Errors in weighing

Change in the condition of the containing vessel or of the substance between successive weighings. 

The first source of error is occasioned by change in weight of the containing vessel (a) by absorption or loss of moisture, (b) by electrification of the 

Now consider the case of a solid, such as potassium chloride, under the above conditions. The density of potassium chloride is 1.99. If 2 g of the salt are weighed, the apparent loss in weight ( = weight of air displaced) is 2 x 0.0012/1.99 = 0.0012 g. The apparent loss in weight for the weights is 2 x 0.0012/8.0 = 0.000 30 g. Hence 2g of potassium chloride will weigh 0.0012 — 0.000 30 = 0.00090 g less in air than in vacuo, a difference of 0.05 per cent. 

It must be pointed out that for most analytical purposes where it is desired to express the results in the form of a percentage, the ratio of the weights in air, so far as solids are concerned, will give a result which is practically the same as that which would be given by the weights in vacuo. Hence no buoyancy 

The density of the air will depend upon the humidity, the temperature, and the pressure. For an average relative humidity (50 per cent) and average conditions of temperature and pressure in a laboratory, the density of the air will rarely fall outside the limits 0.0011 and 0.0013 g mL 1. It is therefore permissible for analytical purposes to take the weight of 1 mL of air as 0.0012 g. 

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Care should be taken to avoid powerbil suction or air flow on the mixer or the weigh hopper from which the ingredients feed into the mixer. If the dust-collection suction on the mixer is too strong, vital ingredients may be sucked out. If the dust-collec tion suction on the weighing system is too strong, errors in weighing may result. 

Only a small amount of potassium iodate is needed so that the error in weighing 0.14-0.15 g may be appreciable. In this case it is better to weigh out accurately 4.28 g of the salt (if a slightly different weight is used, the exact molarity is calculated), dissolve it in water, and make up to 1 L in a graduated flask. Twenty-five millilitres of this solution are treated with excess of pure potassium iodide (I g of the solid or 10 mL of 10 per cent solution), followed by 3 mL of IM sulphuric acid, and the liberated iodine is titrated as detailed above. 

To make such a solution, we could calculate exactly how many moles of acid to add to water, but this method is generally difficult, since even small errors in weighing the acid can cause wide fluctuations in the pH. Furthermore, we cannot easily weigh out one of acid oxides such as NO. Anyway, the pH of a weak acid does not clearly follow the acid s concentration (see p. 254). 

Accuracy and precision depend on the propagation of error starting from the error in weighing, volumetrically preparing the sample, and delivering the titrant to the sample. 

Figure 8 is a plot of r89M4 vs. particle size. We define rlt j as the ratio of equivalent fissions of i to equivalent fissions of j. These ratios have several advantages (1) they are independent of the percent active particles, (2) they are independent of errors in weighing the samples, and (3) in a truly representative sample of weapon debris, rltj = 1 for all i and j. Figure 8 presents two surprises (1) for Zuni, r89,144 is generally lower in the more distant sample, and (2) the Zuni 85-km. curve below 150/x and the Bravo curve above 50/x have positive slopes (and appear to... 

Corner, M. Errors in Weighing not Inherent in the Balance. In Proceedings of the International Symposium on Microchemistry, Birmingham, 1958. Oxford Pergamon Press 1960. 

Each of these factors is dependent on the other two, and the failure of any one of them can affect the accuracy of a weighing. For example, do not expect great accuracy from a balance that is located above a radiator. Likewise, do not expect accuracy from a balance which has just demonstrated poor precision. Finding the source of errors in weighing is a step-by-step process. You must rule out each problem before moving on to the next level. 

If the percent recovery is less than 99.0%, check the condition of the sieves and for possible errors in weighing, and repeat the test. If the percent recovery is not less than 99.0%, calculate the percent retained on each sieve by the formula... 

Richards and his coadjutors have proved one of the fundamental errors in the work of Stas, for years regarded as a model of accuracy, to have been the employment of excessive quantities of substances. His object was to avoid errors in weighing, but his method necessitated a concentration of the solutions such as induced occlusion of extraneous material in his precipitates. This experimental defect and other sources of error have been avoided by the American school, with Richards as its leader, and a new era in the field of atomic-weight determination has been initiated. 

The number of particles on the sieve (load). The smaller the sieve loading the more rapid the analysis too low a load however leads to errors in weighing and unacceptable percentage losses. 

The results may be expressed in terms of the nominal size, although it is preferable to use calibrated sieves. A reference set of sieves should be used after every fiftieth analysis for comparison purposes in order to detect wear. In essence, the smaller the sieve loading, the more rapid is the sieving operation. The low weights however lead to errors in weighing and intolerable percentage losses. 

Note 2. The weights and volumes, usied arc quoted to one decimal place, since this is the level of accuracy required for preparative. experiments which results from the techniques used, distillation, recrystallization, extraction, etc., and the noh-quantitative conversion of the reactants to products may be more significant than errors in weighing. 

It should have a high formula weight to minimize the effect of error in weighing. 

A high formula weight means a larger weight must be taken for a given number of moles. This reduces the error in weighing. 

What is the true molecular weight (15) What error in the molecular weight would be caused by an error of a centigram in weighing the fiask when filled with carbon dioxide (16) How great an error in weighing the fiask full of water would have caused the same error in your final result ... 

Problem 1J3. If the indeterminate error in weighing on a laboratory balance is 0.003 g, what size sample should you take to keep the relative error to 1.0% ... 

Evaluation of errors and accuracy in combustion calorimetry High-precision combustion calorimetry is considered to be one of the most difficult experimental procedures [37]. The precision required in combustion experiments would have to be 0.01-0.02% in order to have an uncertainty in the enthalpy of formation of approximately 1 kJ mof, which is the precision necessary to obtain reliable thermochemical data. This includes errors in all three different parts of a combustion experiment. In the calorimetric part there are errors in weighing the water in the calorimetric jacket and also in the temperature measurements. In the chemical part there are errors in weighing the sample and in the data for auxiliary materials (benzoic acid, cotton. Vaseline, polythene, etc.), errors in the combustion process caused by production of either carbon monoxide or soot and, in the case of compounds with S or N, errors arising from the production of SO and NO instead of SO2 and NO2. Important errors may arise from sample impurities, water being one of the most important and difficult, because many compounds are hygroscopic. In the third part there are errors in the corrections to the standard state. Thus errors in any part of experiment should be kept imder 0.01%. 

The use of large samples and large titrations not only minimises sampling errors but also reduces the relative errors in weighing and titration, and is to be preferred to the use of small samples and small titrations, circumstances permitting. A weighing error of 1 mg on a 10 g sample is 0.01% relative on a 0.1 g sample it is 1% relative. An error of one drop, or about 0.05 ml, on a 25 ml titration is 0.2% relative on a 5 ml titration it is 1% relative. 

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