School of Science
http://hdl.handle.net/10500/2736
2015-08-29T06:24:25ZStudy of the effect of salt solutions on the kinetics of sucrose inversion as monitored by polarimetry
http://hdl.handle.net/10500/18902
Study of the effect of salt solutions on the kinetics of sucrose inversion as monitored by polarimetry
Makwakwa, Tlou Auguston
The acid-catalyzed inversion of sucrose is often taken as an example of a first order reaction. It is, however, influenced by many factors such as temperature, type of acid used, concentration of sucrose, and the concentration of acid. What has received little attention so far is the influence of addition, in particular, other salts to the reacting solution. In this study, the influence of different salt solutions on the kinetics of sucrose inversion rate was studied at 29 °C by use of optical rotation measurements. The salts chosen for this study are readily soluble in sucrose solution and they provide an opportunity to study the interaction of electrolytes in aqueous solution of sucrose as well as their effects on the inversion of sucrose kinetics. The rates are found to be influenced by the concentration of the salts. No significant differences was measured when the salt were dissolved either in the sucrose or in the acid solutions.
The influence of added salts to saccharide solutions was determined by evaluating the difference between the rotation of pure saccharides solutions and the rotation of pure saccharide solutions with salts. The changes in optical rotation were compared to the Hofmeister series.
The saccharide-salt systems containing acidic salts (Na2HPO4 or NaH2PO4) were found to be dependent on the pH. Changing the molar ratio of sucrose and salt added also had an influence of the change in optical rotation.
2014-06-01T00:00:00ZThe use of effect sizes in credit rating models
http://hdl.handle.net/10500/18790
The use of effect sizes in credit rating models
Steyn, Hendrik Stefanus
The aim of this thesis was to investigate the use of effect sizes to report the results of
statistical credit rating models in a more practical way. Rating systems in the form of
statistical probability models like logistic regression models are used to forecast the
behaviour of clients and guide business in rating clients as “high” or “low” risk borrowers.
Therefore, model results were reported in terms of statistical significance as well as business
language (practical significance), which business experts can understand and interpret. In this
thesis, statistical results were expressed as effect sizes like Cohen‟s d that puts the results into
standardised and measurable units, which can be reported practically. These effect sizes
indicated strength of correlations between variables, contribution of variables to the odds of
defaulting, the overall goodness-of-fit of the models and the models‟ discriminating ability
between high and low risk customers.
2014-12-01T00:00:00ZSynthesis of Polyaryl-substituted Bisquinazolinones with potential photophysical properties
http://hdl.handle.net/10500/18789
Synthesis of Polyaryl-substituted Bisquinazolinones with potential photophysical properties
Mmonwa, Mmakwena Modlicious
3,5-Dibromo-2-aminobenzamide was reacted with 1,3-cyclohexanedione derivatives in the presence of iodine as catalyst in toluene under reflux to afford novel 6,8-dibromo-2-[3-(2´-alkyl-1´,2´,3´,4´-tetrahydro-6´,8´-dibromo-4´-oxoquinazoline-2yl)propyl]quinazolin-4(3H)-ones in high yields. Suzuki-Miyaura cross-coupling of the latter with arylboronic acids in the presence of Pd(PPh3)2Cl2–Xphos catalyst complex and K2CO3 as a base in dioxane-water mixture (3:1, v/v) afforded the corresponding polyaryl-substituted bis-heterocycles in a single step operation. The resultant compounds were characterized using a combination of NMR (1H and 13C) and IR spectroscopic techniques, as well as mass spectrometry. The electronic absorption and emission properties of these polyaryl-substituted bis-heterocycles comprising 2,3-dihydroquinazolin-4(1H)-one and quinazolin-4(3H)-one moieties linked by a flexible carbon chain were measured in dimethylsulfoxide (DMSO) and acetic acid by means of UV-Vis and fluorescence spectroscopic techniques. The absorption spectra of the resultant polyaryl-substituted bis-heterocycles showed blue-shift in acetic acid and red-shift in DMSO, while their emission spectra are blue-shifted in DMSO and red-shifted in acetic acid. The 4-methoxy groups on aryl-substituents caused red shift on π‒π* transition of the aryl-substituents. Moreover, it was also observed that as the propyl linkage becomes more substituted, the absorption and emission intensities decrease.
