

Precipitated lead chromate tends to change colour during filtration, becoming orange. This is due to the formation of the red basic salt, PbaCrOg or PbCr04.PbO, by hydrolysis, which is the more likely to take place when the salt is prepared from basic lead acetate. ... 

Double salts of lead chromate with alkali chromates have been obtained. The potassium salt, K3Cr04.PbCr04 or KaPb(Cr04)2, is formed as a yellow amorphous precipitate when SO c.c. of a saturated solution of potassium chromate is mixed with 10 c.c. of saturated lead acetate solution. The ammonium salt, (NH4)2Pb(Cr04)2, is prepared similarly, and both salts are decomposed into their components by %vater. 

Use Preparation of lead salts, lead chromate pigments, analytical reagent. 

K. Fischbeck and E. Einecke found that the cathodic polarization of ferrous, cuprous, calcium, and magnesium chromites produces chromic acid, whilst the other chromites are unaffected, and natural chrome ironstone behaves in a like manner, but other commercial chromites are reduced on cathodic polarization, and yield chromic acid on anodic polarization. Chromites behave as an intermediate electrode. 0. Unverdorben observed that chromyl fluoride, prepared by heating a mixture of fluorspar, lead chromate, and sulphuric acid, when passed into water, furnishes an aq. soln. of this oxide. The soln. was treated with silver nitrate, and the washed precipitate of silver chromate decomposed by hydrochloric acid. A. Mans said that anhydrous sulphuric acid or fuming sulphuric acid is not suited for the preparation because of its volatilization with the chromyl fluoride. 

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