2014-11-01T00:00:00ZModellering van afhanklikheid in die lineêre model : 'n meteorologiese toepassing
http://hdl.handle.net/10500/18737
Modellering van afhanklikheid in die lineêre model : 'n meteorologiese toepassing
Nieuwoudt, Reina
As deel van die weermodifikasie-eksperiment in Bethlehem, Suid-Afiika, is 'n reenmeternetwerk
geinstalleer, en word die neerslagwaardes R; wat by 43 reenmeterstasies waargeneem is, vergelyk
met die waargenome radar reflektiwiteit Z;. Alhoewel radar ruimtelike en tydskontinue metings van
reflektiwiteit bied wat onmiddellik by een sentrale punt beskikbaar is, is die akkuraatheid van radar
om reenval te meet onseker as gevolg van verskeie potensiele foute in die omskakeling van
reflektiwiteit na reenval. Dit word aanvaar dat reenmeters akkurate puntwaarnemings van reenval
gee en daar bestaan eenstemmigheid dat die kombinasie van die twee metodes beter is as enigeen
van die metodes afsonderlik. In hierdie studie ondersoek ek die toepassing van die veralgemeende
lineere model as 'n beramingstegniek.
Vorige studies gebruik die log-log transformasie, d. w.s. logZ = logA + b(logR) van die Z = ARb
verwantskap om die koeffisiente A en b met behulp van kleinste-kwadrate-regressie te bepaal.
Die implisiete aanname hiermee is dat die foute ongekorreleerd is.
Met die inverse verwantskap R = czd d.w.s. logR = logC + d(logZ) neem ek aan dat die
waarnemings nie onafhanklik is nie sodat die regressiekoeffisiente bereken word met behulp van
die metode van die veralgemeende lineere model. Om die ruimtelike afhanklikheid van die reenmeterwaarnemings
te modelleer, word eksperimentele variogramme uit die data bereken en gepas
met teoretiese variogramme wat gebruik word om die variansie-kovariansiematriks te vu!.
"Gemiddeld" vaar hierdie metode beter as gewone regressie vir analises wat reenmeters wat verder
as 45km vanaf die radarstel is, insluit.
Residu-stipping wys dat die afstand van die meter vanaf die radarstel as 'n afsonderlike onafhanklike
veranderlike in die regressievergelyking ingesluit behoort te word, d.w.s. die beraming
verbeter met logR = 3-0 + a,(logZ) + a2(afstand). Hierdie meervoudige regressiemodel stem ooreen
met die teoretiese model van Smith en Krajewski omdat e -- afstand as 'n praktiese manifestasie van
die foutproses [e.,, (ij)] beskou kan word. Omdat E(ez) = eE<ZJ e'"a' as Z 'n lognormaalverdeling het, kan die sydigheid wat ontstaan
wanneer antilogaritmes geneem word, reggestel word deur die beraamde reenval met e112
"' te
vermenigvuldig.
Die studie !ewer 'n bydrae met die afleiding van 'n beramingstegniek wat die beraming van
neerslag uit radar betekenisvol verbeter.; In a study of a rain-gauge network that was installed for a weather modification experiment in
Bethlehem, South Africa, precipitation values R; observed at 43 gauging stations are compared to
the observed radar reflectivity Z;. Although radar provides spatial and temporal measurements of
reflectivity that are immediately available at one location, the accuracy of radar estimation of
rainfall is uncertain due to various potential errors in the conversion from reflectivity to rainfall.
Rain-gauges are assumed to give accurate point measurements of rainfall and there is general
agreement that the combination of systems is better than either system alone. In this study I
explore the application of the general linear model as an estimation technique.
Previous studies have used the log-log transform, i.e. logZ = logA + b(logR) of the Z = ARb
relation, and applied least-squares regression analysis to determine the coefficients A and b. This
implicitly assumes that the disturbances are uncorrelated.
Working with the inverse relation R = czd i.e. logR = logC + d(logZ) and assuming that the
observations are not independent we compute the regression coefficients using generalised least
squares. To model the spatial dependence of the rain-gauge observations we compute
experimental variograms from the data and fit them with theoretical variograms which are then
used to fill the variance-covariance matrix. "On average" this method performs better than
ordinary regression for the analyses that included rain-gauges further than 45km from the radar
set.
Residual plotting revealed that distance of the rain-gauge from the radar set should be included as
a separate independent variable in the regression equation, i.e. logR = ao + a1(logZ) + a1(distance)
improved the estimation of rainfall as it performs better than ordinary regression. This multiple
regression model agrees with the theoretical model of Smith and Krajewski in the sense that
e "'distance is a practical manifestation of the error process [ e,, (ij)].
Showing that E( ez) = el!.(!.) e 112
"' if Z has a lognormal distribution, the bias when taking antilogs can be removed by multiplying estimated rainfall by e1
'
2a'.
The contribution of this study is the derivation of an estimation technique which significantly
improves the estimation of rainfall from radar
Text in Afrikaans, abstract in Afrikaans and English
1997-06-01T00:00:00Z